CENOZOIC DEPOSITS AND LAND FORMS
THE RECORD OF CENOZOIC HISTORY
The present section of this report deals with the Cenozoic history of the southern Guadalupe Mountains and their surroundings. Here, a different method must be adopted from that used in interpreting Permian history. For Permian time, a relatively complete record is contained in the rocks which form the present mountains, and this record can be dealt with, step by step, by following the stratigraphic sequence upward. For Cenozoic time, the stratigraphic record is incomplete and scattered, being represented in the southern Guadalupe Mountains by various unconsolidated deposits. Gaps in the record must be filled in by interpreting the land forms, the sequence of tectonic events, and the stratigraphic record in nearby regions.
Spreading over the consolidated rocks of the southern Guadalupe Mountains are unconsolidated deposits of later Cenozoic age (shown on plate 22). They are generally found in the lower places where they form either a thin veneer over previously graded rock-cut surfaces (pediments), or a thick fill in areas of decided tectonic relief where the bedrock lies far beneath the surface (bajadas). The unconsolidated deposits, which have an obvious source in the present mountains, consist of fragments washed in from the higher parts of the area that were being eroded while the deposits were forming.
Although these deposits were laid down after the mountains had attained nearly their present form, the aspect of the mountains is still relatively youthful. Their escarpments are high and straight and the canyons that trench them are deep and V-shaped. The plains that surround them are generally bajadas, characteristic of the early phases of degradation of a mountain area. Pediments, which are characteristic of more stable conditions, occupy only small areas. Moreover, some of the older unconsolidated deposits are faulted and tilted, indicating that the mountains continued to be uplifted after the deposits began to be spread over the region.
The unconsolidated deposits are doubtless all of later Cenozoic age. Deposits that lie near the present streams are obviously of Recent age; others, which are now dissected and disturbed, must be as old as the Pleistocene. Still other deposits, perhaps of Pliocene age, may lie beneath the surface of the Salt Basin west of the mountains, for deposits of that age are known in the more dissected desert basins nearby. In the Salt Basin, however, they are wholly concealed from view, as the younger deposits that cover the floor of the basin have been penetrated very little by erosion.
The higher-standing parts of the area have been undergoing erosion ever since the mountains were uplifted, and have not been covered by deposits. Although their slopes are still being worn back, certain relic features persist, inherited from earlier periods. Some of those in the mountains are probably older than any unconsolidated deposits now visible in the plains, and may date from the early phases of the uplift of the area, or even before.
Some of the surface features of the Guadalupe Mountains region are of modern origin, but most of them have been in growth throughout a long span of Cenozoic time. During most of this time the surface features were shaped by processes conditioned by an arid climate. The extensive mountain areas composed of limestone and the widespread interior drainage system could not have persisted as well in a humid climate, nor would the mountain ridges have retained their present harsh outlines or be so poorly mantled by soil. The deposits of the old debris aprons (bajadas), like those forming today, consist of slightly decayed rock fragments, and the subsoil on both young and old land surfaces is impregnated by caliche, a product of soil formation that exists only in regions of scanty rainfall.
A few of the surface features of the region seem to be relics of processes no longer at work. Such processes existed in part during interludes of more humid climate in Pleistocene time; in general, however, the interludes were too brief to have left much of a mark on the land scape.
Because of the fact that present and past conditions are closely related, I feel it desirable, before taking up the Cenozoic history, to consider the modern landscape and processes at work on it. The landscape and the processes are probably similar to those of the past and their understanding will aid in the interpretation of Cenozoic history.
The southwestern arid region of the United States, lying south of the middle of the temperate zone, has short, mild winters, and relatively high temperatures during most of the year.97 In the Guadalupe Mountains, periods of freezing weather are of short duration, and frost action is much less effective than at higher latitudes.
To judge from the record of nearby stations, the rainfall of the Guadalupe Mountain region varies from 10 inches on the plains to nearly 14 inches on the mountain summits,98 the increase in rainfall with altitude being clearly reflected by the upward increase in the density of the vegetation. Like other high-standing desert ranges, the mountains are a gathering ground for clouds, and they are likely to capture much rainfall that otherwise would never reach the ground.
More than half the normal year's rainfall comes in July, August, and September, and is of the convectional, thunderstorm type. "The individual afternoon thunderstorm does not cover much territory. Clouds gather over a mountain range, where the instability of the air becomes particularly great; thunder begins to roll at noon, or in early afternoon, and a short, brisk downpour covers part of the land that has lain in the shadow of the thunder heads."99 These rains are not necessarily torrential. Although the run-off that follows them is rapid, this is less because of the volume and rapidity of the downpour than because of the barrenness of the land that receives it.1 Occasional rainstorms during the summer and other seasons are of cyclonic type. They last longer and cover a wider expanse of territory than the thunderstorms. Evaporation is rapid, and surfaces wet by the rains dry out rather quickly afterwards, thereby reducing the effectiveness of the precipitation as an aid to plant growth.
Mean annual figures express only poorly the actual rainfall of the arid region, for actual rainfall fluctuates from year to year within wide limits. During some years the rainfall, largely by an increase in the number of cyclonic storms, may be so excessive as to create temporary subhumid conditions; during others, it may be so deficient as to create desert conditions.2 Such fluctuations seem to take place in 5- to 10-year periods, and indications of still longer cycles of 50 to 100 years are suggested by tree-ring records, and by meteorological observations which are as yet insufficient for any final conclusion.3
Because of their height and exposed position, the Guadalupe Mountains are swept by strong winds, which increase in frequency and violence during the drier years. The full force of the gales is directed against the mountain crests, saddles, and plateau surfaces; because of the strong relief, some points in the canyons and near the bases of steep slopes are sheltered from winds from certain directions.
SOILS AND VEGETATION
The surface of the area is more or less mantled by typical arid-climate soils, which are thin, poor, calcareous, and generally impregnated by caliche at depth.4 The soil profiles of the area, however, may not result entirely from processes now at work, but may reflect a climate of the near past, which differed somewhat in the amount of rainfall, and other features.5
Within the Guadalupe Mountains and its foothills are extensive tracts of bare rock and rough, stony land, interspersed with patches of immature, residual soils, generally full of rock fragments (Ector series).6 Despite the somewhat greater rainfall of the mountain areas, soils there do not have an opportunity to reach maturity because they are constantly being washed away. The higher summits and sheltered slopes of the mountains support a sparse forest growth, resulting not so much from favorable soil conditions as from favorable rainfall. In the lower, drier parts of the mountains, where the soils are more extensive, the surface is thinly carpeted with grass, interspersed with sotol, lechuguilla, other woody shrubs, and a few trees.
On the detrital aprons that fringe the Guadalupe Mountains are more extensive, transported, calcareous soils (Reeves series).7 On the higher parts of the alluvial slopes they are gravelly loams and gravelly fine sands. Farther out are silty clay loams, fine sandy loams, and areas of fine sand that are blown into low dunes by the wind. The alluvial slopes support variously spaced clusters of creosote bushes and other woody shrubs, between which is a sparse grass cover. At the bases of the alluvial slopes is the nearly level expanse of the Salt Basin, which is mantled by a deep, gypsiferous, alkaline soil (Reeves chalk),8 on which is a moderately thick growth of wiry yeso grass, salt grass, and mesquite.
The inadequate soil cover and sparse vegetation greatly facilitate run-off. The cover of vegetation however, is somewhat more effective in resisting erosion than might be supposed. Saner9 has pointed out that in the similar area of southeastern Arizona "The rainfall regime is such that it favors the development of an adequate cover of vegetation prior to the heavier rains. The grasses are dormant until the summer rains set in, but then get underway with great rapidity. The heavier summer rains are almost never at the beginning, but rather toward the end of the rainy season, when the vegetation is already well established."
Climatic fluctuations have a largely unknown but probably important influence on the vegetative cover. "A sharp desert year may have more effect on crops, tree seedlings, and soil erosion than half a dozen normally moist years. Likewise, a single exceptionally wet year may start a grass cover that will survive less favorable years."10 During extended dry periods, the cover may be so reduced as to permit extensive soil erosion. Bryan and Albritton11 report ancient, now filled arroyos in the flood plains of some New Mexico and Texas streams, that were cut during such dry periods, long before the region was occupied by white settlers.
Conditions of this sort probably have been intensified by the manner in which the land has been used since white settlement. By a combination of drought and overgrazing, the grass and shrub cover in places has been seriously depleted. Much soil erosion is in evidence, and formerly level alluvial flats are now penetrated by steep-walled arroyos.
STREAMS AND THEIR WORK
RELATION TO BASE LEVEL
Streams that flow east from the crest of the Guadalupe and Delaware Mountains drain into the Pecos River, a through-flowing stream at the base of the slope, 50 miles away. Because these streams are members of a through-flowing system, they are adjusted to either a constant or gradually lowering base level. Material transported by them is ordinarily carried out of the region, and no doubt eventually finds it way to the sea.
Streams on the west slope have steeper gradients and shorter courses than those on the east slope and are reducing the asymmetry of the mountain block by cutting headward into the area drained by the east-flowing streams. Because they drain into the Salt Basin, a region of interior drainage, they cannot take material out of the region, but must deposit their loads at the bases of the steep slopes. They are probably adjusted to a slowly rising base level, although accretion of new material on the basin floor seems to be taking place very slowly at the present time. It is even possible that the floor of the Salt Basin is being lowered by deflation, but to the east toward which the dominant winds must blow, the enclosing mountains seem to rise too steeply for much material to be blown over them, and so out of the region.
Because of the arid climate, no streams flow permanently in the area, except in sheltered canyons within the Guadalupe Mountains. During most of the year the stream channels of the region are dry gravel washes, and what water exists percolates beneath the surface. Only after rains do the channels spring into action, and for a short time become filled by rushing torrents. During their brief existence the torrents pick up the gravel and finer detritus and shift it downstream, and by so doing corrade the bed and banks of the channel.
The streams of the region are thus effective agents of erosion only during parts of the year, yet they dominate the sculpture of the landscape. Except on the floor of the Salt Basin, the whole surface is penetrated by channels, each traversing a valley or detrital slope of its own creation. Not only have the streams been able to cut valleys and build up detrital slopes, but many of them, particularly those draining eastward from the crest of the mountains have been able to cut down to grade or have reached that condition of balance "in which the ability of transporting forces to do work is equal to the work they have to do."12
Between the stream channels that penetrate the region is a complex of sloping surfaces, across which storm waters and weathered rock fragments travel and are collected and carried away by the streams below. Some of them are gently inclined and form plains of various sorts, which are graded with respect to the streams and related agencies at work on them. Other surfaces rise steeply, forming the foothills and mountains, and are as yet unconsumed by the attacking streams. One might suppose, because of their steepness and height, that such surfaces were unstable. Actually, except where they have recently been unbalanced by tectonic or climatic changes, they themselves are graded for the processes at work on them, and their inclination is just steep enough for material to be carried across them.13
The steeper slopes, although graded, remain steep because the processes at work on them are less effective than on the gentler slopes. The fragments that lie on them, being close to their sources, are little broken down, and being of relatively large size, are not easy to transport. At the same time running water performs less work, for it has not yet gathered into streams, but is deployed in sheets, rills, and streamlets. In humid regions, where the steep slopes are mantled by residual soil, its downward creep is the chief transporting force. In dry regions, such as the Guadalupe Mountains area, where the soil cover is thin or absent, gravity is a dominant force, acting directly rather than as an aid to soil or stream movement. The steep slopes thus stand at an angle only a little less than the angle of rest of the fragments that cover them. The fragments can almost slide or roll of their own weight, and need but little water to urge them forward.
In granitic mountains of arid regions, it has been observed that the steeper graded slopes change with an abrupt angle into the gentler graded slopes at their bases.14 In nongranitic mountains, however, such as those in the area of this report, the steep slopes characteristically grade into the gentle ones through a concave arc.15 This difference in profile is probably caused by the difference in sorts of weathered materials that lie on the two types of surfaces. Those in nongranitic mountains are likely to be smaller, and subject to more rapid disintegration as they move down the slope, than are those in granitic mountains. Hence, the grade needed to transport them lessens down the slope, resulting in a curved profile. In the nongranitic area of the southern Guadalupe Mountains such features are characteristic, and the mountain slopes change gradually rather than abruptly into the plains at their bases.
The gentler-graded slopes have a lower angle because of the greater effectiveness of the transporting processes. Fragments that reach them have been in process of transport for a longer time, and hence have been reduced in size by breakage when falling, by abrasion when carried by water, and by weathering when at rest. Here, running water has gathered into streams of small to large size. The gentler slopes are, to a large degree, graded with respect to the streams, either by cutting down the bedrock, or by building up the areas below grade through deposition. The worn-down bedrock surfaces of arid regions are called pediments,16 and the built-up surfaces underlain by deposits are called bajadas.17
CONTROL OF DEGRADATION BY STREAMS
Degradation of such an area as the Guadalupe Mountains, made up of graded surfaces of varying degrees of steepness, takes place by the propagation of activity backward and sideward from the streams that drain it, thereby extending the gentler slopes, which become adjusted to the more effective transporting agents, at the expense of the steeper slopes, which are adjusted to the less effective transporting agents. By cutting downward, the larger streams renew the activity of the smaller ones on the adjacent pediments and bajadas. They are not only lowered, but are also extended mountainward, thus reviving the activity of sheet wash and gravity on the graded mountain slopes. As a result, the mountain slopes retreat in their turn.
