The Geology of Chaco Canyon, New Mexico In Relation To The Life And Remains Of The Prehistoric Peoples Of Pueblo Bonito Smithsonian Miscelleanous Collections Volume 122, Number 7

GENERAL RELATIONS

Chaco Canyon lies in northwestern New Mexico on the upper reaches of Chaco River, a tributary of San Juan River (fig. 1). Chaco River, about 100 miles long, is an ephemeral stream such as is characteristic of arid regions. Its sandy bed throughout the greater part of the year is dry and the stream is dignified by the name of river only because of its considerable length and the violence of its floods. The stream begins in the high plains country north of Chacra Mesa at an altitude of 6,900 feet and flows a little north of west for 68 miles. Here the course changes sharply to the north and the river flows nearly parallel to, and on the east side of, the ridge known as the Grand Hogback for 26 miles and thence, breaking through the Hogback in a narrow canyon, it reaches San Juan River in 7 miles. The total length of the stream is thus about 100 miles, of which, however, only 15 or 20 miles of the upper course lies in a canyon worthy of the name. About 12 miles of this canyon, the portion with which we are concerned, is shown on the accompanying map (fig. 1).

Fig. 1. Map of Chaco Canyon showing present and post-Bonito channels.

Chaco Canyon lies in the southwestern part of the great Plateau province which occupies northwestern New Mexico, northern Arizona, western Colorado, and eastern Utah. The province is noted for its extensive flat surfaces, long lines of cliffs, and deep canyons. The flat surfaces are in part developed on the more resistant beds of nearly horizontal sedimentary rocks, although in part they consist of large outflows of lava, and in part they are the remnants of extensive plains of erosion. In northwestern New Mexico the largest unit of the Plateau province is the San Juan Basin, a vast area in which the rocks dip gently from the periphery toward the center. Chaco Canyon lies near the southern part of this area with a dip of about 2° to the north and east.

Sandstone and shale are the characteristic rocks. The shale is eroded into broad, flat surfaces or gently sloping valleys; the sandstone stands out as ridges or plateaus, bounded, especially on the south, by cliffs. The order and succession of these rocks have been studied by a number of geologists² interested primarily in the occurrence of coal or of vertebrate fossils.

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²Holmes, 1877; Endlich, 1877; Schrader, 1906; Shaler, 1907; Gardner, 1909; Sinclair and Granger, 1914; Matthew, 1897; Brown, 1910; Bauer, 1917; Bauer and Reeside, 1921; Reeside, 1924.

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Chaco Canyon is cut in the Cliff House sandstone, the upper member of the Mesaverde group. This sandstone member is 369 feet thick as measured by Reeside on Meyers Creek, a few miles northwest of Pueblo Bonito. It is underlain by dark shale containing thin sandstone and coal (Menefee formation) which crops out in the cliffs on the south side of Chaco Canyon and in a few places on the north side. The Mesaverde group is overlain by the Lewis shale which forms the plain north of Pueblo Alto and has a thickness of about 70 feet. Above the Lewis shale lie the Pictured Cliff sandstone and higher formations.

The Cliff House sandstone consists of two massive sandstones separated by relatively thin bedded sandstone. Consequently, weathering tends to produce two cliffs separated by a bench of gentler slope. The lower of these two massive sandstones is buff-Colored and about 140 feet thick. The cliffs which make the northern wall of Chaco Canyon are carved from this rock by processes considered more in detail on pages 18-20.

CLIMATIC CONDITIONS AFFECTING GEOLOGIC PROCESSES

The climate of the Chaco country is arid, but such a simple statement does not adequately summarize the effect of climate on the geologic processes. Aridity has many gradations from the almost total lack of rainfall characteristic of parts of the Libyan desert of Africa, and of certain areas on the west coast of Peru, to the tempered aridity of California where trees and grass thrive in areas having relatively low rainfall. Aridity is thus an inclusive term embracing climates having varying amounts of precipitation up to a quantity fixed arbitrarily around 20 inches of rainfall a year. The many shadings and gradations of aridity are dependent on such factors as the proportion of the precipitation that may occur as rain or as snow, on the distribution of precipitation throughout the year, and on the incidence of rainfall whether in hard showers or gentle drizzles. Similarly the daily or seasonal range of temperature and the extremes of heat and cold with their incidence and duration are all factors in aridity.

