CENOZOIC DEPOSITS AND LAND FORMS (continued)
After the older land forms had been carved and were mantled by deposits, degradation of the region was renewed, and the land forms and deposits were thereby dissected. During this time of degradation, the mountains were given the form they now possess. The younger land forms and deposits came into existence during the time of degradation; those now in the process of formation have been described in an earlier section (pp. 126-138), and need be mentioned further only to place them in their historical perspective. In addition, some features will be described that are older than the modern features and younger than the older topographic features and deposits.
EVIDENCE FOR FAULTING
In the Guadalupe and Delaware Mountains, later Pleistocene time was one of some tectonic instability. Reference has already been made to the fact that in the foothill area the older fanglomerates and the gravels deposited on older pediments have been displaced by faults, which are indicated in some places by low fault scarps and in others by fault planes traversing the deposits (pp. 113-114). These faults are all west of the Border fault zone and it is probable that this zone moved again at about the same time. This movement is indicated by relations along the black limestone bench that flanks it on the east, described below.
BLACK LIMESTONE BENCH
Along the west base of the Guadalupe and Delaware Mountains from the latitude of Guadalupe Peak southward, the black limestone at the base of the Permian succession projects as a bench that rises a few feet to more than 300 feet above the downfaulted rocks to the west. Above the bench are gentle slopes carved from the overlying sandstones. A bench has formed on the black limestone because its resistance to erosion is greater than that of the sandstones that overlie it. The surfaces of the bench are boulder-controlled slopes, adjusted to the movement of weathered black limestone fragments across them.
The bench is prominently in view along the base of the escarpment in the panorama, plate 5, A, and the right-hand end of the panorama, plate 5, B. On plate 14, B, a nearer view of the bench, its relation to the Border fault may be seen. A profile across the bench is given on figure 23, B.
The bench seems to be more intimately related to the Border fault zone than are the higher parts of the escarpment. Its western edge is a nearly straight line that follows the traces of the faults throughout its entire distance, and it is scarcely dented by the streams that have incised narrow gorges across it. Thus it has receded very little from its original tectonic surface. In places, older fanglomerates and gravel deposits lie against the black limestones on the upthrown sides of the Border faults (pl. 14, B) in such a manner as to suggest that the fanglomerates had been faulted against the black limestones. In one place, older gravels appear to be displaced by one of the faults of the zone (right-hand end of fig. 24, A). Black limestone fragments are absent from the older fanglomerates and gravels west of the Border fault zone, which contain only rocks from the higher parts of the escarpment. On the other hand, such fragments are abundant in the younger rocks of the same district.
These relations suggest that the black limestone bench may have been partly or wholly concealed at the time the older slope, fanglomerate, and pediment deposits were laid down, and that it did not reach its present height until later, when renewed movements on the Border fault zone took place (stage 3, fig. 22). Such movements may have amounted to several hundred feet in places (fig. 23 B). The face of the black limestone bench is probably a slightly eroded fault scarp, much younger than the greatly eroded fault scarp or fault-line scarp that forms the higher part of the escarpment.
RELATION OF FAULTING TO EROSIONAL FEATURES
The faulting just described interrupted the development of the older features and deposits, which had been forming during a long period of crustal stability. The displacements were relatively small, amounting to a few hundred feet at most, but they were sufficient to cause the dissection of the various older features on the west-facing escarpment and the foothills to the west. Because of the movements, the features were placed in a new relation to the adjacent drainage, and may have been shifted upward relative to the base-level of the Salt Basin. either by depression of the basin or uplift of the mountains.
EROSIONAL AND DEPOSITIONAL FEATURES
DISSECTION OF OLDER FEATURES AND DEPOSITS
The older topographic features and deposits have not only been dissected where they are faulted, but in parts of the area where they are not faulted. Thus, the streams in the canyons of the Guadalupe Mountains have cut more than 100 feet below the valley-side shoulders on their walls, and now flow in narrow, inner gorges, with imperfectly graded channels. In like manner, the gravel plain to the southeast is trenched as much as 100 feet by narrow gorges that in places extend into the underlying bedrock. Farther south broad plains flank the larger streams; they are pediments adjusted to the grade of these streams. The plains stand at a lower altitude than the older gravel plains and older pediment remnants, and represent a new cycle of base leveling at a lower level.