Degradation of the Guadalupe Mountains region has not yet reached an advanced stage, largely because the mountains are still geologically young. Although most of the surfaces are graded, steep slopes still dominate, and pediments are narrow. The gentler slopes along the western edge of the mountains are mostly bajadas "characteristic of disturbed conditions."18 They are formed during the early stages of degradation of a tectonically unbalanced region by the effort of streams to attain a graded slope.
KINDS OF SLOPES
In the Guadalupe Mountains region, the steep slopes of the mountains and foothills are carved from limestones, sandstones, and other stratified sedimentary rocks. The inclination of the slopes depends to a large degree on the nature of the rocks from which they are carved.19 Rocks that are massive and little jointed weather out in large blocks that are difficult to transport, and hence form steep slopes or cliffs. The rocks that break up into small fragments are worn back into surfaces with a lower inclination. Here, as in the country described by Bryan,20 the mountain slopes can be classified, according to steepness, into cliffy slopes, boulder-controlled slopes, and rain-washed slopes.
Because of the arid climate and consequent ineffectiveness of solution weathering, the limestones of the region are resistant to erosion. Some of those in the Guadalupe Mountains, belonging to the Capitan and Goat Seep formations, are so indistinctly bedded that they behave as massive rocks. In most places they form steep, graded, boulder-controlled slopes, but on the west side of the mountains, where underlying, poorly resistant sandstones are laid bare to erosion, sapping at their bases has maintained them in cliffy slopes.
In other parts of the region the limestones and sandstones are well bedded. Some of these well-bedded rocks give rise to weathered blocks so large that they form boulder-controlled slopes almost as steep as those of the massive limestones. The sandstones of the Delaware Mountain group form steep slopes of this sort in places, even though they consist largely of thin-bedded material, because layers supplying large weathered blocks are either interbedded with them or overlie them. Elsewhere, the thin-bedded sandstones are cut back into gentle rain-washed slopes graded for the transportation of their own fine-textured weathered detritus.
Because of the general absence of soil creep in the dry region, the different classes of slopes tend to maintain their identity, even when erosion is far advanced.21 Massive rocks continue to project as cliffy slopes. Boulder-controlled slopes and rain-washed slopes stand at nearly the same angle during their retreat, instead of being worn down to more subdued surfaces.
The rocks uncovered on the slopes already contain planes of weakness (bedding and joints) which determine to a large degree the size and shape of the fragments into which the rock will subsequently break. When the rocks are exposed to the weather, the processes of disintegration and decay work inward along these planes, loosening the intervening fragments and modifying their surfaces. Weathering takes place mainly by chemical, and partly by physical processes.
The calcitic and dolomitic limestones weather mainly by chemical processes and especially by the dissolving action of water, although its work is retarded by the dry climate. In South McKittrick Canyon, the openings of numerous solution caverns can be seen on the mountain sides, and the stream itself is so charged with calcium carbonate dissolved from the limestones of its drainage area that it is depositing masses of travertine in its channel. At one place in Pine Spring Canyon, solution of the limestones along joints has etched these rocks into groups of pinnacles. Elsewhere, solution has produced less striking forms, yet it has been at work on nearly every outcrop, producing jagged surfaces, and dividing the rock into blocks along widened joints.
Physical processes of weathering have aided in breaking down the limestones, as many of their surfaces are exfoliated. Ledges and residual blocks are in places bounded by curved surfaces, and contain incipient curved cracks within the rock. Broken spalls lie near them on the ground. The exfoliating blocks range from a few feet to 5 or 10 feet in diameter. In the area of Goat Seep limestone in the western foothills of the mountains, the spalls have angular outer surfaces, deeply pitted by solution, but smoothly curving, fresh, clean-cut inner surfaces, indicating that blocks previously shaped by solution had suddenly been split. Analyses of the spalls here and elsewhere show them to consist of calcium or magnesium carbonate, with few impurities.
These features are not easy to explain, for both calcitic and dolomitic limestones have a low coefficient of expansion, which would make their breaking by normal temperature changes unlikely. Moreover, they do not contain minerals that change in volume during chemical decomposition, and thereby set up strains within the rock. It is possible that some of the exfoliation was caused by the heat of brush or forest fires.22 Some of the most prominent spalling is in exposed, rocky areas, that do not support much vegetation, where its origin is not clear.
The thicker-bedded sandstones weather mainly by such physical processes as exfoliation and granular decay. Weathering along joints and the sapping of weaker beds beneath break them into great, rectangular blocks. Exfoliation reduces many of these blocks to rounded boulders or shapes them into pedestal rocks. Exfoliation shells are constantly developing. Thus, some Indian pictographs in a shallow recess near Chinamans Hat are partly destroyed by the scaling off of thin sheets of the rock on which they were painted. Some chemical processes work hand in hand with the physical processes. Ferruginous material tends to concentrate as a desert varnish near the surface of the sandstones, forming a resistant brown crust over the softer, more friable rock. Later weathering has sought out weak places in the coating, and has cut out pockets that extend behind it.
The thin-bedded sandstones weather, largely by physical processes, into fine-textured debris, such as chips and plates broken out along closely spaced bedding planes and joints, and into sandy soil produced by granular disintegration.
PROCESSES THAT LOOSEN WEATHERED BLOCKS
After being split by weathering, the rock debris is set free in various ways from its parent ledges, and is made available for transportation. Large amounts of material are no doubt thus released by the weathering away or washing out of the rocks that support them, especially if the supporting rocks belong to a poorly resistant stratum. Some are loosened during the colder months by frost. Thus, on a warm, sunny day that followed a period of freezing weather in November 1934, H. C. Fountain and I saw and heard many rocks fall from the cliffs near Guadalupe Peak. Water that had run into crevices and frozen there had pried the rocks apart, and the melting of the ice was no doubt letting the newly broken rocks fall. The work of frost in this region, however, is less effective than at higher altitudes and latitudes.
Some fragments may be broken from their parent ledges by catastrophic forces. Fresh scars that dot the slopes of the Guadalupe Mountains, where blocks have recently come off, have been pointed out to me by local residents as marks left by lightning. It is true that lightning plays about the mountain sides during every thunder storm, and trees riven by lightning bolts can be seen on most ridges. The suggestion that lightning made the scars in the rocks, however, seems to be based on inference rather than observation, and it is unlikely that it could break free any more than small rock masses. Some blocks previously loosened by weathering may be shaken free by earthquakes. Thus, W. B. Lang reports that during the Valentine earthquake of 1931, whose intensity in the area was VI, there were slides of rock in the McKittrick and Dog Canyon areas.23 Mr. A. J. Williams, who lived near the cliffs on the west side of the mountains, however, did not observe any rock falls from them at this time.
Some of the largest rock falls in the area have come from a place near the top of the cliffs on the west side of the Guadalupe Mountains a few hundred yards north of El Capitan. According to Mr. J. T. Smith, of Frijole, one of them took place about 1920, when so large a mass was suddenly loosened that its impact shook the windows of his ranch, 4 miles away. An other mass fell from the same place in December 1934, when H. C. Fountain and I were in the region. The cause of these rock falls is undetermined, but they seem to come from a place on the cliffs that has been much weakened by weathering.
TRANSPORTATION OF MATERIAL ON SLOPES
After being set free, the weathered fragments are moved down the slopes by transporting agents. On the cliffs, where they can fall to the base without hindrance, transportation is by gravity alone. Elsewhere, although fragments may be able to roll for short distances, they must be continually urged forward by rain wash. Most of the mountain sides are graded to a combination of gravity and washing. According to Bryan,24
The material in transport on the boulder-controlled slopes consists of boulders and smaller fragments, down to pebble size, generally of angular shape and either scattered singly or gathered in waste streams approximately one boulder deep. Soil is almost absent, and soil creep plays little part in the movement of detritus. In most places, the bedrock is scarcely or not at all concealed by the surficial material.
On boulder-controlled slopes carved from interbedded thin- and thick-bedded sandstones, large blocks from the thicker layers strew the surface in great numbers. They seem to be so lightly placed that one expects their movement will be rapid; in fact, Mr. Walter Glover reports that where United States Highway No. 62 has been cut into one of the slopes of Guadalupe Canyon, several sandstone blocks have fallen on the road in recent years. Close study of the sandstone blocks on the slopes, however, shows that in most places their movement must be very slow. Most of them rest on flat faces or have been rather firmly anchored by the surrounding debris. Few of them can be pushed by hand from their present positions, and those few are generally caught again by other blocks after rolling a few feet down the slope. On one of the slopes of Guadalupe Canyon not far from the cut of the highway a large block has an Indian pictograph on one face. This pictograph is in such a position that the block cannot have moved appreciably in the hundred years or more since it was made.
Large blocks lie on the lower slopes of Pine Spring and McKittrick Canyons. They are of massive limestone, and some are more than 30 feet across. They do not seem to be in the process of movement at the present time. They may have rolled to their present positions after having been released from the steeper slopes above. Probably they have not moved since, except during violently torrential rainstorms.
On the slopes carved from the nonresistant rocks, such as thin-bedded sandstone and anhydrite, weathering has produced more soil than elsewhere. Movement of material by soil creep here may be of some importance. At any rate, the land surface, particularly in the anhydrite area, is reduced to subdued, gently rounded forms, which resemble the soil-cloaked slopes of humid regions more closely than do any others in the area.
The most prominent cliffs in the area are those near Guadalupe Peak and El Capitan at the south end of the Guadalupe Mountains, which form the top of a high escarpment, and are themselves 500 to 1,500 feet high. They are carved from calcitic and dolomitic limestones of the Capitan and Goat Seep formations, whose bedding planes are either so indistinct, or so welded together, that they have little influence on the weathering of the rock. The rock is traversed by several sets of joints, many of which extend through the full height of the cliff.
In horizontal plan, the cliffs consist of several segments of north-northwest trend, parallel to the dominant joint set of the region, and of shorter offsets of west-northwest trend. In vertical profile, they consist of smooth, fresh, vertical parts that follow single joint planes, separated by craggy parts carved from the rocks between the joints. The craggy parts are more weathered than the vertical parts, and in places some vegetation has obtained foothold upon them.
At their tops, the cliffs intersect gentler slopes that drain to the east. The angle of intersection is generally sharp, as though the cliffs were being cut back more rapidly than the slopes. The profile of the cliff summits as seen from the west is undulatory, each low place marking the beheaded end of an east-draining valley.
The map relations of the cliffs are shown on plate 3, and in greater detail on plate 9. Their structure is shown on the sections of plate 9, and section KK', plate 17. For views of them, see the aerial photograph, plate 1, and the panoramas, plate 5, A (which shows their appearance from the south), and plates 5, B and 12 (which show their appearance from the west).
The relations of the cliffs to jointing can be seen on the structure map, plate 20, where the pattern of the cliffs can be compared with observed joint trends. The joints are suggested on plate 12, A, where many of the vertical lines are drawn along actual joint planes. The undulatory profile of the cliff summits is well shown in the view from the west, plate 5, B.
Through most of their length, the cliffs surmount a slope 500 to 2,000 feet high, carved from the underlying less resistant sandstones of the Delaware Mountain group. The sandstone slopes are more or less mantled by blocks of limestone that have fallen from the cliffs above and have gathered into long waste streams between projecting rock spurs. The heads of the waste streams conceal the top of the poorly resistant sandstones and generally extend up to, but not above, the base of the overlying cliff-making formations. In places, however, the waste streams extend headward above their base along recesses cut along cross joints.
The cliffs in this district owe their prominence to the lofty position of the massive limestones, and to the poor resistance to erosion of the beds which underlie them. The cliffs stand high above the base-level of the Salt Basin to the west, in whose drainage system they lie, and are separated from its flanking bajada by mountain slopes thousands of feet high. Ordinarily the cliff-making rocks would wear back to graded, boulder-controlled slopes as they do in other parts of the area, but here they are continually renewed by the rapid erosion of the beds beneath. The beds beneath, moreover, form steeper slopes than they would assume without the capping of massive rock, because they are graded for carrying away not their own weathered fragments, but the larger, more unwieldly fragments from the cliffs above.
Views of the slopes below the cliffs can be seen on plate 12, A, the waste streams appearing most prominently below El Capitan and Guadalupe Peak. Waste streams now in the process of formation (labeled "younger slope deposits") are shown on plate 22. Figure 23, B, shows profiles of the slopes below the cliffs, including both a surface being cut in the present cycle, and one formed in a past cycle. The profiles of figure 23, A, show hypothetically the successive stages in the erosion of cliffs like those described, which stand above slopes carved from less resistant beds.
The slopes below the cliffs seem to have just attained grade (stages 3 and 4, fig. 23, A). Before grade was reached, the slopes were being cut back, either from an original steep fault surface (stage 1), or from a graded surface of a previous cycle. Then (stage 2) they were free of waste because blocks that fell on them from the cliffs could roll to their bases. The poorly resistant beds next beneath the cliff-making formations were thus exposed to erosion, allowing the cliff bases to be sapped, loosening slices of rocks from the cliffs. During this period, there was little time for weathering at the tops of the cliffs, and the cliffs cut off the graded slopes to the east at an acute angle.