Climatic elements directly affect various subprocesses involved in the weathering of rocks and indirectly influence the nature of streams which act as the agents of removal and of transportation of weathered rock. Slight differences in degree of aridity often have marked influence in the growth of a vegetative cover, one of the greatest single factors influencing and delimiting erosive and sedimentary processes. In the account of these processes given hereinafter it will be seen that the scant vegetation of an arid region is a necessary prerequisite to the relative intensity of action, or even the existence, of many of the subprocesses. It follows, therefore, that any past or anticipated climatic change, provided it is sufficient to alter the existing vegetation, may have relatively large effect on geologic processes.

The available rainfall records of the Navaho Country up to the end of 1913 were collected by Gregory (1916, pp. 51-59) and the factors of climate in Chaco Canyon are now being recorded by the National Park Service. Herein only such general elements of climate are described as seem necessary for the purpose of defining climate in respect to geologic processes.

The climate of the Plateau province may be considered moderately arid. On the higher portion, between the valleys of San Juan River and the Little Colorado, there is greater precipitation than in the lowlands. In the mountains doubtless as much as 20 inches may fall each year, but current rainfall stations are all on lower ground. St. Michaels, Ariz., altitude about 6,950 feet, has a mean of 13.72 inches based on records for 29 years out of a period of 68 years; Crownpoint, altitude 6,800 feet, has 10.93 inches, based on an incomplete record extending over 11 years. At lower elevations, especially to the north and south of Chaco Canyon, the precipitation is less. Holbrook, Ariz., altitude 5,069 feet, has 9.38 inches with 25 years of record out of a total of 33 years. Places in the San Juan Valley have a lower rainfall: Fruitland, N. Mex., altitude 4,800 feet, 6.38 inches with 7 years of record; Farmington, N. Mex., altitude 5,220 feet, 9.23 inches with 7 years of record; Aneth, Utah, altitude about 4,700 feet, 4.96 inches. It seems likely the Chaco Canyon district has a precipitation similar to that at Crownpoint with a little less rainfall on the floor of the canyon which is 300 to 400 feet lower than the adjacent cliffs. For the purpose of this study it will be assumed that Chaco Canyon has a mean of about 10 inches.

A large part of this precipitation falls during the so-called summer rainy season in July and August. This period is characterized by sharp local or general rains from cumulus clouds or thunderheads. The rate of rainfall is high but the storms seldom last long. The incidence of the rains is also variable in time and space. Small areas are deluged and adjacent areas are left dry. The rains may come as early as June or as late as September, or may be inconsiderable in amount for a whole summer.

Gregory (1916, p. 63) summarizes many observations as follows:

The area covered by a shower is frequently only a few square miles, and on two occasions showers of 20 to 30 minutes' duration resulted in wetting less than 300 acres. Many of the showers result in a heavy downpour, and the total precipitation for a month is not infrequently the result of a single shower. . . . Generally the intense heat preceding a shower is reestablished within an hour or two after rain has ceased, especially at elevations below 6,000 feet. . . . Lightning is the almost invariable accompaniment of summer showers and constitutes a real danger to travel. . . . My records of thunderstorms for the Navaho Reservation during the field seasons 1909, 1910, 1911, and 1913 are 38, 26, 33, and 23, respectively, and it is believed that the annual number exceeds 40.

The winter precipitation falls gently and is likely to be widely spaced in time, but on the average totals nearly as much as the summer rainfall. At elevations above 6,000 feet there are 17 to 25 inches of snow, and at lower elevations some snow is possible each winter.