The widespread dissection of the older features resulted from the interaction of numerous factors, whose relative importance is difficult to evaluate. It is possible that the mountains were uplifted at the time of the later faulting along the Border zone and in the foothills. If so, they were shifted upward relative to the base levels of the streams that drained them. Climatic changes also probably took place, some of which encouraged downcutting by the streams. Moreover, as degradation of the mountains progressed, the size of materials carried by the streams decreased, and the streams tended to cut down to increasingly lower gradients.
On the east slope of the mountains, in the area drained by the Pecos River, dissection by streams also took place as a result of lowered base levels caused by deeper cutting of the river. Aerial photographs of the area east of the mountains, in the Gypsum Plain and Rustler Hills, indicate that at some place along each of the major streams draining toward the Pecos there are abrupt descents from wide alluvial valleys upstream to steep-sided, headward-cutting gorges downstream. Some of the major streams have more than one such descent. These descents represent impulses toward renewed downcutting that are being generated upstream from the river along each tributary. In addition to normal downcutting of the river, such impulses may have been influenced in part by eustatic changes in sea level that are known to have taken place during Pleistocene time.
The dissection of the older topographic features and deposits furthered the development of subsequent streams (shown by a separate symbol on pl. 22), although some of these streams may have come into existence during earlier periods.
On the east slope of the Delaware Mountains, the structure is such that the surface is made up of many strike belts of poorly resistant sandstone, lying between belts of more resistant sandstone and limestone. Along them drainage leading into the larger consequent streams has cut headward to form subsequent streams, such as Bell Canyon (pl. 22). In other places in the same area belts of poorly resistant sandstone are faulted down to the same altitude as more resistant rocks. Along one of these belts, the Getaway graben, a depression was hollowed out by two subsequent tributaries of Getaway Canyon. The more resistant limestones to the east and west rise above it in resequent fault-line scarps, whose tops are remnants of an older pediment, probably of the same age as the gravel plain farther north (pl. 22).
On the west slope of the Delaware Mountains a number of belts of weak rock extend along fault lines, perhaps because of the close spacing of joints. In this area, during dissection of the older features, subsequent streams were cut in the weak belts; the largest of them is the stream of Guadalupe Canyon (pl. 22).
Along the sides of some valleys that trench the gravel plain southeast of the Guadalupe Mountains are terraces that lie between the plain and the present channels. They record pauses in the dissection of the plain.
Along Lamar and Cherry Canyons east of the D Ranch Headquarters are remnants of a gravel-capped, rock-cut terrace 50 feet above the present stream and 50 feet or more below the surface of the gravel plain (pl. 22). The deposits on the remnants consist of limestone fragments, derived from the Guadalupe Mountains, that were either washed out from the mountains at the time the terraces were formed or were reworked from the older deposits of identical composition on the gravel plain.
In Glover and Getaway Canyons, two headwater tributaries of Delaware Creek, are terraces of different character. Here, remnants of alluvium lie on the sides of the present valleys, as much as 50 feet above the present channels or within 100 feet of the hilltops whose surface is equivalent to the gravel plain. The alluvium consists of fine-grained limestone gravel and buff silt. In this region, after the valleys were first cut, they were filled to a considerable depth and then reexcavated. Terraces probably of similar structure but consisting wholly of coarse gravel lie along Pine Spring Canyon for about a mile west of Pine Spring (pl. 22).
The terraces in Glover, Getaway, and Pine Spring Canyons are the only examples that have been observed in the region of the sort of alluvial terraces that have been described in other parts of the southwest.61 Such terraces are especially prominent along the Pecos River near Roswell, N. Mex. They are supposed to have been formed by successive stream-cutting and stream-filling as a result of changes from wet to dry climate. The development of alluvial terraces in the area studied is poor, probably because the area lies near the sources of the streams that drain it and too far from the Pecos River to have been much affected by temporary changes in its regimen.