Now that the slopes below the cliffs have been graded (stage 3), a mantle of talus and other waste has spread over them, which is just thick enough to be kept in motion by gravity and rain wash. This has so concealed the poorly resistant beds beneath the cliffs that the cliffs are being cut back more by weathering than by sapping. In places, the weathering of recesses in the cliffs has permitted the waste to encroach upward onto the cliff-making formations themselves (stage 4). Weathering of the tops of the cliffs has become important, tending to round their angle of intersection with the slopes behind. Material that is now falling from the cliffs comes from their tops, rather than their bases. To judge from the processes now at work, continued erosion will cause the waste streams to be extended farther up on the cliff-making formations (stage 5), and cause the cliff tops to be lowered by weathering.
BOULDER-CONTROLLED SLOPES ON MASSIVE ROCKS
The cliffs into which the massive Capitan and Goat Seep limestones have been carved are exceptional features in the region. In most places the same rocks form sloping mountain sides with an average inclination of 30 to 35 degrees. Such slopes are well developed in the southeast part of the Guadalupe Mountains, in the area drained by Pine Spring and McKittrick Canyons, and form the whole surface of the Patterson Hills southwest of the mountains.
A general view of the slopes of this sort can be seen in the aerial view, plate 18. Note the similarity in angle of slope on all the ridges. More detailed views of the boulder-controlled slopes which form the walls of North McKittrick Canyon can be seen in plate 16, B.
These slopes have been carved from rocks of the same composition and with the same spacing of bedding planes and joints as those which make the cliffs. Slopes rather than cliffs have formed because the underlying poorly resistant sandstones are scarcely or not at all exposed at the bases of the limestones, Instead of surmounting a long sandstone slope, the slopes on the massive limestones rise almost directly from the stream beds, pediments, or bajadas below, so they cannot be steepened by sapping at the base. They are therefore graded slopes, adjusted to the transportation across them of their own weathered rock fragments by gravity and rain wash.
On broader view, the mountain sides cut on massive rocks are smooth, but in detail they are a complex of bouldery rock surfaces, discontinuous ledges, clifflets, and patches of stony soil and slope wash. The clifflets form where the rocks are most massive and least jointed, and are not especially maintained by undercutting below. Ridge crests carved from the massive rocks alone are likely to be serrate or slightly rounded, but many of those in the southeastern Guadalupe Mountains are capped by flat-topped remnants of the overlying, well-bedded Carlsbad limestone. (Summits of both kinds appear in pl. 18.)
BOULDER-CONTROLLED SLOPES ON BEDDED ROCKS
Below the cliffs on the west side of the Guadalupe Mountains are slopes several thousand feet high, carved from bedded sandstones of the Delaware Mountain group. Similar slopes form most of the surface of the west-facing escarpment of the Delaware Mountains to the south. The slopes have been carved for the most part from soft, friable, thin-bedded sandstones that weather into small fragments. They would have been cut back to a low angle were it not that they are graded for the transportation across them of large, unwieldy fragments. In the Guadalupe Mountains, these fragments are of massive limestone and have fallen from the cliffs above. Elsewhere, they are of thick-bedded sandstone and limestone that come from layers interbedded in the thin-bedded sandstone. Slopes controlled by such boulders are likely to be as steep as those carved from the massive limestone.
Along the Delaware Mountains escarpment, most of the interbedded thick layers are of massive sandstone in beds as much as 100 feet thick, but near the rim of the escarpment there are several limestone beds. The thick-bedded sandstones form benches and lines of clifflets on the mountain sides, and one of them projects as a broad shelf about halfway up the slope below El Capitan. Where there has been considerable dissection, the massive beds form the caps of flat-topped mesas and castellated buttes, and where the strata are tilted, as in the foothills west of the mountains, they rise in hogback ridges.
For a general view of slopes of this sort see the panorama, plate 5, A, where they form most of the escarpment of the mountains below the cliffs, in the center and right-hand parts of the view. A more detailed view of the slopes below El Capitan appears on plate 1, and of the slopes in the Delaware Mountains farther south on plate 14, C. The latter shows some of the characteristic butte-and-mesa topography of the area.
Steep slopes carved from bedded rocks of another sort form most of the surface in the northwest part of the southern Guadalupe Mountains. These rocks are dolomitic limestones of the Goat Seep and Carlsbad formations, which lie in thin, even beds, with occasional breaks of softer, more marly or more sandy material. Slopes carved from them are not as steep as those carved from the massive limestones, and they have a very different aspect. Weathering has accentuated the already well-marked bedding planes, so as to give the mountain sides a banded appearance. Each white band is a ledge of limestone somewhat thicker than the rest, and each intervening dark band is a soil-covered slope cut on thinner-bedded limestone, sandstone, or marl.
A typical view of slopes of this sort can be seen in the panorama, plate 14, A. Their appearance can be compared with that of boulder-controlled slopes cut on massive rocks on plate 16, B, where slopes of bedded rock form most of the canyon wall to the left, and slopes of massive rock most of the canyon wall to the right.
Ridge crests on the bedded dolomitic limestones are likely to be gently rounded, spreading out into flattish surfaces on the broader divides, and narrowing into castellated walls between closely adjacent valleys. Few of the ridge crests follow any single bedding plane, but near Cutoff Mountain where the rocks are steeply tilted, broad dip slopes have been cut on some of the surfaces of the limestone beds in the Goat Seep and Bone Spring formations.
Besides the high, steeply sloping mountain sides that dominate the landscape in the southern Guadalupe Mountain region, there are other surfaces, as yet not reduced by streams, that have angles of 20 degrees or less. Slopes of this type form the summit and east slope of the Delaware Mountains in the southeast part of the area studied, and the hillsides of the Gypsum Plain to the east. They have been cut from thin-bedded sandstones of the Delaware Mountain group, from limestone layers interbedded with them, from anhydrite of the Castile formation, and from Quaternary gravels that in places spread over the bedrock.
A typical view of such topography can be seen in the panorama, plate 4, A, where the sandstones form the slopes and valley bottoms, and the limestones and gravels the mesa tops.
The thin-bedded sandstones are similar to those from which the steeper escarpments to the west have been carved. Here, however, few of the harder, thicker, interbedded layers are exposed on any individual hillside. Few large blocks are therefore contributed to the slope deposits, and the slopes are graded mainly for the transportation of fine-textured debris. Where unprotected by harder beds, the sandstones are worn back into gently rounded, grass-covered hills.
Where harder limestones are interbedded, the overlying poorly resistant sandstones have generally been stripped from the limestone surfaces, and the limestones form the caps of flat-topped mesas or of gently sloping cuestas. At the edges of the mesas and cuestas the limestones break off in low cliffs or chains of ledges, below which are slopes formed on the underlying sandstone, whose angles are steeper than those where no capping is present.
The Quaternary gravels, which spread as a sheet over the rocks of the Delaware Mountain group for several miles southeast of the edge of the Guadalupe Mountains (pl. 22), are almost as resistant to erosion as the hard limestone and sandstone layers interbedded with the thin-bedded sandstones. The deposit consists of closely packed cobbles and pebbles of resistant limestone washed out from the Guadalupe Mountains, cemented in many places by caliche. Even where not cemented, however, the gravels are probably so porous that water falling on them mostly sinks in, and erosion by rain wash and rills is therefore retarded.25 Because of their resistance to erosion, the now dissected remnants of gravel stand as sloping plains scored by narrow ravines or, where more greatly reduced, as flat-topped patches on the divides, not unlike the mesas carved from the hard layers of the bedrock.
The Guadalupe and Delaware Mountains and neighboring ranges of the arid region, composed of slopes of the sorts just described, rise abruptly from gently inclined plains which surround them like pedestals. To one who travels through the region, the mountains appear to dominate the scene, and the plains seem foreshortened to the eye. By comparison with the diverse and rugged mountainsides, their surfaces appear featureless and monotonous. Actually, the plains of the arid country occupy as wide an area as the mountains, and their surfaces, although less impressive, are equally diverse in form and origin.26
Large areas of the plains, particularly near the mountain bases, are dominated by the work of streams, which, in an effort to accomplish a graded slope, have shaped the plains into characteristic profiles. In part the plains are bajadas which have been built up by the deposition of detritus, and in part they are pediments, which have been carved out of the bedrock. West of the bajada that fringes the western base of the Guadalupe Mountains is the broad, nearly level floor of the Salt Basin, a typical desert bolson.27 Here, there is little evidence of stream work, and many of the features found on its surface seem to have been shaped by the wind.
ORIGIN OF BAJADAS AND PEDIMENTS
Bajadas are graded depositional surfaces built up by streams.28 Streams lay down deposits where they lose the power to transport the loads that they were carrying in their upper courses, either by loss of volume or loss of gradient. Volume is lost at the edge of the mountains partly because the mountains are the chief source of rainfall, and streams are not renewed on the plain, and partly because the streams sink into previously formed deposits, or dry up by evaporation. Gradient is lost because the stream enters either a region whose slopes have been changed by tectonic activity, or one in which the slopes were planed off to a low gradient under earlier, more favorable climatic conditions. Deposition continues until streams reach grade, and are able to carry their loads across the slope.
During the earlier stages of the degradation of a region after tectonic activity has ceased, when much coarse detritus is available and when many surface irregularities must be smoothed out, streams occupy themselves mainly with building up the depressed areas. Most of the plains along the mountain bases at that stage are bajadas or constructional surfaces. Later on, as the period of crustal stability lengthens, the streams begin to plane off the lower margins of the uplifted blocks, and wear down their rocks into pediments or erosional surfaces. Along the flanks of the Guadalupe and Delaware Mountains, which are still geologically young, most of the plains are bajadas, and pediments are narrow or absent. Pediments, however, have developed extensively during times of stillstand in the past. They are continuing to form, and if there are no great tectonic or climatic disturbances in the future, their area will be further extended.
Because pediments and bajadas are graded surfaces, they are cut down or built up until there is a balance between the transporting forces and the load to be carried. The strength of the transporting forces and the nature of the load varies, however, in response to renewed tectonic activity, to changes in the amount of rainfall, or to reduction of the mountain masses by degradation. Some of the larger tectonic or climatic fluctuations that occupy a long span of time so disturb the balance between transporting forces and load that they are reflected in the land forms. As a result, pediments may be dissected by accelerated erosion, and bajadas covered with sheets of material supplied by renewed aggradation. Also, bajadas may be dissected and pediments aggraded, thereby superimposing one contrasting type of land form upon the other. Such land forms of mixed origin are common in the Guadalupe Mountains region, suggesting a varied geomorphic history in the near past.
The pediments of the area have been cut only on the less resistant rocks, such as the sandstones of the Delaware Mountain group. They are most extensively developed where that group crops out on the east slope of the Delaware Mountains, and in the foothills to the west. Here, broad pediments have been cut in the past, and narrower ones are now being cut at lower levels. The limestones of the Guadalupe Mountains have been planed off very little, and most of the streams that drain them still flow in V-shaped canyons. The pediments of the area appear to have been cut by streams rather than sheet floods.29
The areal distribution of the pediments that are forming today is shown on plate 22, where they occupy the areas labeled "younger pediments" and "stream alluvium and cover of younger pediments." A small pediment area in the Delaware Mountains, thinly covered by deposits, can be seen along the stream channels in the foreground of plate 4, A. A more extensive pediment area, west of the Delaware Mountains, extends across the foreground of plate 5, A. Here, most of the lower ground is covered by deposits whose surfaces merge northward into a bajada which may be seen in the distance.
The pediments on the east slope of the Delaware Mountains are strips of flattish ground spread out side-ward from the flood plains of the streams, into which they merge at their lower edges. In places they are mantled only by coatings of residual and transported soil; elsewhere they are thinly covered by fine-grained alluvium. Older pediments, later buried under a sheet of gravel and now dissected to depths as great as 100 feet, occur in the same area and fringe the southeast base of the Guadalupe Mountains.
The pediments west of the Delaware Mountains form an irregular network that penetrates the foothill ridges of harder rock and mesalike remnants of an older, higher-standing, gravel-capped pediment. The pediments have been extended sideward from streams that flow westward across the foothills to the bajada at the edge of the Salt Basin. Owing, perhaps, to a gradual rising of the base level of the streams, most of the pediment areas are covered by alluvial deposits, probably of small thickness. Toward the divides, however, the rock surface is laid bare, and rises here and there in low protuberances, which probably mark the sites of former buttes, whose hard cappings have now been eroded away.
A bajada extends as a wide belt along the west side of the Guadalupe and Delaware Mountains, between the base of their west-facing escarpment and the floor of the Salt Basin. The unconsolidated deposits which compose the bajada are spread over a bedrock surface that probably has many irregularities of tectonic origin, and in places the deposits may be very thick. A somewhat similar area extends along the base of the Reef Escarpment, southeast of the Guadalupe Mountains. Here, however, the unconsolidated deposits are spread over the surface of the older pediment noted above, and are of no great thickness. They are now being dissected, and no further deposition is taking place on them at the present time.