The distribution of precipitation throughout the year and its effect on agriculture is best expressed in the following table compiled by Gregory and amplified in a quotation also from him (ibid., pp. 61-62):

It will be noted that the season of least rainfall, April to June, is the growing season for most crops, and that therefore the seasonal distribution of rain is unfavorable for agriculture or for the vigorous reproduction of many grasses. Half an inch of rain per month for the period April, May, and June is an unusually large precipitation for most parts of the reservation, and during many years the combined precipitation of these three months is less than one-half inch. Moreover, plants obtained only a portion of this meager supply, for evaporation is most effective during the clear, dry, hot days of early summer. The moisture in the ground, supplied by the rains of winter supplemented by the scattered showers of spring, is sufficient to allow seeds to germinate and to send their stalks above ground, but is insufficient to bring a crop to maturity. The rainfall of July becomes therefore the critical factor in the life of the Navaho. If his prayers to the rain gods are answered his corn crop is assured, and grass springs up from the desert floors; if his prayer is denied the crop is a failure. . . . For a large part of the reservation corn, without irrigation, fails to mature every second or fourth year.

The variation in rainfall from year to year is of the greatest importance. The amount ranges between half the normal and twice the normal. For the 29 years of record at Fort Defiance and St. Michaels the year of greatest rainfall was 1854 with 22.44 inches; the year of lowest rainfall was 1900 with 6.52 inches. It is obvious that in years of severe drought like 1900 almost nothing grows. Such years are periods of starvation for a population dependent on agriculture or on the pasturage of animals.

Similarly the native vegetation must be able to resist these extremes of drought and precipitation. In general, sagebrush and scattering grass grow in the dryer areas, and perennial grasses where precipitation is more generous. With a slight additional increase of rainfall, cedar (juniper) forms sparse groves and a total precipitation of 15 to 20 inches is adequate for the open pine forests of mountain areas. These vegetative zones are, however, not strictly bounded by lines of equal rainfall because slope, exposure, and soil are all factors in the growth of plants. Near Chaco Canyon the flat parts of the plateau are generally underlain by clayey soils derived from shale or by loams formed by the admixture of sand from the sandstone beds with clay from the shale areas. These soils, under the influence of the local climate, support a fairly continuous cover of perennial grasses. The outcrops of sandstone have a rough and broken topography without soil or with only a thin sandy soil. Here grow scattered cedars, occasional woody bushes, and patches of "sand grass" but large portions of such areas are bare rock. The floor of Chaco Canyon supports a growth of greasewood (chico) with, in areas overflowed by storm water, a fair growth of perennial grass. A few cottonwood trees have survived from the period when the stream bed was shallow and are evidence that, with a slightly higher water table or less interference by man, domestic animals, and floods, many of these trees would again grow in the valley.

The temperatures of the region are, when expressed in yearly or monthly means, those of a temperate region. Yearly means range from 47.6° F. to 60.6° according to the altitude of the station. The annual and daily ranges in temperature are, however, very great. The maximum range recorded for various stations in the region is as follows: Fort Defiance—St. Michaels, 122° (98° to —24°); Fruitland, 124° (110° to —14°); Holbrook, 127° (106° to —21°); Crownpoint, 103° (98° to —5°). Temperatures exceeding 100° normally occur for 10 to 20 days each summer and 5 to 6 days of below-zero weather are likely each winter. The daily ranges in temperatures may amount to as much as 40° to 50° and, although doubtless effective in producing the disruption of rocks, are somewhat mitigated in their effect on man and beast by the low humidity of the air.