Alluvial deposits on the flood plains of the modern streams occupy relatively small tracts in the area of this report, and only the larger of them have been mapped (shown as "stream alluvium and cover of younger pediments" on pl. 22). The largest areas are along Delaware Creek on the east slope of the Delaware Mountains, and near Guadalupe Arroyo in the foothills west of the Delaware Mountains. The alluvium consists mostly of buff or brown clay, somewhat impregnated by caliche, with lenses of fine gravel. Along Delaware Creek it is about 25 feet thick, but on the west side of the mountains it may be somewhat thicker. Away from the flood plains, the alluvial deposits grade into a relatively thin sheet that forms the cover of younger pediments.
YOUNGER SLOPE DEPOSITS AND FANGLOMERATES
The character and origin of the younger slope deposits and fanglomerates have already been discussed (pp. 133, 135-136), and need not be repeated here. These deposits seem to have formed in much the same manner as the older slope deposits but later than the period of renewed faulting and uplift in which the older ones were dissected. The younger fanglomerates, which form the bajada west of the mountains, were probably built up at the same time, as a result of the renewed faulting.
Deposition of the younger fanglomerates gave rise to a new generation of consequent streams (shown as "streams consequent on bajada surface" on pl. 22). Streams like them no doubt existed on the bajada ever since the first uplift of the mountain area, but because they are constantly shifting, the streams now seen there have occupied their present positions for only a relatively short time.
In some places material washed out from the mountains has filled the depressions between the mountains and the foothill ridges to such an extent that streams consequent on the bajada have been able to flow over these ridges at their lowest places. In this way they have acquired new courses across barriers in the original tectonic surface.
In some places younger deposits are still gathering on slopes, pediments, and bajadas, but in others they are now being dissected. Dissection of the younger fanglomerates on the bajadas has already been discussed (p. 136).
Dissection of younger slope deposits is taking place south of El Capitan, as shown on plate 1. Here, two waste streams (indicated by the letter b) are trenched by ravines to depths as great as 50 feet, and in places cut into bedrock. Some of the steeper slopes between the waste streams, only lightly covered by deposits, are scored by gullies, and between them the surfaces are broadly rounded. Similar features were observed on the east slope of the Delaware Mountains, notably on the cuesta formed by the Lamar limestone member of the Bell Canyon formation northeast of the junction of Bell and Lamar Canyons. The sandstones forming the slope of the cuesta are generally stripped of all soil and deposits, but here and there are remnant patches of an older, rounded, soil-covered surface.
These features may be relics of climatic changes in the geologically recent past. The rounded, soil-covered slopes were formed during a time of relatively humid climate, and the dissection that followed probably took place during a time of relatively dry climate. The dissection seems to be considerably older than the arroyo cutting described below.
The alluvium in many of the flood plains of the area has been trenched to depths as great as 20 feet by steep walled arroyos. According to Mr. Walter Glover, a local resident, the arroyos near Getaway Gap have been cut since about 1905. Before that time, the valley bottom at the upper end of the gap was a smooth flat, easily crossed in all directions by a wagon, whereas since then the arroyos have widened so much that a wagon can now be driven along their channels.
The arroyo cutting resembles that which has recently taken place in many other parts of the arid southwestern United States.62 It seems to have resulted from modern depletion of the vegetation cover, thereby quickening run-off and soil erosion. This depletion probably happened because of overgrazing of the country by stock, for in the regions where I have observed it, arroyo cutting has taken place within a few score years after the country was settled. Periods of drought in recent years have greatly increased the overgrazing, for the cattle that remained on the land during the dry periods were forced to crop the grass down to its roots, and to eat plants such as the prickly pear and sotol that they usually avoid. It is entirely possible, however, that the artificial depletion of the vegetation merely accelerated a natural depletion resulting from an increasingly dry climate, and that conditions favorable to soil erosion existed at the time of the arrival of the first settlers.