The bajada on the west side of the mountains is shown on plate 22, where it occupies the area labeled "younger fanglomerate." Note the pattern of the streams on its surface, which are shown in a separate symbol. The bajada forms the foreground of plate 5, B. Its relations to the underlying rocks are shown on sections AA' and BB', plate 3. A schematic section across it, based on several actual profiles, is shown on figure 22, A.
The bajada west of the mountains is 2 to 4 miles wide and rises 500 to 1,500 feet from the floor of the Salt Basin to the bases of the mountains. In the northern part of the area studied, it is a succession of coalesced alluvial fans, interrupted only at wide intervals by rock ridges, which project above it like islands. Each fan slightly indents the mountain front at its apex where a canyon leading down from the mountains drains onto its surfaces. Each fan merges sidewise with adjacent fans, and ends forward on the floor of the Salt Basin. The slopes of the fans are concave upward (fig. 22, A). Those with the flatter gradients and longer radii are fed by canyons that drain the larger areas in the mountains. As most of the canyons in this district drain only a few square miles of area, all the fans tend to be of nearly equal size and gradient.
Between the fans in many places are strips of ground with gentler gradient that extend toward the mountain front (not separated from the fanglomerate areas on pl. 22). Some of these strips have been shielded from depositing streams so long that they are smooth and soil covered.30 Other interfan strips down which streams have recently been deflected from the fans, are choked by coarse detritus. All the soil-covered strips will doubtless be covered by such material at some time in the future. Down-slope from some of the interfan strips into which streams have been deflected are small, secondary alluvial fans.
Farther south, where the foothill ridges are higher and more continuous, the bajada is more irregular. Material washed out from the Guadalupe Mountains has accumulated behind the foothill ridges until drainage can overflow the lower saddles, or be deflected around the ends. Instead of forming a continuous slope, different segments of the bajada are thus interrupted by ridges. As a result of the deflection by the ridges, drainage tends to be concentrated in places and dispersed in others, so fans of greatly different sizes have been built west of the foothills. Two fans with large radii and low gradients have thus formed at the north and south ends of the Patterson Hills, where many streams coming from the mountains approach one another. In the intervening area there are smaller, steeper fans, built by streams draining from the Patterson Hills alone.
Along the south edge of the area studied, where the high-standing rocks of the foothills are poorly resistant to erosion and have been worn down to pediments and low ridges, the bajada occupies a relatively narrow area between the foothills and the floor of the Salt Basin to the west. South of the area studied, the foothills gradually disappear, and a bajada again extends up to the base of the Delaware Mountains.
The material on the bajada is derived mainly from the escarpment of the Guadalupe and Delaware Mountains to the east of it. It includes resistant fragments of bedded or massive limestone, which predominates toward the north, and of bedded sandstone and black limestone, which predominate toward the south. Fragments of similar rocks are contributed to a smaller extent by the foothill ridges. Near the head of the fans, angular blocks 10 feet or more across are common, and are clustered in trains that extend out from the canyon mouths above. No doubt these blocks were washed out by exceptionally large floods or mud flows. Between the trains is an unstratified aggregate of finer-textured, angular debris, probably laid down during times of more normal stream flow. Farther out, trenches eroded in the bajada expose finer-textured deposits, consisting of alternating beds of cobble or pebble conglomerate, and of loesslike, in part gypsiferous, clay. Stratification is parallel to the bajada surface, gently inclined to the west. The surface of the bajada, even to its edge, is a gravelly soil, which supports a characteristic vegetation of lechuguillas, yuccas, daggers, and creosote bushes.
As a result of the conical shapes of the fans, due to excess deposition opposite the canyon mouths, streams that flow across them radiate from the apices. (These are shown as "streams consequent on bajada surfaces" on pl. 22.) The placing of the main channel is fortuitous, and depends on minor depositional irregularities. On some fans the main channel leads directly down the slope along the crest; on others, it is deflected sideward, near or against the mountain front, toward the adjacent interfan area. The channels increase in number outward, but this increase is less from the bifurcation of the main channel than from the implantation of new channels, whose heads are on the fan surfaces. Many channels, instead of bifurcating, come together down the slope.
Some of the channels that cross the fans are anastomosing, gravel-floored washes, whose surfaces are nearly level with the interstream areas. Near them the boulder deposits consist of fresh or slightly weathered fragments, suggesting that such channels are actively depositing material today; however, by far the greater number of the channels entrench the fan surface. Near the apices of the fans, the trenches reach as much as 50 feet in depth, but they are shallower down the slope. Some of the entrenched channels are possibly the normal beds and banks of the larger torrents that occasionally follow them.31 However, boulders near the entrenched channels are usually much weathered, soil covered, and overgrown by vegetation, as though the processes that placed them there had now ceased, and as though the entrenched streams no longer overflowed the sides of their channels. If so, the streams have now been cut down to a gradient lower and flatter than that which had controlled the building of the bajada.
Such down cutting may have been brought about during the normal progress of degradation of the mountain area, and without an interruption by tectonic or climatic causes.32 At first, when the mountains were newly uplifted, their slopes were high and steep, and the material delivered to the surrounding bajadas was coarse textured. The surfaces of the bajadas were then graded to a relatively steep angle. Later on, after the mountain slopes were worn back, material carried across them was weathered to smaller fragments before reaching the bajadas. The streams then adjusted their grade to a lower angle of slope. As a result, they entrenched the upper parts of the fans, and shifted the material thus picked up to lower places on the slope. The bajadas in the Guadalupe Mountains region seem to have passed into this later stage of development.
FLOOR OF SALT BASIN
West of the bajada that slopes down from the escarpment of the Guadalupe and Delaware Mountains is the broad floor of the Salt Basin. The bajadas on its periphery do not merge with the central floor by a gradual flattening of profile and diminution in texture of the deposits toward the axis of the basin. Instead, the bajada descends 100 feet or more in the last mile to its edge, and then gives place to the basin floor which is essentially horizontal (fig. 22, A). With this change in gradient, there is a corresponding abrupt change from coarse- to fine-textured soil, although in places this relation has been modified by drifting of the surface material by the wind. Soil changes are emphasized by the vegetation, for the creosote bush assemblage of the bajada gives place within a few feet along the boundary to the yeso grass assemblage of the basin floor.
Within the area studied, and over wide expanses elsewhere, the floor of the Salt Basin lies between 3,620 and 3,640 feet above sea level. There is not sufficient gradient for water discharged on it to flow in any particular direction, and it has no drainage channels. Ground water stands nearly level, within a short distance of the surface. At Williams Lower Ranch west of the Patterson Hills, it is reached by a well at a depth of 28 feet, and it is probably at or a little below the surface in the alkali flats to the west, whose altitude is 22 feet lower than that of the ranch. Richardson33 reports similar depths to ground water elsewhere in the basin.
The basin floor within the area studied is but a small segment of the whole expanse, which extends about 5 miles farther north and 35 miles farther south, and has a maximum width of 15 miles (pl. 23). The Salt Basin itself continues farther north and south than the ends of the floor, but here the outer edges of the bajadas from either side meet at the center in broad axial troughs. Aerial photographs indicate that the troughs are followed by more or less definite stream channels that drain toward the lower-lying floor in the central segment of the basin.
The floor of the Salt Basin is underlain by fine-grained unconsolidated deposits which probably extend to great depths. It does not seem to be receiving any important amounts of new material at the present time. Most of the detritus washed out from the mountains and foothills is deposited on the bajada and does not reach the floor. The lower edge of the bajada, where it meets the basin floor, seems to be the outer limit of effective stream action at present. The chief process now at work on the basin floor is the wind, which is not bringing in any new material, but is shifting about what is already there. Instead of leveling the surface, the wind is increasing the relief, scooping out hollows, and piling up material. The processes that leveled the floor are, therefore, no longer at work, and the floor has been inherited from an earlier period. As indicated in a later section, the floor was probably the bottom of a lake or succession of lakes which filled the lowest part of the Salt Basin in Pleistocene time (pp. 151-152, 156-157).
Brief field observations on the surface features of the basin floor were made during the present investigation; they have been subsequently studied by me in aerial photographs. They closely resemble those in other nearby desert basins, which have been described and interpreted in some detail by Meinzer.34 The features comprise alkali flats, sand dunes, clay hills, meadows, and beach ridges. They are distinguished by separate patterns on the map, plate 22.
The alkali flats, locally known as salt lakes, are among the most conspicuous features of the basin floor. Several occur in the western part of the area studied; one is well displayed where crossed by U. S. Highway 62. Outside the area studied, the flats are not adequately shown on any published map,35 but they are strikingly exhibited on aerial photographs. The alkali flats within the area studied are part of a series of flats that extend 8 miles westward, 4 miles northward, and 15 miles southward (pl. 23). The larger flats lie west of the area studied, where some are over 5 miles long and 2 miles wide. The alkali flats of the series have a curious arrangement, tending to lie in chains on the east and west edges of the basin floor, with ground as much as 20 feet higher in the intervening central area. This higher ground with its flanking depressions is a relatively old feature, for aerial photographs show that it is marked by concentric beach lines, perhaps dating from the lacustrine period of late Pleistocene time (pl. 23).
The surfaces of the alkali flats are level expanses of alkaline clay, bare of any vegetation. For short periods after rains, they are likely to be covered by thin sheets of water, and for somewhat longer periods the clay is damp and sticky, and dotted with saline pools. At other times their surfaces are dry, hard, and sun cracked, and mirages often give them the appearance of containing water. All the flats are coated with a thin efflorescence of various salts. On one of them, which lies a short distance west of the area studied and from which salt has been dug for many years, the efflorescence is continually forming, and is renewed within a few weeks after it has been stripped away (p. 161).36
The flats are bordered at the edges by low, steep banks 10 to 20 feet high, which are scored in places by small ravines that are cutting headward into the surrounding country. These ravines indicate that the banks are being maintained or even cut back at the present time. The banks pursue a highly irregular course, curving around numerous promontories and fingerlike embayments. In the middles of some of the flats are island-like areas of higher ground with similar steep banks along their edges. On the large alkali flat in the southwest part of the area studied (pl. 22), the greatest irregularity of the edges is on the west side; the eastern edge is nearly straight.
The alkali flats probably originated as shallow or intermittent lakes that filled slight depressions in the basin floor. Old beach lines on higher ground nearby indicate that the flats lie in relatively ancient depressions. Lacustrine action is not effective at the present time, as the flats are covered by water only at long intervals, yet the surfaces remain smooth and open, and the steepness of the banks at the edges is being maintained. As suggested by Meinzer,37 the flats are probably swept clearand are even being extendedby wind erosion. The surfaces of all the flats in the area, whether connected or not, stand at an altitude a few feet above 3,620 feet, or near ground-water level. Apparently the wind has carried away all the dry earth above ground water level, and has found an effective downward limit of cutting on the surface of the saturated earth below.
The next most conspicuous features of the basin floor are the sand dunes. These are well developed in the area studied, perhaps to a greater extent than elsewhere in the Salt Basin (pl. 23). In places the sand is white, and consists of grains of crystalline gypsum; elsewhere it has a reddish hue and consists of quartz. The two minerals are not mingled, and their dunes occupy separate areas.
The dunes of quartz sand are found in two areas, one north and the other south of the ends of the Patterson Hills. They spread over the edge of the basin floor and up the slopes of the bajadas to the east. The dunes reach a maximum height of 30 feet in the northern area. They are of irregular form and are separated by irregularly placed depressions. Mesquite and yucca commonly grow between the dunes, but many of the dune surfaces are bare and ripple marked. The bare sand is so loosely placed that even moderate winds can blow it about. The dune sand on the bajada thins gradually toward the mountains, and the easternmost dunes are separated by depressions in which the underlying deposits of the bajada are exposed. East of the main dune area, some sand is banked against the eastern or lee side of low rock foothills that project from the bajada.
The sand appears to be moving eastward up the bajada slope, urged forward by winds from the west. According to local residents, the sand is encroaching year by year on the land to the east. There is thus a steady conflict between the eastward-blowing winds and the westward-flowing streams of the bajada. As the wind is at work for longer periods than the streams, it fills their channels with sand during the dry periods. One channel was traced through the dune area. Up the slope, east of the dunes, it is a gravel-floored arroyo with steep banks 20 feet high. Within the dunes it narrows and its bed contains only tiny pebbles. It lies in a shallow swale, across which the sand has drifted in many places. Apparently the stream that flows in it during each freshet loses its vigor in the dune area, perhaps because much of its water sinks in the sand and what remains must work to clear the channel of drifted sand.
The source of the quartz sand seems to be the unconsolidated deposits along the western edge of the bajada and the eastern edge of the basin floor. The basin deposits are more sandy in some places than others, the sand having been deposited by streams draining areas of sandstone in the mountains to the east. It is significant that there are no areas of quartz sand dunes west of the Patterson Hills where the basin deposits contain little mountain-derived detritus.