The growing season, or number of days between the last killing frost of spring and the first killing frost of autumn, ranges at various stations from 89 days to 161 days. In general, localities of lowest altitude have the longest growing season but there is at all stations a variability from year to year in the length of the growing season that may be shorter than the mean by as much as a month. Fort Defiance, at an altitude of 7,000 feet, has experienced killing frost in every month of the year except August. Obviously these variations in the length of the growing season add an additional hazard to agriculture in a region where rainfall is scant and also highly variable in incidence and amount. The data also give an index of the probability of changes in temperature that cross the frost line and these changes are the ones effective in the disruption of rock by frost action.

EXCAVATION OF CHACO CANYON

One who climbs the north wall of Chaco Canyon to Pueblo Alto is rewarded by magnificent views of a region that appears to be flat on all sides. To the south, beyond the canyon, he sees a vast plain from which rise a few low hills and, far to the southwest, high mesas that close in the horizon south of Crownpoint. To the north, the valley of Escavada Wash is a prominent feature bordered by ragged bluffs, but beyond lies a plain similar to the one on which he stands. This high level plain occurs generally on the more elevated parts of the San Juan Basin and is more or less independent of the hardness of the underlying rock. Canyons divide this plain into several parts that are obviously remnants of a once continuous erosion surface that formerly extended over the entire region. The plain has been too little studied to warrant strict definition or to hazard correlation with the Mojave peneplain which Robinson (1907) believes to have existed over the whole of northwestern New Mexico and northeastern Arizona.

Bryan and McCann (1936) imply that this surface is older than the Ortiz surface and other surfaces which, in the drainage of the Rio Puerco (of the East), are graded to the Rio Grande.

From the evidence near Chaco Canyon it seems possible to postulate two or more erosion cycles during each of which the region was reduced to very low relief. Whether a single peneplain or a more complex series of erosion cycles will be demonstrated by further work in the area, Chaco River and the adjacent streams gained their courses in a region of such moderate relief that the direction of flow was more or less independent of the distribution of hard and soft rocks. After a general uplift of the Plateau country, the "Canyon Cycle of Erosion" was initiated and the great canyons of the Colorado River system were cut.

FIG. 2.—Northwestern New Mexico showing the location of Chaco Canyon and Pueblo Bonito.

Chaco River, a distant and rather feeble tributary of the Colorado, also lowered its bed. In some places it excavated canyons and in others fairly broad valleys. That its canyon cutting was not continuous is evidenced by a well-marked erosional terrace near the mouth of Escavada Wash, a terrace capped by gravel largely derived from the local rocks and lying at an elevation about 150 feet above Chaco River. How important or general this pause may have been awaits field work over a larger area.

The general course of the river appears to have been controlled by undiscovered factors on the ancient plain already mentioned, but details of the carving of the rocks within Chaco Canyon, as we see it, result from the interaction of forces of erosion normal to the climate and the structure of the rocks.

The most notable feature of the canyon is its asymmetry. The north, or rather northeast, wall is steep and but little indented; the south or southwest wall is gentler and broken by branching canyons. Asymmetry is not an uncommon feature of valleys and canyons in New Mexico that have an east-west trend. For example, the relatively smooth, boulder-strewn slope of the south wall of Canyon del Rito de los Frijoles, near Santa Fe, contrasts strongly with the sheer cliffs of its north wall, in which Indians carved caves for occupancy in pre-Spanish times. Yet this canyon, cut in lava and tuffs having a slight dip downstream, is essentially alike in the two walls. The south side, however, is shaded for much of the day, a condition that leads to lower evaporation both of rain and snow, and consequently plants thrive. Small bodies of soil are held in place by grass and bushes; chemical erosion is promoted; talus heaps become overgrown with trees and mantle the rock slopes. In contrast, the north wall with its slope exposed to the sun is relatively dry. The mechanical forces of erosion are in full swing here and debris once loosened from the wall falls clear from rock surfaces which are thereby again exposed to the weather.