As already indicated, the center of the Salt Basin, beyond the edges of the bajadas on either side forms a remarkably even floor, which stands at an altitude a little above 3,620 feet (pp. 136-138). It marks the extent of the gypsiferous clay hills and intervening meadows mapped as Reeves chalk in the soil report.63 This floor is a former lake bed, which from time to time in the past was covered by standing water. On it are many features formed by a lake that is probably of late Pleistocene or early Recent age.
The lacustrine features are most clearly indicated on aerial photographs. Old beaches stand out clearly as curving, concentric bands, encircling the margins of the floor and the sides of low protuberances on the floor itself (pl. 23). The beaches can be seen also when the floor is viewed from the mountain tops to the east, but their pattern and character is less evident. Such features are difficult to recognize on the ground, but they have been studied in the field a mile southwest of the old P X Ranch within the area of this report, and near the mouth of Victorio Canyon east of the Sierra Diablo south of the area of this report.
On aerial photographs the beaches appear as low, light-colored ridges, a few hundred feet to nearly a quarter of a mile across, that extend as bands along the contours, bending outward around the outer ends of alluvial fans, and recessed between them. The highest beaches lie about 40 feet above the lowest points on the floor, or at an altitude of about 3,660 feet. They are indefinite and discontinuous, and hence probably older than the lower beaches. The most definite beaches lie at a lower altitude and about 20 feet above the lowest points on the floor; others lie both above and below. Although the beaches are not far apart in altitude, the very gentle slopes on the floor cause them to be in places as much as a mile apart laterally.
The two beaches studied in the field are both parts of the 20-foot beach. At the locality southwest of the P X Ranch the beach is a narrow embankment of gypsiferous clay which rises about 10 feet above its surroundings and is about 20 feet higher than the nearby alkali flats on the lowest part of the floor. At the locality east of the mouth of Victorio Canyon, the outer edge of the bajada is cut off in a scarp 10 to 20 feet high, which descends steeply from the bajada to a flat meadow containing alkali flats. The scarp is composed of buff loam, with a capping of gypsiferous clay. In places, the top of the scarp is a few feet higher than the surface of the bajada behind it.
HISTORY OF LAKE
The beaches indicate the one-time existence of a lake which was at first about 40 feet deep, and covered the whole expanse of the basin floor. Later, the lake receded but maintained a depth of about 20 feet for a considerable period, when well-marked shore features were formed. During the 20-foot period, slightly higher areas within the floor of the basin rose about lake level, such as the higher ground west of the area studied, between the chains of alkali flats on the east and west sides of the basin. The gypsiferous clay of the clay hills and the brown clay of the meadows, which are the characteristic surface material of the basin floor, are probably lacustrine deposits, later shifted somewhat by the wind.
These lacustrine deposits were built up in standing water to form the nearly level surface of the basin floor, and may have been laid down over the outer edges of the bajadas, thus causing the sharp boundary between the topography and soils of the two features.
After most of the waters of the lake had disappeared and most of the floor of the basin was uncovered, a few remnants in the form of intermittent water bodies remained at the lowest places on the floor. These low places were somewhat enlarged by subsequent wind action and form the alkali flats that are a characteristic feature of the modern basin floor.
The lake in the Salt Basin is probably of the same age as that which once filled the Estancia Basin of central New Mexico64 where there are many well-preserved shore features. Antevs65 suggests that the lake in the Estancia Basin existed during the "pluvial period" which came at the end of the Pleistocene.
The limestones of the area studied contain numerous caves, but there are no large ones comparable to Carlsbad Cavern and others in the Carlsbad and Capitan limestones not far to the northeast. Most of the caves observed in the area studied are shallow openings, recesses, and shelters.