Dunes of gypsum sand are much less extensive than dunes of quartz sand, in fact there is only one large tract in the Salt Basin. This tract lies on the floor of the basin within the area studied, and has an area of about four square miles. The northeast end of the tract is a crescent-shaped ridge a mile across, made up of white, shifting dunes, bare of vegetation, with an appearance similar to the well-known White Sands area of the Tularosa Basin in New Mexico.38 The northeast side of the ridge is steep-faced, and the white sand ends abruptly along its base, as though the dunes were moving northeast. Farther southwest, the dunes are low and considerably masked by vegetation. Aerial photographs indicate that the lower dunes extend 5 miles southwestward, nearly to the edge of a large alkali flat west of the area studied. In this area they extend across beach ridges and other older features.
The sand of the gypsum dunes in the Salt Basin, like that in the Tularosa Basin, is probably derived from the gypsiferous clay blown out of the alkali flats that lie to the west and southwest. As Talmage suggests, the sand may have been derived from crystals that had grown in the clay.
Elsewhere in the Salt Basin, particularly south of the area studied, are numerous low, rolling hills of gypsiferous clay which likewise were probably heaped up by the wind. Here, however, the gypsum is not granular and was probably derived from earthy rather than crystalline material. Wide areas in the basin on its west side, and smaller tracts between the dunes and clay hills farther east, are flat meadowland underlain by brown clay. The difference in texture between the clays of the meadows and the earthy gypsum of the hills becomes evident after a rain, when the clays are wet and sticky, and the gypsum relatively dry and hard. The meadows are covered with a thick growth of wiry yeso grass and salt grass, and in the moister places there are clumps and groves of mesquite.
Beach ridges are present in many places on the floor of the Salt Basin, and show prominently on aerial photographs (pl. 23). They are relics of Pleistocene time when the floor was covered by standing water and are discussed under a later heading (pp. 156-157).
Having now reviewed the modern landscape and processes at work on it, attention will be directed to features formed in older Cenozoic time, proceeding from the oldest features to the youngest.
The earliest period, probably pre-Pleistocene, is very poorly recorded, and many gaps must be filled by inference and deduction. The oldest topographic feature in the Guadalupe Mountains, the summit peneplain, is probably not Cenozoic, but pre-Cretaceous. It antedates the uplift of the mountain area and serves as a convenient datum from which to gage the effects of later events. The next younger features in the mountains are the courses of certain streams which appear to have flowed down the slopes of the initial uplift, and to have been consequent on these slopes. These streams were probably older than the faulting of the area. At the time that these streams were taking their courses in the mountains, deposits were probably forming in the lower country east and west of the mountains. Not much is known about the deposits near the Guadalupe and Delaware Mountains, but comparable deposits exposed in surrounding areas have been dated by fossils as of Pliocene age. A period of movement later than the initial uplift is suggested by a second generation of consequent streams. During this later movement, faulting dominated, for the second generation of consequent streams flow in fault troughs.
Anyone who crosses the Guadalupe Mountains is soon aware that their summits are notably even-crested. For considerable distances the trails, which afford the only means of travel through the region, cross a succession of broad swales and gently rounded hilltops, much overgrown by timber, with here and there a glimpse into a steep-walled canyon incised to great depth below. If the trail descends into such a canyon, it is likely to rise on the other side to another patch of relatively flat ground of about the same altitude as the first.
The even crests seem readily explainable at first. The rock of most of them is the well-bedded limestone of the Carlsbad, and for short distances the crests are cut on single layers of the bedded rock. However, when any ridge is viewed in panorama, the bedded rocks on its sides are seen to dip at a low angle to the southeast and to rise and terminate on its crest to the northwest. The rocks at the summits are thus of different ages from place to place. Toward the southeast, near the rim of the Reef Escarpment, the rocks belong to the top of the Guadalupe series, whereas a few miles to the northwest, they belong to the base of the upper part of that series, nearly 1,000 feet lower (fig. 15, B).
The even crests of the mountains are therefore not wholly caused by the bedding of their rocks; instead, they seem to be remnants of a formerly more continuous surface that extended across the edges of the rocks. This surface was no doubt formed by erosion when the geography of the region was very different from that of today; it is probably an ancient peneplain. Remnants of this peneplain have persisted in the Guadalupe Mountains because the resistance of the limestones in the upland has retarded the widening of the canyons that now incise it. Boulder-controlled slopes rising from the bottoms of two adjacent canyons thus rarely meet at their upper ends, but are separated by fairly broad dividing ridges. Degradation has no doubt been at work on the ridges, yet it has proceeded so slowly that the divides still retain something of the form of the original surface.
Although the remnants of the peneplain bevel the underlying strata, the peneplain seems to have shared the tilting and faulting of the mountains themselves. On the northeast side of the mountains, in the vicinity of McKittrick Canyon, the crests rise southwestward from altitudes of 7,000 feet or less, to 8,000 feet or more, at the rate of several hundred feet to the mile. Near the headwaters of the canyon, where the rocks are much faulted, the crests remain at accordant heights in single fault blocks, but stand higher or lower in adjacent blocks by an amount corresponding to the throw of the intervening fault. No higher ground appears to have projected above the peneplain except what has been raised by subsequent movements.
Plate 18 is an aerial view across the region in which the summit peneplain is preserved. Note the occasional patches of level ground on the divides, such as that in the right foreground and that near the center in the middle distance. The irregularity of the surface toward the background results from faulting. On sections EE', HH', and II', plate 17, tracing of the beds that form the canyon rims and ridge crests shows that they are truncated northwestward by the summit peneplain. Displacement of the peneplain by faulting appears on plate 14, A, where it forms the sky line in the background, and also the summits of the lower ridges in the foreground. The structure in this vicinity is shown in the right-hand part of section AA', plate 3.
POSITION OF SUMMIT PENEPLAIN IN SURROUNDING AREAS
The crests of the southern Guadalupe Mountains, outlined by the summit peneplain, project far above the surrounding areas. Southward and southeastward the land surface descends abruptly over the Reef Escarpment to the Delaware Mountains and Gypsum Plain 1,000 feet or more below. Because remnants of the peneplain extend without change in attitude to the rim of the escarpment, and because there is no tectonic break along its edge, any former extension of the peneplain in this direction must have lain high above the existent land surface. All traces of it have been destroyed because the rocks of the region are sandstones and anhydrites, less resistant to erosion than the limestones of the Guadalupe Mountains, and have been degraded to lower levels. Between the streams, the rocks have been worn down to gentle, rain-washed slopes rather than to steep, boulder-controlled slopes. These slopes meet at their upper ends in divides that stand far below the original surface of the country.
The southeastward termination of the summit peneplain along the Reef Escarpment can be seen on plate 4, A. Note the accordant summits on the skyline, at the top of the escarpment, and the low, rolling hills and shallow valleys along its base, which are characteristic of the country to the south. The same relation is shown diagrammatically on figure 20, A, and in profile on figure 21, A. On the latter, note the difference in altitude between the summit peneplain and the land surface southeast of it.
The land also descends northward and northeastward from the southern Guadalupe Mountains into the northern Guadalupe Mountains, but without an abrupt topographic break. The same limestone plate as that which caps the southern Guadalupe Mountains extends into this region, and here also, remnants of the peneplain are probably preserved on the mountain summits. Queen Mesa, one of the summit areas northeast of the area studied, is of considerable extent. West of the southern Guadalupe Mountains, the limestone plate is preserved in the lower ridges of the Brokeoff Mountains and Patterson Hills, but has been much disturbed by faulting and tilting. Extensions of the summit peneplain, which have no doubt been correspondingly faulted and tilted, probably at one time outlined the crests and slopes of these ridges, but degradation has now almost obliterated their original form.
The northward extension of the summit peneplain beyond the area studied can be seen on plate 14, A. Note the even sky line in the distance, formed by-the peneplain, and the manner in which it joins Queen Mesa to the north (left). Some idea of the wide extent of the upland surfaces in the northern Guadalupe Mountains can be gained from figure 2.
AGE OF SUMMIT PENEPLAIN
The summit peneplain is the oldest land form preserved in the area, and is probably older than the uplift of the mountains themselves. No remnants of original higher ground project above the peneplain as one would expect if the peneplain had been carved from a previously existing mountain area. Further, the peneplain is now tilted away from the axis of uplift of the mountains, and is displaced by faults to the same extent as the rocks that underlie it. It is true that the peneplain bevels the underlying rocks, but it bevels them in a northwestward direction, unrelated to the present trend of the uplift. This beveling is related to a southeastward tilting of the Permian rocks which took place during the subsidence of the Delaware Basin in Permian time.
The summit peneplain, therefore, is younger than the Permian and is probably older than the uplift of the mountains. It may be of Mesozoic age and may be the exhumed surface on which Cretaceous rocks were once deposited. Similar surfaces, on which remnants of Cretaceous rocks are still preserved, form the crests of the Glass Mountains,39 the Sierra Diablo, and other ranges of trans-Pecos Texas. In the Guadalupe Mountains, so far as known, no remnants of the former cover of Cretaceous rocks remain.
The summit peneplain of the Guadalupe Mountains is probably older than peneplains that have been described in the mountains to the northwest, in New Mexico. One of these peneplains has been observed high up in the Sacramento Mountains (Sacramento plain) and another in the San Andres Mountains (see fig. 1).40 Both surfaces stand lower than the highest summits of the ranges. They were probably formed after the first uplift of the mountains, but before the main uplift; a Pliocene age has been suggested for them.
FORMER COVER OF SUMMIT PENEPLAIN
If the summit peneplain is of pre-Cretaceous age, it was probably covered at one time by Cretaceous sediments, similar to those whose remnants are now found in surrounding regions. Such sediments may have been less resistant to erosion than the underlying limestones, and hence easily stripped away (as suggested on secs. 1 and 2, fig. 22, B). Cretaceous rocks may have covered the area when the mountains were first uplifted, but proof for this suggestion is not available.
OLDER CONSEQUENT STREAMS
Viewed broadly, the streams of the Guadalupe and Delaware Mountains flow east and west from the tectonic and topographic crest of the range, and their gross pattern is consequent to the original tectonic surface. In detail there are many complications, resulting from modifications since the mountains were uplifted. Some streams have been deflected along fault troughs, others have been cut as subsequents on weak beds, and still others flow across alluvial slopes without regard to the bedrock structure beneath. Under this and succeeding headings the complex history of the streams of the area will be analyzed.
The stream pattern of the Guadalupe Mountains as a whole can be seen on figure 2. The pattern in the immediate vicinity of the area studied is shown in more detail on figure 19. On plate 22, the streams of the area studied are classified according to possible origin. Note especially the "streams consequent on tilted rock surfaces" and "streams consequent on tilted fault blocks"; these form the basis of the present discussion.
STREAMS CONSEQUENT ON TILTED ROCK SURFACES
The larger streams on the east slope of the mountains all pursue east-northeastward courses, and are probably consequent streams whose direction of flow was determined by the slope of the mountain block. However, only those to the north, in the limestone upland of the Guadalupe Mountains, preserve any semblance of their original aspect. Farther south, in the Delaware Mountains, erosion has worn down the whole area far below the level of the summit peneplain, and the streams have been able to modify their original courses in harmony with later conditions.
The streams to the north, in the limestone upland, flow in deep, narrow canyons, and join one another at nearly right angles, forming an open dendritic pattern (fig. 19). Most of them have winding courses, consisting of a succession of inclosed meanders.41 They probably have much the same pattern as they did when first formed, and furnish information as to the probable nature of the original consequent drainage.
Typical stream valleys of the limestone upland are shown on the aerial view, plate 18. The area drained by them is indicated by stippling on figure 19. On this map note the broad similarity between the direction of flow of the streams in the upland and those to the south, and yet the great contrast between their detailed patterns.
The origin of the inclosed meanders of the streams in the limestone uplands is uncertain. They may be entrenched from previously meandering courses, formed on an original surface of low relief; they may have become ingrown (incised) by sideward cutting at the same time that the streams cut downward, or they may have originated from a combination of these and other conditions.42 Distinctive criteria that would suggest one type or the other are generally lacking. In McKittrick Canyon some sideward cutting is going on, but it appears to be relatively ineffective because of the great height of the canyon walls; moreover, the ridges between the meanders do not have the low-angled profile of slip-off slopes, as though there had not been much sideward migration in the past.
The dendritic pattern of the drainage, and the possibility that the inclosed meanders result at least in part from entrenchment suggests that the surface down which the consequent streams originally flowed probably had a lower gradient than the present one, and that at the time they were formed the mountains did not stand as high as they do today.
The streams of the limestone upland appear to be superimposed on the Reef Escarpment (fig. 20). The east-northeastward courses of the streams that cross it intersect the northeastward course of the Reef Escarpment at an acute angle. Because of the acute-angled intersection, the notches cut in the escarpment by the streams characteristically have narrow, serrate southwest walls and blunt-angled northeast walls. Such features occur on South McKittrick Canyon in the area studied, and on Big, Gunsight, and Slaughter Canyons in the region to the northeast (fig. 2). These relations suggest that the streams began their courses at a much higher level than that at which they are flowing today, above the varied bedrock that forms the present mountains, and perhaps near or above the surface of the summit peneplain.