Chaco Canyon also has an almost east-west course and is subject to the same influences. A more important factor in creating a difference in the slopes of the canyon walls is, however, a northeasterly dip of the rocks. This inclination varies from 1 to 2 degrees with the result that the base of the Cliff House sandstone lies at or below the floor of the canyon on the north side, whereas it is from 50 to 100 feet above on the opposite side. In consequence the south cliff is undermined with relative ease by sapping of the underlying soft sandstone and shale. The fall of blocks is also assisted by a slight inclination of the bedding planes. Consequently, numerous and relatively large branch canyons have developed. In the vicinity of Fajada Butte a large tributary drainage has completely destroyed the south canyon wall and enters through a valley broader than that of Chaco River (pl. 2, upper).

PLATE 2. Upper: Fajada Butte from Pueblo Una Vida, with the present arroyo dimly seen beyond the ruin, and, at the right, the treeless plateau extending southward toward Crownpoint (Photograph by Neil M. Judd, 1920.) Lower: A small ruin in a northern branch of Chaco Canyon between Una Vida and Wejegi. Seepage has deposited an incrustation of gypsum along the rear wall of the cave. (Photograph by Neil M. Judd, 1926.)

PLATE 3. Left: Carved by wind and water, this niche reveals the characteristic cleavage planes of the lower Cliff House sandstone in the north wall of Chaco Canyon. Right: A feature at section 5 was this slab-sided Pueblo III fireplace 5 feet below the present surface. (Photographs by O. C. Havens, 1924.)

PLATE 4. Upper: Near the mouth of the Escavada Wash the lower Cliff House sandstone in the north wall of Chaco Canyon has been scoured and blasted by wind-driven sand. Dunes have blocked the old road to Farmington. Lower: Rainwater percolating through sandstone often results in a type of weathering called "stonelace." On the rock in the background water issuing from holes has left vertical streaks. (Photograph by O. C. Havens, 1924.)

PLATE 5. Upper: Looking down Chaco Canyon from Pueblo del Arroyo. The irregular mass of Penasco Blanco is seen on the horizon at left center. At the right a sunlit cliff in the middle distance marks the mouth of Rincon del Camino; between it and the standing figure are the broken walls of Ruin No. 8. Lower: A newly fallen section of hank immediately west of Pueblo del Arroyo. (Photographs by O. C. Havens, 1925.)

On the north side of the Chaco the base of the Cliff House sandstone lies below the canyon floor from Escavada Wash to Mockingbird Canyon. This part of the escarpment is characterized by having a sheer cliff surmounted by a bench and a more gentle cliff above. Its tributary canyons are generally less than half a mile in length and many are mere indentations in the cliff, of the type commonly called "rincons." From Mockingbird Canyon upstream the base of the sandstone lies above the valley floor and the lower cliff is benched rather than sheer, the tributaries are longer and the aspect of the canyon wail is more like that of the southwest wall.

Excavation of the canyon is a process long since interrupted, for the main stream nowhere runs on rock today, nor is it cutting laterally against the walls of the canyon. The process of canyon cutting was succeeded by a period of alluviation resulting in deposition of a valley fill to the level of the present valley floor (see below). Filling of the canyon has also been interrupted by formation of the present arroyo (p. 35).

ALLUVIATION OF THE CANYON FLOOR

After cutting Chaco Canyon to a depth somewhat greater than at present, the stream changed its habit and began to deposit more material than it removed. The gradual character of this filling and details of the process are here recounted at some length. This change from erosion to sedimentation was not confined to Chaco Canyon. Other canyons of the Plateau province and other streams throughout the Southwest were also filled and alluviation was the characteristic process up to a time within the memory of man. The isolation of Chaco Canyon has prevented the accumulation of definite historical data on characteristics of the canyon during this recent period of alluviation. However, canyons of adjacent parts of the Plateau province furnish reliable and analogous data since they lie at similar elevation, and are cut in like rocks under similar conditions of climate and settlement.