Most of the caves here and elsewhere in the Guadalupe Mountains were probably formed when the topography was approaching its present form. The smaller ones occur in the present canyon walls and escarpments. The larger ones could have been cut to their present size and depth only by underground drainage whose outlets were near the levels of the modern streams. The time of cave formation was probably related to times of still stand expressed elsewhere by gravel plains, terraces, and other surface features.
According to interpretations made in this report, the Guadalupe Mountains did not begin to assume their present form until the beginning of Pleistocene time, and the development of the present surface features took place during the Pleistocene and Recent. Because of their close relation to surface features, the caves of the region also probably formed during these epochs. This conclusion has been previously suggested by Gardner.66
Some caves in the Guadalupe Mountains contain vertebrate bones and archeological material. One of them, the Indian Cave on the Williams Ranch, south of El Capitan, lies within the area studied. Its contents have been described by Ayer67 as including not only various living species, but also the extinct horse, dire wolf, and ground sloth (dung only). She states:
The Burnet Cave, in the Guadalupe Mountains near Three Forks, north of the area studied (fig. 2), has yielded still larger collections. The faunal and archeological material from it has been described by Howard and Schultz.68 The vertebrates include extinct species of bear, horse, camel, musk-ox-like bovid, and bison. According to these authors:
EVIDENCE FOR RECENT CLIMATIC CHANGES
In various places in the preceding descriptions, reference has been made to features that probably formed as a result of certain climatic conditions, or of changes in climate. Some of them are relatively ancient, and perhaps of Pleistocene age; others are of relatively recent age. Such interpretations of climatic conditions are not absolute, because of possible complications resulting from other factors, but evidence regarding the climatic conditions affecting younger features appears to be more obvious than for the older. The various features indicate various things and not all of them are in harmony, and not all of them took place at the same time. Evidence is still too scattered and indefinite to fit the observed features into any comprehensive climatic history.
EVIDENCE FOR CLIMATIC CHANGES IN AREA STUDIED
A formerly more humid climate is suggested by the evidence of lacustrine conditions on the floor of the Salt Basin in late Pleistocene time. Humid climate is suggested also by rounded, soil-covered slopes on some of the mountain sides and cuesta faces. A change to a drier climate is suggested by dissection and partial stripping away of this cover. Arroyo cutting in the alluvial deposits, though perhaps mostly the result of overgrazing, may have been influenced by increased dryness within modern times.
A formerly colder climate is suggested by the nature of the vertebrate faunas mentioned above, which came from two caves in the Guadalupe Mountains. Their nature may be explained partly by other factors, but these other factors probably do not account for all the features observed in the faunas.
EVIDENCE FOR CLIMATIC CHANGES IN NEARBY AREAS
Possible recent climatic changes in the southwestern United States have been discussed at some length by Huntington,69 who concluded that within the last few thousand years the climate has become distinctly more arid than before. Geomorphological, botanical, and archeological evidence is cited, not all of which is entirely convincing. Much more evidence has been accumulated since Huntington's publication appeared, but not all of it agrees with his conclusion. He has pointed out, however, that large climatic changes are the net effect of much more complex minor fluctuations, and some of these minor fluctuations may have been strong enough to have left some record of their passing.
Huntington70 discusses the results of the work of E. E. Free on the alkali flats and sand dunes of the Tularosa Basin west of the Sacramento Mountains (fig. 1). Free recognizes three or more sets of gypsum deposits of different ages in the basin, all presumably of aeolian origin, and all perhaps indicating periods of dry climate, similar to that under which the White Sands of the area are now forming.71
The conclusions of Antevs72 regarding the pluvial period at the end of the Pleistocene have already been mentioned (p. 157). He suggests that the extinct lake in the Estancia Basin and others near Clovis, N. Mex., were formed during this period. There is supposed to have been a moister climate than at present. Aeolian sand that covers the lake deposits is cited as evidence that there was a later change toward more arid conditions.
Bryan and Albritton73 have discussed features in the alluvial deposits of New Mexico and Texas that suggest climatic fluctuations, some of which probably took place within the last few thousand years. In the Davis Mountains area three alluvial formations supposedly laid down during humid periods are recognized; they are separated by unconformities due to erosion, which supposedly occurred during drier periods. During some of the erosion periods, channel trenching took place which resembles that going on today.