Some of the streams are probably antecedent to the faulting within the mountains, because they cross upraised fault blocks with little deflection. Thus, within the area studied, South McKittrick Canyon crosses the Lost Peak fault zone from the downthrow to the upthrow side (pl. 22), and some streams in the Brokeoff Mountains to the northwest cross other faults in a similar manner. The streams on the east slope of the mountains, where the rocks are not faulted, may have originated at the same time as those mentioned, but this correlation cannot be determined definitely.
Some of the east-flowing streams of the limestone upland may once have headed farther west than they now do, and west of the present west-facing escarpment. Two of them, which drain Pine Spring and South McKittrick Canyons, now head on the rim of the escarpment and have lost their original sources by an eastward recession of the rim. The original sources, however, were not much farther west, for near the point of beheading both streams are split into numerous branches (pl. 22), no one of which would have been capable of draining a former large territory to the west.
The evidence listed suggests that the streams of the limestone uplands are consequents resulting from the first uplift of the mountain area. The slope of the uplifted surface was low enough for them to acquire an open dendritic pattern, and the surface was probably not faulted. It probably lay above the rocks that now form the Reef Escarpment. The stream pattern suggests that the surface after the original uplift had the form of a broad dome, whose axis lay near the present crest of the mountains. Its eastern flank is indicated by the east-northeastward flowing streams, and the west flank by the streams of the Brokeoff Mountains which drain northwestward or westward to the Salt Basin (fig. 19).
STREAMS CONSEQUENT ON TILTED FAULT BLOCKS
In the western part of the Guadalupe Mountains, where the rocks are broken by faults, there is another type of consequent steam. Here, many streams follow the downfaulted areas and are evidently consequent on tilted fault blocks.
The age relations of the two sets of consequent streams is suggested along South McKittrick Canyon. This canyon crosses the Lost Peak fault zone without deflection, is probably older than the faulting, and is antecedent to the upraised fault block to the east. On the downthrown side to the west, it is joined from the north and south by two tributaries which probably originated on the downfaulted surface. The tributaries are therefore younger than the faulting.
Streams that belong to this post-faulting generation are prominently developed north of the McKittrick Canyon area. Here, the stream in West Dog Canyon follows the downthrown side of the Lost Peak fault zone and the stream in Dog Canyon follows the downthrown side of the Dog Canyon fault zone (fig. 19). Farther north both streams pass into the limestone upland of the Brokeoff Mountains and appear to cut across the fault blocks. Perhaps they were relatively short consequent streams at first, and acquired large headward extensions when fault blocks sank across their upper courses.
If the streams consequent on tilted rock surfaces can be correlated with the initial uplift of the Guadalupe and Delaware Mountains, it is possible that the streams consequent on tilted fault blocks are to be correlated with the main uplift of the mountains, in which faulting apparently was a dominant feature. It cannot be determined whether the major movement on the Border fault zone took place at this time, as distinctive geomorphic features in that area have been obliterated. If such major movement took place on the Border zone, the streams draining the west-facing escarpment of the southern Guadalupe Mountains are of the same generation as those in Dog Canyon and West Dog Canyon.
DEPOSITS CONTEMPORANEOUS WITH PRE-PLEISTOCENE (?) TOPOGRAPHIC FEATURES
As indicated above, the initial uplift of the Guadalupe and Delaware Mountains may have taken place immediately before the development of the streams consequent on tilted rock surfaces, and long before the development of streams consequent on tilted fault blocks.
At the time of this initial uplift, many of the ranges of the Sacramento section43 were sheeted over by poorly resistant Cretaceous and other Mesozoic rocks. In the Guadalupe Mountains, this cover may have overlain the surface of the summit peneplain. The Mesozoic sediments were no doubt stripped rather rapidly by the streams until the hard Paleozoic limestones beneath were exposed. Streams overloaded with such material probably deposited great quantities of it in the structurally lower areas roundabout. Some of it probably filled the depressions between the ranges, and some was spread as a vast detrital apron over the surface of the Llano Estacado east of the mountains.
DEPOSITS OF THE GUADALUPE MOUNTAINS REGION
Within the Guadalupe Mountains region, deposits that formed in response to the first uplift of the ranges are poorly known.
The Salt Basin west of the Guadalupe Mountains probably received a large volume of the early deposits, but its surface has been eroded so little that none is exposed. As indicated by wells drilled there, the unconsolidated deposits beneath the surface of the Salt Basin reach a great thickness.44 They are probably similar to those exposed in the Hueco Bolson, the next desert basin to the west (fig. 1). The deposits in the Hueco Bolson are gray to flesh-colored silts, in part gypsiferous, with some sandy lenses, and near the bordering mountains are interbedded with fanglomerates and mudflow deposits. They probably accumulated in an enclosed depression, not drained as today by a through-flowing stream. They were perhaps deposited in a shallow, intermittent lake. The fanglomerates along the edges were no doubt deposited on bajadas that fringed the primitive mountain ranges.
The deposits of the Llano Estacado, now exposed east of the Pecos River, 90 miles away, perhaps once extended farther west, over the present river valley, and up to the bases of the Guadalupe and Sacramento Mountains beyond. In the Sacramento Mountains, according to Nye,45 the subsummit or Sacramento Plain can be projected eastward across the present valley, and beneath the deposits on the other side. It was probably carved by streams that were at the same time laying down deposits farther east.
AGE OF DEPOSITS
Older unconsolidated deposits laid down in intermontane areas of the Sacramento section, and in the Llano Estacado to the east, contain vertebrates in a few places that are considered to be of Pliocene age.46 Over wide areas the deposits resemble one another so closely in lithologic character, conditions of deposition, and degree of deformation, that they are probably all of about the same age. Near El Paso, the older unconsolidated deposits are overlain unconformably by gravels that contain vertebrates considered to be of early Pleistocene age by Hay.47
If these older deposits are Pliocene, if they formed in response to the initial uplifts of the mountains of the Sacramento section, and if the Guadalupe and Delaware Mountains had a history similar to the Sacramento section as a wholea rather extensive series of assumptionsthen the older deposits serve to give an approximate date to the initial uplift of the region. Such a dating serves to justify assigning the features and deposits so far discussed as pre-Pleistocene (?).
In the Guadalupe Mountains, a period of movement later than the initial (Pliocene?) uplift is suggested by the second generation of consequent streamsthose consequent on tilted fault blocks. During this second period of movement, faulting was apparently a dominant feature. The second period of movement was probably a major uplift, comparable to major uplifts farther northwest in the Sacramento section, which are assigned to a post-Santa Fe (late Pliocene or early Pleistocene) age. According to Bryan:48
This major uplift probably gave the Guadalupe and Delaware Mountains their present tectonic form and accelerated the degradation of the mountains. Processes were initiated that carved them into the outlines we see today.
After the uplift, degradation went on without any recorded pause until the streams on the east slope of the mountains had cut 1,000 feet or more below the summit peneplain, and until other parts of the region had been correspondingly reduced. This degradation was followed by a major period of still stand during which streams widened their valleys, broad pediments were formed, and the west slope of the mountains was shaped into a gently rounded surface. Afterwards a part of these erosion features was covered by deposits. Some of these deposits remain today and are the oldest Cenozoic deposits exposed in the region. They are probably of early Pleistocene age.
The early Pleistocene (?) deposits are complex in that they are scattered over the area in many diverse situations. On the east slope of the mountains they form a thin sheet of gravel, spread out on the plains southeast of the Reef Escarpment. These deposits were laid down on a pediment, extensions of which may be seen farther south, and which may be related to benches or shoulders in the canyons to the north. On the west slope of the mountains deposits occur, not on a stream-graded surface, but on steep slopes. Farther west, beyond the base of the escarpment, are fanglomerates laid down on bajadas at the edge of the Salt Basin.
All these deposits are of some antiquity, as they lie above present stream grade and have been dissected. The fanglomerates west of the mountains also have been disturbed and faulted, indicating that they are older than the last tectonic movements in the mountain area.
GUADALUPE AND DELAWARE MOUNTAINS
Fringing the base of the Reef Escarpment, along the southeast side of the Guadalupe Mountains, and extending out for several miles into the Delaware Mountains and Gypsum Plain to the southeast is a gravel-covered plain, or pediment, which records an extended period of planation and deposition. Throughout its extent the plain is trenched, to depths ranging from a few to more than 100 feet, by streams which in places expose the underlying bedrock in their channels. The plain is therefore a product of conditions no longer existing in the region.
The extent of the gravel deposits is shown on the map, plate 22, where two subdivisions are distinguished. "Higher gravels," apparently older than the main deposits, occupy relatively small areas. The main body is designated as "gravels deposited on older pediments." Two views across the gravel plain toward the Reef Escarpment are shown on plate 4. On plate 4, B, the surface is little dissected and probably has much its original form. Below Pine Top Mountain it has the form of an alluvial fan. Plate 4, A, shows the appearance of the plain where dissection is more advanced. Much of the flat-topped surface in the middle distance is a part of it, although in places benches of bedrock rise to about the same level. Figure 21, A, shows a profile across the plain and includes not only the main deposits but also some remnants of the higher gravels.
Rising above the main gravel plain in places are small, flat-topped remnants of an older set of gravel deposits (the "higher gravels" of pl. 22). They stand 50 feet or more above the main plain from which they are separated by rock-cut slopes, and lie 150 feet or more above present drainage. They are made up of fragments derived from the Guadalupe Mountains which resemble those in the main or younger gravel deposits. The patches of higher gravels are probably remnants of deposits formed in restricted level-floored stream valleys, rather than remnants of a nearly continuous gravel plain like that described below.
The deposits of the main gravel plain ("gravels deposited on older pediments" of pl. 22) consist of fragments washed out from the Guadalupe Mountains. The most abundant pieces are of massive (Capitan) limestone, but also include some bedded, light-gray (Carlsbad) limestone, and dark-gray (Pinery) limestone. Sandstone fragments from the Delaware Mountain group are not common.
The fragments are subangular to subrounded. Near the mountains blocks up to 6 feet across are enclosed in the finer material, and along McKittrick Draw, 3 miles from the mountains, blocks 3-1/2 feet across are present. Some miles away from the mountains, however, cobbles and pebbles a few inches across prevail. Near the mountains the material is poorly sorted and poorly bedded; farther out the gravels lie in regular beds, with some intercalated layers of buff clay. At a deep cut in Pine Spring Canyon about a mile west of Pine Spring, there is at the base a 10-foot bed of flat-lying reddish clay, above which is 100 feet of cobbles and boulders with inclined layers that slope down the sides of an alluvial fan. Elsewhere, the gravels rest directly on the rock surface below. Over wide areas the gravels are loosely cemented by caliche.
The gravels are thickest near the Guadalupe Mountains, from which they were derived, and thinnest to the southeast, where they probably come to a feather edge. Near the Guadalupe Mountains, some stream cuts show exposures of gravel 100 feet or more thick. Their present upper surface appears to have been their original depositional surface, for at the mouths of some of the canyons leading out from the mountains they are heaped into low alluvial fans. Besides the alluvial fan in Pine Spring Canyon just mentioned, another, below Pine Top Mountain, is noted on plate 4, B. They are not rock fans, as stream cuts show that the gravel deposits are thicker beneath the fans than elsewhere.
No vertebrate bones have been seen in the gravel deposits, but here and there the deposits contain mollusks. A small collection of the mollusks, made by H. C. Fountain, has been identified by J. P. E. Morrison of the United States National Museum. The species are listed below, along with those contained in a collection of living shells from the same area, made by Fountain for comparative purposes.
Mollusks from gravel deposits of the Guadalupe and Delaware
1. Flat-lying reddish clay at base of deposit 1 mile west of Pine Spring on north side of Pine Spring Canyon.
2. Coarse gravel on east side of Bell Canyon 1 mile northwest of Hegler ranch house.
3. Extensive terrace along Bell Canyon near prominent bend 1 mile north of Hegler ranch house.
4. Living species from sheltered places along cliffs high up on south wall of Pine Spring Canyon.
According to Morrison, all the species listed are living forms, and are within their present ranges. The Guadalupe Mountains are near the northern limit of the present range of the species of Humboldtiana. A slight difference in climate from that of the present is suggested by lot 3, with its fresh-water forms, for water is not permanent in this part of Bell Canyon today; the difference may have resulted from only a slight variation from the present annual rainfall. The fossils listed do not confirm the geomorphologic evidence that the gravel deposits are old, but according to Morrison they do not deny it. He believes that the assemblage could well be of Pleistocene age.
ROCK SURFACE BELOW GRAVEL DEPOSITS
The surface of the bedrock below the gravel is fairly even in most places, and was no doubt a pediment of wide extent. At a few places, however, irregularities are observed in it. South of Rader Ridge, over the belt of outcrop of the South Wells limestone member of the Cherry Canyon formation, the gravels are only a few feet thick, but, as shown by stream cuts, they thicken to more than 100 feet a short distance to the west. At this place, the South Wells limestone apparently projected above the pediment as a low cuesta, which was afterwards entirely buried under the sloping sheet of gravel.