During the surveys of Powell and Dutton in 1878-1880, the canyons were undergoing alluviation as attested by the following statement (Dutton, 1882, pp. 228-229):

Most of these lateral canyons . . . are slowly filling up with alluvium at the present time, but very plainly they were much deeper at no remote epoch in the past. The lower talus in some of them is completely buried and the alluvium mounts up on the breasts of the perpendicular scarps. In some cases a smooth floor of alluvium extends from side to side of what was originally a canyon valley.

Such conditions no longer exist and had, even in Dutton's time, ceased in certain parts of the San Juan drainage. At present every main canyon in the area is occupied by an arroyo with vertical banks from 10 to 100 feet high. The streams now run at a level lower than the flat floors Dutton described by an amount equal to the height of their banks. These arroyos began at or near the lower end of canyons and progressed headward by a receding fall. The upper (or falls) portion of an arroyo is ordinarily marked by a vertical bank or a chaos of jagged, vertical banks and great blocks of detached alluvium. In many canyons the head of the arroyo has not yet completed its advance and, generally, only in the lower and larger tributaries have branch arroyos been formed.

In the undissected parts of canyons and in minor tributaries there can still be seen at work the processes by which their flood plains were built up during the period of alluviation that has so recently been brought to a close. The flat floor of such a canyon slopes downstream at a grade dependent on the ratio of the volume of water in floods to the supply of sediment carried. In general there is no well-marked central channel, but numerous small discontinuous and branching channels mark the central part of the canyon floor. Low alluvial fans consisting of sediment carried in from minor tributaries diversify the plain. Some of these fans are so large they have partly dammed flood waters of the main canyon and thus created temporary lakes or leveled broad stretches of the canyon floor. In such places perennial grasses may grow in quantities great enough to be cut for hay. However, the features of these canyon floors constantly change, since floods vary in volume, in quantity of material carried, and in the time of their occurrence in the annual cycle. Obviously a flood that occurs after a long dry season during which vegetation has been reduced to a minimum by the dryness of the soil will readily change the minor features of the plain over which it flows. However, generally speaking, the parched soil will absorb so much that only when it is present in large volume will floodwater be able to run the full length of a canyon. On the other hand, a heavy rain and small flood in early summer may produce such a rapid growth of vegetation that the plain will, for the rest of the summer, be protected against erosion by much greater floods.

Obviously, also, floods down the axis of a canyon tend to produce a slope more or less uniform at any locality but of decreasing declivity—the so-called graded slope of a stream. Floods in the tributaries, however, tend to dump material on the floor of the main canyon and interrupt this "graded slope." Opposite the fan of a tributary, therefore, the main stream may have a marked channel that is constantly renewed and shifted in position. Such discontinuous channels must be sharply differentiated from the arroyos characteristic of dissection.

Discrimination of these minor channels from continuous channels that indicate a new cycle of erosion is not always easy. Both are commonly described in the Southwest as arroyos. The character of discontinuous channels can best be explained by an example. The Cañada del Magre, located about 90 miles southeast of Pueblo Bonito, is a narrow, flat-floored canyon tributary to the Rio Puerco (of the East). Its walls are of buff Cretaceous sandstone similar in color, massiveness, and porosity to the Cliff House sandstone. A tributary, the Cañada de Bernardo, is similar. In 1909 a steep-walled gully about 25 feet deep extended from the arroyo of the Rio Puerco across the abandoned flood plain of that stream and a quarter of a mile within the Cañada del Magre where it ended in a chaos of blocks of alluvium and an intricate dry falls. This was the head of the new arroyo representing the readjustment of grade of the Rio Puerco—a readjustment gradually affecting all its tributaries. Upstream from this arroyo head the floor of Cañada del Magre was a grassy flat for a distance of about half a mile. There the center of the flat became sandy and this sandy stretch was enclosed in low banks which gradually increased upstream until they were about 6 feet high and which closed in from the sides until they were only 10 feet or so apart. Here there was an other dry falls, more or less grassed over. Some grass also grew on the floor of the arroyo. Above this latter falls was another grassy flat, succeeded at irregular distances by similar gullies and similar grassy flats. It is evident that these discontinuous channels are merely a phase in the process of transportation and deposition on the floor of such a valley. If the grade is locally out of adjustment but the grass cover prevents the easy removal of a thin sheet of alluvium over the whole floor, more sand and clay become lodged in the grass. Alluvium thus continues to be deposited even though the floor is above grade. Finally the strong currents of an exceptional flood break through the grass cover and initiate a gully which then increases headward rather easily. The overpour at the head of the gully so increases the ability of floods to erode that the gully is carried deeper than necessary and its lower end begins to fill with coarser sediment derived from the upper end. The manner in which these gullies disappear can only be inferred from the characteristics of certain ones which are very broad and low-banked at the downstream end and shallow and almost obliterated by the growth of grass at the upper end. Evidently lateral erosion in the lower and middle parts of a gully removes material lying above normal grade, and since the head of the gully stands below this grade it is, with the help of vegetation, gradually filled in.