Evidence for climatic fluctuations based on other features has been suggested. Cave silts, wind-polished rocks, and sand dunes of various ages are cited as evidence for dry periods, which may correspond to unconformities in the alluvial sequence above noted.74 Bryan75 has attempted a tentative interpretation of soil profiles and weathered slopes near Alpine, Tex., in terms of climatic changes. A succession of an early, long period of aridity followed by moister conditions and finally by modern, drier conditions, is suggested.
EVOLUTION OF THE MOUNTAIN AREA
The evolution of the surface features of the Guadalupe and Delaware Mountains can be considered under the headings of structure, process, and stage.76 The mountains have the structure of an uplift, much broken by faults. The structural surface has been acted on by subaerial processes of degradation, under the influence of an arid climate, and dominated by the work of streams. Degradation has reached a stage wherein considerable modifications may now be seen in detail, although the original structure is still reflected in the broader configuration.
Changes in the aspect of the mountains following their original uplift have been brought about partly by renewed uplift and faulting during several succeeding periods, and partly by the erosion of a large amount of material from the upraised areas, some of it being deposited in the adjacent depressed areas. Poorly resistant rocks, of which no trace now remains, may at the time of the uplift have covered the summit peneplainthe oldest land form in the area; moreover, toward the south, a great thickness of sandstone and anhydrite below the level of the peneplain has been stripped off the mountain summits.
The escarpment that forms the western side of the mountains, although outlined by the faults along its base, is not as high as the tectonic relief of the rocks that compose it (fig. 22, B). Its crest has been lowered by erosion, and its base raised by the deposition of unconsolidated material on the bajada to the west. It is also not as steep as the original tectonic surface, as it has been cut back into graded slopes.
The Guadalupe Mountains lie in the Basin and Range province, "characterized by isolated, subparallel mountain ranges rising abruptly above desert plains."77 The origin of the surface features in the province has long been debated.78
As worked out by Gilbert, Davis, and others, the ranges are composed of rocks that had previously been more or less deformed and degraded, and originated as uplifted blocks, outlined on one or more sides by faults that cut across the older tectonic features. The adjacent plains are believed to be underlain by rocks that were depressed so far at the time of the uplift of the ranges that they have been entirely buried by detritus washed out from the uplifted areas. The faults along the edges of the ranges are therefore seldom exposed to view, but must be deduced from evidence afforded by the land forms. This interpretation has been challenged by Spurr, Keyes, and others, who consider that the ranges have resulted from the differential erosion of a previously deformed terrain.
As may be seen from the interpretations of the Guadalupe and Delaware Mountains that are made in this report, these mountains correspond, at least generically, to the type of Basin-Range origin advocated by Gilbert and Davis, although possessing many specific features of their own. They depart from the ideal in that their rocks are only mildly deformed, in the probable absence of remnants of the prefaulting topography (assuming that the summit peneplain is pre-Cretaceous), and in the complications resulting from several periods of upheaval and faulting. The Guadalupe Mountains are, therefore, one of the "Basin-Range types" in the sense used by Davis.79
The conclusions reached for the Guadalupe and Delaware Mountains should not, however, be considered as favoring the general application of the interpretations of Gilbert and Davis to all the mountains of the Basin and Range province. Each range in the area has tectonic peculiarities of its own. Study of the ranges in recent years demonstrates that some, such as the Guadalupe Mountains, have indeed been raised by block faulting, but that others have been raised by arching and warping, and that some have been shaped largely by erosion.
In a region as vast as the Basin and Range province, the tectonic features and geologic history of whose parts is so varied, one is led to suspect that the characteristic surface features have not been caused by any one tectonic process, so much as by the all-pervading dry climate, which has allowed the drainage to remain poorly integrated, and has prevented the surface from being worn down to the subdued forms of humid regions.
Last Updated: 28-Dec-2007