A significant comparison can be made between the profile of the gravel surface, the profile of the pediment on which the gravels rest, and the profile of the present streams, which entrench them both (fig. 21, B). The present streams have a concave upward profile, which is steepest in the Guadalupe Mountains, and gentlest in the plains to the southeast. The surface of the gravels is also concave upward, but apparently more so than the stream profiles, as the streams entrench it to depths of 100 feet or more southeast of the mountains, and less than 50 feet near their base. The surface of the pediment beneath the gravels is still more concave. Like the gravel surface, it stands well above the present streams southeast of the mountains. Near the mountains, however, it lies near or below the stream channels, many of which fail to penetrate it. As all three profiles were probably formed by streams flowing at or near grade, the differences in concavity suggest changes in conditions of stream equilibrium.
STREAMS CONSEQUENT ON GRAVEL DEPOSITS
The streams that drain the gravel plain were developed on its depositional surface and are consequent to it (shown as "streams consequent on gravel deposits" on pl. 22). They thus belong to a later generation than the two sets of consequent streams previously described (pp. 140-143). When the gravel deposits were being laid down, each consequent stream that flowed east-northeast out of the Guadalupe Mountains aggraded its course and built up a low alluvial fan at the foot of the Reef Escarpment. As deposition progressed, the streams were deflected this way and that from the canyon mouths. Later, the new courses became fixed by renewed entrenchment. The net result has been to deflect streams draining east-northeast from the mountains to a more easterly or southeasterly course on the plain (fig. 19).
The streams consequent on gravel deposits follow straight courses and are closely spaced and nearly parallel. As a result, the gravel deposits are scored by a series of ravines which run side by side for long distances and join each other at acute angles. The streams tend to radiate from each canyon mouth of the Reef Escarpment, in the manner of streams on alluvial fans. Some streams on the gravel plain which are fed by the same canyon of the mountains thus diverge widely from one another away from the mountains. In this manner, a part of the drainage from Pine Spring Canyon flows east into Cherry Canyon, and part flows south into Delaware Creek, the bifurcation taking place near the foot of the mountains at Pine Spring Camp.
With renewed dissection of the area, the streams consequent on the gravel deposits have been superimposed on the bedrock beneath. In places they cross the hills of the former topographic surface, as along the outcrop of the South Wells limestone member of the Cherry Canyon formation south of Rader Ridge (pp. 145-146). Southeast of the present gravel area, some streams now flowing on bedrock were probably superimposed on it through a former sheet of gravel, afterwards destroyed by erosion.
Within the area studied, the gravel deposits extend southeastward into the Delaware Mountains for about 4 miles from the Reef Escarpment. Farther southeastward, they have been removed by Delaware Creek and its tributaries. Where the gravels are lacking, either by nondeposition or erosion, many of the even-crested hilltops stand at about the same level, and are probably remnants of the same older pediment as that on which the gravels rest (shown as "older pediments" on pl. 22). The even crests are conspicuous on each side of the depression carved from downfaulted rocks near Getaway Gap, and also along the rim of the Delaware Mountains. The rim maintains its height even where the resistant Getaway limestone member that caps it fades out into poorly resistant sandy beds. The possibility that the rim to the south is part of an older pediment is confirmed by relations farther north, near Guadalupe Pass, where the rim is capped by older gravel deposits.
Along Pine Spring, McKittrick, and other canyons that drain the limestone upland of the Guadalupe Mountains are features that probably formed at about the same time as the gravel plain to the southeast. These canyons have been incised several thousand feet below the summit peneplain that forms their rims. Their walls, which are boulder-controlled slopes cut on massive or rudely bedded rock, rise from the channels to the rims at angles of 30° or more. The slope, however, is not continuous but in many places seems to have a two-storied profile, resulting in valley-side shoulders 100 feet or more above the present stream channels.
When viewed from about midheight on the canyon wall, each spur projecting into the canyon is seen to sweep down from the rim to a rounded shoulder near its lower end, and then to plunge 100 feet or more in steep rock slopes to the channel below (shown as "valley-side shoulders" on pl. 22). The aspect of the upper part of the canyon is thus broad and open, whereas its lower part is narrow, tortuous, and steep-sided. In detail, these features are complex. All the shoulders and the canyon walls above and below them are greatly modified by weathering and erosion, and few of the shoulders stand at exactly the same height. Some are only 100 feet above the channel, and others are as much as 300 feet above it (fig. 21, A).
The valley-side shoulders are not caused by any difference in the nature of the rocks, for the rocks are all rather uniformly massive and are of different ages from place to place along the canyons. The shoulders apparently record a time in the past when the downcutting of the canyons ceased long enough for some widening of their banks to have taken place.
Proof that some of the shoulders were formed during a pause in downcutting is given by relations at Devils Hall in Pine Spring Canyon. Here, on one side or the other of the channel and about 100 feet above it, are narrow benches floored by stream gravel, above which the higher slopes are in places over-steepened, as though by sideward cutting of the former stream. By means of the gravel remnants, a former meandering course can be reconstructed. Across this course the present stream passes through Devils Hall, in a straighter course that follows a line of weaknesses caused by closely spaced joints. Other less well preserved valley-side shoulders occur farther up the same canyon, but they lie at different heights above the stream channel. Whether they belong to a single epoch of valley widening contemporaneous with the gravel-capped benches at Devils Hall, or to several epochs, cannot be determined.
FAULT SCARP VERSUS FAULT-LINE SCARP
The steep west-facing escarpment of the Guadalupe and Delaware Mountains is so closely associated with the Border fault zone that it probably is genetically related to it. The escarpment originated either from an exposed surface of tectonic origin which has since been modified by erosion (fault scarp), or from the erosion of weak beds from the downthrown side, leaving the strong beds on the upthrown side to form the present escarpment (fault-line scarp).49 It probably came into existence during the major uplift of the mountains in late Pliocene or early Pleistocene time.
If any weak beds ever lay on the downthrown side of the fault they could not be a part of the succession now exposed in the region, for the strata exposed on the downthrown side, along the base of the escarpment, comprise the Carlsbad and Capitan limestones, the Lamar limestone member of the Bell Canyon formation and the Castile formation which lie at the top of the known section. The beds named are the ones that lay immediately beneath the summit peneplain. They may have been covered by weak Cretaceous rocks that overlay the peneplain even at the time of the faulting. If so, immediately after the faulting these weak rocks for a time covered a part of the fault surface (as suggested in stage 1, fig. 22, B), but they were removed rather rapidly, and were redeposited in the deeper parts of the Salt Basin farther west (as shown in stage 2 of fig. 22, B).
As shown earlier (p. 110), however, the displacement along the Border faults ranges from 2,000 to 4,000 feet. It seems very unlikely that a sequence of beds as thick as this covered the rocks now found in the region at the time of the faulting, or that fault surfaces of such height were entirely concealed by them. It is therefore probable that the present escarpment is at least in part a fault scarp. After the faulting its height was probably increased by the removal of weak beds from the lower part of its surface, in which case the part originally covered is a fault-line scarp.
EROSION OF ESCARPMENT
After the face of the escarpment was laid bare, either by the original faulting or by subsequent erosion, processes of degradation set to work on the tectonic surface, and caused its rim to be shifted toward the east. The tectonic surface, as indicated by the faults exposed along its base, probably dipped at angles of 70° or more to the downthrown side (stage, fig. 23, A), whereas with the exception of the cliffs, the graded slopes formed from it as a result of slope retreat have angles of 45° or less (stage 3, fig. 23, A). Streams draining the escarpment have much steeper gradients than those draining the country behind it, so they are able to cut actively headward. By a combination of slope retreat and headward cutting, the rim of the escarpment has receded a mile or more east of its original position.
The escarpment has not only receded, but its top has been lowered to a greater or less degree, as indicated by the occurrence from place to place along the rim of bedrock of different ages. Near Guadalupe Peak and El Capitan the upper part of the escarpment is formed by the same limestones that spread as a plate over the Guadalupe Mountains, on whose surface the summit peneplain has been cut. In this area the rim has been lowered very little below the summit peneplain, remnants of which extend to the rim, although any weak beds that overlay the peneplain (stage 1, fig. 22, B) have long since been removed. Farther south, however, the rim was worn down a great distance below its original height while the Delaware Mountains and Gypsum Plain to the east of it were being degraded. Most of the lowering of the rim in this area was accomplished before the gravel plain to the east was formed, as gravel remnants cap the rim near Guadalupe Pass (fig. 24, A).
The streams that drain the escarpment (indicated as "streams of complex origin" on pl. 22) are probably mainly consequents that took their courses down the original tectonic surface. Their history, however, has been complex, for there have been several periods of movement, and each movement has modified the pre-existing surface and thereby influenced the streams that drain it. Moreover, by headward cutting the streams have acquired obsequent extensions at the expense of streams draining eastward from the rim. Other streams may have acquired new courses on the surfaces of deposits laid down over the bedrock on the escarpment or west of it.
On the west side of the southern Guadalupe Mountains, between Shumard Peak and El Capitan, are many steeply sloping, dissected remnants of slope deposits (shown as "older slope deposits" on pl. 22) which indicate a well-marked pause in the erosion of the escarpment. On the south slope of El Capitan similar deposits form the caps of ridges and mesas and stand high above the channels of Guadalupe Canyon and other streams. Apparently these deposits were formerly continuous with remnants of gravel on the rim of the Delaware Mountains near Guadalupe Pass, east of Guadalupe Canyon. This interpretation suggests that the older slope deposits are of about the same age as the older gravels of the Delaware Mountains.
Older slope deposits are not present on other parts of the west-facing escarpment, either north of Shumard Peak in the Guadalupe Mountains or south of El Capitan in the Delaware Mountains.
The relation of the gravel plain of the Delaware Mountains to the older slope deposits is suggested on figure 24, A, where the gravels on the rim of the Delaware Mountains near Guadalupe Pass are shown on the farthest section, and the slope deposits on the tops of ridges and mesas are shown in the nearer sections. The amount of subsequent erosion can be determined by their relation to the profile of Guadalupe Canyon, also shown on the figure. Their relations to present topography are also suggested on plate 1, where they are designated by the letter a.
The older slope deposits farther north, on the west side of the mountains below Guadalupe Peak are shown on plate 12, A, and on the profiles of figure 23, B.
The gravel remnants below Guadalupe Peak lie on the smoothed faces of spurs projecting from the escarpment between the waste-covered embayments at the heads of the present canyons. They stand several hundred feet above and forward from the embayments, but like them have a slope of about 30° (fig. 23 B). The upper ends of the remnants are several hundred feet below the bases of the cliffs that surmount the escarpment, and stand slightly forward from them, as though they were formed when the cliffs had not receded as far east as now. The lower ends flatten over the top of the black limestone bench at the edge of the escarpment, whereas the younger waste streams extend down into the canyons cut into the bench.
The slope deposits of the remnants consist of unsorted angular blocks of massive (Capitan) limestone from a few feet to more than 10 feet across, which in many places are rather firmly cemented by caliche. Many of the blocks are deeply pitted by weathering, as though they had not been disturbed for a long period. The deposits have a thickness of as much as 10 feet, or about that of the diameter of the largest boulders embedded in them. The fragments have all fallen or rolled from the cliffs above in the same manner as those in the younger waste-streams.
The position of the remnants of older slope deposits suggests that they formed under conditions similar to those under which the younger slope deposits are now forming. Both sets of deposits are composed of the same type of material, and have the same type of slope (fig. 23, B). Most of the remnants are now on the points of spurs, between recesses covered by modern waste-streams. At the time of their deposition, this relation was probably reversed in places, and remnants on the present spurs accumulated in the recesses of the earlier time. In general, however, the older deposits appear to have accumulated on a surface more subdued than the present one, and with fewer rock spurs projecting from it, and fewer canyons (such as Guadalupe Canyon) cut below it.
Some indication as to the age of the deposits can be obtained south of El Capitan. Here, on a canyon wall 150 to 250 feet below the nearest remnants of the older slope deposits, is the Indian Cave (fig. 24, B), which has yielded a fauna that includes a number of extinct late Pleistocene or early Recent vertebrates.50 The fossils will be discussed in a later section of the report (p. 158). The relations imply that deep erosion took place after the slope deposits were laid down and before the fauna accumulated in the cave, in which case the slope deposits are probably of Pleistocene age.
Ever since the first uplift of the mountain area, material eroded from its west side has been washed out and deposited in or along the edges of the tectonically lower Salt Basin. The process was furthered by the lack of through-flowing drainage in the basin. Coarser-textured detritus was laid down as a fanglomerate on the bajada along the edge of the mountains, and was built up until the streams were able to attain a graded profile across it. These processes, however, were probably interrupted several times by renewed uplift or climatic changes.
The bajada on the west side of the mountains is underlain by a complex of fanglomerates, laid down during successive stages of the uplift and degradation of the mountain area. Most of the fanglomerate that now lies at the surface is probably of fairly recent origin (shown as "younger fanglomerate" on pl. 22), but some deposits are exposed in places that appear to be older (shown as "older fanglomerate" on pl. 22).
West of the escarpment near Guadalupe Peak is a tectonic trench a mile wide, lying between the outer bench of the escarpment and the easternmost ridge of the Patterson Hills (pl. 20). It is covered everywhere, except a few rock hills that project from it, probably to great depth, by fanglomerates composed of fragments washed out from the escarpment to the east (pl. 22).
Several miles southwest of Guadalupe Peak some low ridges project above their surroundings in the trench. They are composed of fanglomerates rather firmly cemented by caliche, which appear to be older than those underlying the lower country around them. They consist mostly of great blocks of massive Capitan limestone, but include a few blocks of sandstone from the Delaware Mountain group. They contain no fragments of black limestone from the outer bench of the escarpment, which rises several hundred feet above them a short distance to the east, whereas the surrounding younger fanglomerates contain abundant black limestone fragments. These older fanglomerates resemble the older slope deposits on the escarpment to the east in composition and degree of consolidation, and are probably of the same age.
The western slopes of the ridges of older fanglomerate are gently rounded surfaces, but each one breaks off on its eastern side in a straight, abrupt scarp 25 to 50 feet high. These scarps appear to be fault scarps (as shown in figure 23, B), and indicate that the fanglomerate was disturbed after it was deposited.
OLDER PEDIMENT AND ITS GRAVEL COVER
South of the trench that lies west of Guadalupe Peak, bedrock is exposed in many places, and has been worn down to pediments and low hills. The bedrock is covered in many places by a thin mantle of unconsolidated deposits (shown as "stream alluvium and cover of younger pediments" on pl. 22). Standing 50 feet or so above are terracelike remnants of an older, gravel-capped pediment (shown as "gravels deposited on older pediments" on pl. 22). They are well displayed near Guadalupe Arroyo along United States Highway No. 62, and also occur farther east, toward the base of the Delaware Mountains.
The gravels on the older pediment near the base of the Delaware Mountains reach a thickness of 100 feet, but they thin toward the west, and near Guadalupe Arroyo are less than 20 feet thick.
Near the Delaware Mountains (as in the exposure shown on fig. 17, A), the deposit is a rudely stratified aggregate of limestone cobbles and broken flags, embedded in a buff sandy clay matrix, and interstratified with some beds of clay as much as 5 feet thick. Most of the fragments are dark-colored, bedded limestone derived mainly from the Getaway limestone member of the Cherry Canyon formation, which now forms the rim of the mountains to the east. However, limestone fragments with features characteristic of the Pinery and Lamar limestone members of the Bell Canyon formation much higher in the section can also be recognized. These members do not crop out near the rim of the Delaware Mountains to the east, but they are exposed not far from the gravel areas in the foothills to the west. One gravel remnant 2-1/2 miles southeast of the forks of the Van Horn and El Paso roads contains rounded cobbles of light-gray, massive Capitan limestone. The gravels contain no fragments of the black limestone (Bone Spring) that now crops out east of the Border fault along the base of the Delaware Mountains escarpment, nor of the coarse-grained sandstone (Brushy Canyon) that immediately overlies it.
It is difficult to tell much about the original form of the older gravels and the pediment on which they rest, for they now occur only as remnants. Moreover, some of the remnants seem to have been displaced by faulting. Near the base of the Delaware Mountains, closely adjacent remnants stand as much as 100 feet higher or lower in different fault blocks (fig. 17, B), and in the ravines that cut them they are seen to be traversed by fault planes or to lie in fault contact with the bedrock (fig. 17, A). At one exposure 4 miles south of El Capitan (shown at right-hand end of fig. 24, A), the gravels seem to have been displaced about 60 feet by one of the faults of the Border zone. The remnants farther west, near Guadalupe Arroyo, were probably disturbed in the same manner; for example, one remnant on the south side of the arroyo a mile southwest of the junction of the Van Horn and El Paso roads ends eastward along a straight scarp 40 feet high that is in line with an exposed fault in the bedrock 3 miles to the north.
FLOOR OF SALT BASIN
West of the mountains, and beyond the bajadas that fringe their base, is the level floor of the Salt Basin (pp. 136-138). No outcrops of early Pleistocene deposits have been identified on the floor, and it is not known to what extent they have been covered by later Pleistocene and Recent deposits. The older slope and fanglomerate deposits cannot be traced into the basin from the mountains to the east because the intervening area is covered by later deposits.
The basin floor was probably leveled by deposition in lakes that occupied the central part of the basin from time to time during Pleistocene and perhaps earlier periods. Surface features on the floor indicate that a lake existed there during late Pleistocene time (pp. 156-157). Whether present surface features were shaped entirely by the late Pleistocene lake cannot be determined. As deposition on the floor has proceeded much more slowly than on the adjacent bajadas, it is possible that some of the surface features are inherited from earlier Pleistocene time.
South of the area studied, the basin floor appears to have been deformed. In the latitude of the northern part of the Sierra Diablo, the cross section of the basin is asymmetrical, with the lowest point at the western side, at the foot of the bajada that fringes the high Sierra Diablo scarp (pl. 23). To the east, the floor rises gradually to the more distant and lower Delaware Mountains, which is the reverse of what would be expected if the surface had been shaped by depositional processes alone. Evidently the floor has been tilted toward the west. The tilting is older than the late Pleistocene lake, as its beach lines extend horizontally around the area. It probably took place at the same time as the later faulting along the nearby Sierra Diablo scarp, this faulting probably being of the same age as that which disturbed the older gravel deposits within the area studied.
AGE OF DEPOSITS
The older gravels, slope deposits, and fanglomerates in the vicinity of the southern Guadalupe Mountains contain few fossils, so their age cannot be given precisely. The gravels southeast of the Guadalupe Mountains contain a few terrestrial mollusks which are long-ranging forms that might be either of Pleistocene or Recent age. At Indian Cave, the relation of the older slope deposits indicates that they are much older than the late Pleistocene or early Recent vertebrates contained in the cave (fig. 24, B).
The older gravels, slope deposits, and fanglomerates have one characteristic in common. They are olderperhaps much olderthan the modern and relatively recent features. All have been deeply eroded, and many stand high above present drainage. Some have been faulted and tilted. Although direct evidence is lacking, these relations suggest that they are of early Pleistocene age.
INTERPRETATION OF EARLY PLEISTOCENE(?) FEATURES AND DEPOSITS
The early Pleistocene (?) features and deposits came into existence toward the close of a long period of crustal stability which succeeded the major uplift of the mountains in late Pliocene or early Pleistocene time. The features and deposits seem to record a common historyfirst, a well-marked pause in downcutting indicated by extensive pediments in the lower areas, and mature slope forms on canyon walls and escarpments in the mountains; then, a period of aggradation indicated by deposits laid down on the pediments. This history was controlled by a number of factors. The most important is fluctuation in climate, a characteristic feature of Pleistocene time, which would affect all drainage basins equally. In addition, the emplacement of the Pecos River east of the mountains undoubtedly influenced all streams draining in that direction from the crest.
VOLUME OF EARLY PLEISTOCENE (?) DEPOSITS
Review of the tectonic events and sequence of deposits in the Sacramento section (including the Guadalupe and Delaware Mountains) indicates an anomaly. The initial uplift of the ranges was followed by deposition of great volumes of Pliocene deposits, both in the intermontane basins and the plains to the east. The later and presumably main uplift of the ranges was followed by the deposition of only thin and scattered Pleistocene deposits such as those seen in the Guadalupe and Delaware Mountains.51
The smaller volume of deposits in Pleistocene time is attributed in part at least to the development of such through-flowing drainage systems as the Pecos and Rio Grande, which were able to carry material out of the region. The total volume of deposits, however, may have been small even in such depressions as the Salt Basin which were not connected with through-flowing drainage. The main reason for the smaller volume of deposits in Pleistocene time seems to be that less material was shed from the mountains after the second uplift than after the first because most of the poorly resistant rocks had already been stripped from them, leaving only a core of resistant Paleozoic limestones and other rocks. This suggestion may account for the fact that the Guadalupe Mountains and other ranges of the Sacramento section still project high above their surroundings, even though the main uplift was at least as old as the early Pleistocene, and though subsequent disturbances have been relatively small.
DEVELOPMENT OF PECOS RIVER
A profound change took place on the eastern slope of the Guadalupe and Delaware Mountains during the Pleistocene because of the development of the Pecos River. Previously, drainage had flowed eastward to the aggrading surface of the Llano Estacado and had become adjusted to a relatively high-standing, rising base-level. The Pecos River developed at nearly right angles to the older drainage, and at a much lower level, along the eastern base of the Guadalupe, Delaware, and other mountains of the Sacramento section.52 Drainage on the eastern slope of the mountains was then adjusted to a low-lying, descending base level controlled by the river.
The Pecos River apparently originated in the Edwards Plateau south of the Llano Estacado as a short consequent tributary to the Rio Grande, The gradient of the original stream probably was so much steeper than those of streams flowing east to the Llano Estacado that it was able to extend its original course headwards, thereby capturing the headwaters of each of these eastward-flowing streams in turn.53 Headward cutting toward the north was aided by the fact that a belt of poorly resistant upper Permian and lower Mesozoic rocks lies between the mountains of resistant older rock to the west and the resistant, caliche-capped sheet of Pliocene deposits on the Llano Estacado to the east. At least a part of the capture of other streams by the Pecos was facilitated by large-scale collapse of rocks along this belt of poorly resistant rocks as layers of interbedded soluble salts were removed by ground water.54
These events have not been definitely dated. They are certainly later than deposition of the Pliocene rocks of the Llano Estacado, and are older than deposition of the quartzose conglomerate55 and Gatuna56 which are the oldest formations that fill the valley of the Pecos River. The latter deposits are overlain unconformably by younger Quaternary deposits and are probably of older Pleistocene age. If these deposits are older Pleistocene, the development of the river probably took place in early Pleistocene time.
As a result of the development of the Pecos River, streams flowing east from the crest of the Guadalupe and Delaware Mountains became adjusted to a low-lying, descending base-level controlled by the river, instead of to a high-standing, rising base-level as before. During each successive cycle, such as the pediment cutting and the gravel deposition on the pediments described above, erosion and deposition therefore took place at a lower level than during the preceding cycles. A series of successively lower plains and terraces were thus developed. Moreover, material washed out from the mountains was not deposited in any large volume in the lower country, but much of it was carried out of the region toward the sea.
The fluctuation in conditions suggested by widespread cutting of pediments and other features, followed by deposition on the pediments, was probably caused in large part by a fluctuation in climate. It could not have been due entirely to changes in regimen of the Pecos River for the areas draining into the Salt Basin to show a similar history. Only climatic changes would have equal effect on all drainage basins.
As shown by the relations along Pine Spring and Cherry Canyons (fig. 21, B), the streams that cut the pediments had a more concave profile than the present ones. Concavity of profile results from a downstream increase in the effectiveness of the transporting power, which may be brought about in increase in volume, by decrease in the coarseness of the load, or by both. Each of these factors would be enhanced by greater rainfall; thus streams lose their steep headward declivity in a shorter distance in humid than in arid climates.
The gravel deposits on the pediment apparently resulted from a change in climate toward aridity. The profile of the deposits, as shown along Pine Spring and Cherry Canyons (fig. 21, B) is less concave than the surface on which they rest. Both the gravels on the pediments and the probably contemporaneous slope deposits and fanglomerates are strongly impregnated by caliche, a soil feature characteristic of dry climates. An exposure in Pine Spring Canyon one mile west of Pine Spring (p. 145) suggests that this change may have taken place rapidly. The layer of fine-grained sediments at its base was laid down when little material was being washed off the adjacent mountains. This layer is succeeded by fanglomerates, laid down when erosion of the adjacent slopes was actively renewed and more coarse material was washed in than the stream could carry away.
With the change toward an arid climate, both the volume and the coarseness of the material eroded from the mountain areas was increased. The cloak of vegetation on the mountains was reduced, the soils stripped away, and the bedrock exposed to attack by mechanical weathering. A return to more humid conditions at the end of the period of deposition is suggested by the subsequent dissection of the gravel deposits. These subsequent events are discussed under the heading of later Pleistocene and Recent features.
RELATION OF CLIMATIC FLUCTUATIONS TO PLEISTOCENE GLACIATION
The fluctuations in climate between humid and arid conditions indicated by the early Pleistocene pediments and deposits were probably related to the glacial and interglacial stages of Pleistocene time. A period of humid conditions probably corresponds to one of the glacial stages, and a period of arid conditions probably corresponds to one of the interglacial stages.
Erosion surfaces and unconsolidated deposits along the Pecos River in the nearby Roswell area in New Mexico which are similar to those in the area studied, have been tentatively correlated by Nye57 with the specific Pleistocene glacial and interglacial stages. Such correlations, however cannot rest on a secure basis until studies have been made of much broader areas than those near Roswell and in the Guadalupe Mountains. In particular it is desirable to know more about the geomorphic history of the region which separates these two areas from the nearest centers of Pleistocene glaciation.
Features of probable glacial origin have been reported from the Sangre de Cristo Mountains and the Sierra Blanca in New Mexico,58 but the nearest area in which an extensive glacial history is recorded is in the San Juan Mountains of Colorado.59 Geomorphologic studies of areas not far south of the San Juan Mountains are now being carried on by Kirk Bryan and his associates,60 and as this and other work is extended, more conclusions can be reached as to the Pleistocene history of the region south of the glaciated areas.
Last Updated: 28-Dec-2007