That such discontinuous gullies existed during the alluviation of Chaco Canyon there seems no reason to doubt. The kind of sediment in the floor and the nature of rocks in the drainage area are so similar to those of the Cañada del Magre and other canyons of the Rio Puerco drainage as to preclude the possibility that dissimilar conditions existed. Cross sections of the valley fill exposed in the banks of the modern arroyo show numerous irregularly placed channel deposits from 10 to 40 feet wide and 2 to 10 feet deep that may well have been deposited in discontinuous gullies.

The existence of channels of this type adds much uncertainty to the usefulness of the accounts of travelers in dating the initiation of modern arroyo systems. Thus Simpson (1850) states that in 1849 the banks of the Rio Puerco were 10 feet high and had to be cut down to allow the passage of artillery at a point 7 miles above Cabezon. Banks of similar height at the crossing of Arroyo Torrejon (Torreones) and the Arroyo Cedro are also recorded. Yet even in 1926 the upper portions of the Cañada (en) Medio, a tributary of the Torrejon, and the Cañada de Piedra Lumbre were undissected and their flat floors were farmed by Navaho Indians. A recent investigation (Bryan, 1928a) has, however, shown that the early discontinuous channels of the Rio Puerco were relatively shallow and narrow compared to the existing arroyo which was initiated between 1885 and 1890.

These minor features of the canyon floor, though constantly changing, doubtless had a marked similarity from year to year for there is a balance between the forces involved that could only have changed gradually with the progressive fill of sediment. The experienced observer can easily predict what areas of such a canyon floor will be gently flooded and what areas will be scored and eroded by tumultuous waters. Only gently flooded areas will be suitable for floodwater farming.

The process of filling Chaco and other canyons was doubtless a slow one and for each foot of material permanently added many feet were deposited temporarily and later shifted downstream. The irregularities in bedding produced by this process preclude an estimation of the time involved in deposition by any method now known.

At various places wind is effective in shifting material and thereby adding complexity to minor features of a canyon floor. Much of the material dropped by floods adjacent to channels of greatest flow is incoherent and sandy. It is easily picked up by the wind and piled in low mounds or dunes. Such material is often in motion within an hour after recession of a flood. Nevertheless, these piles of sand are often effective in changing the courses of channels and in spreading floods. Areas so affected are small and usually are confined to three locations, here listed in the order of their relative importance: 1, Adjacent to channels; 2, on alluvial fans of tributaries carrying sandy debris; 3, outcrops of sandstone.

The work of winds in the vicinity of human habitations is notable and is described on pages 21-22.

DATE OF RECENT STREAM TRENCHING

Historical records at Chaco Canyon are meager, and it is impossible to fix precisely the beginning of its present arroyo. That this is recent and that the process is still continuing is self-evident. Many of the tributary canyons, such as Mockingbird, were yet undissected in 1925, although a falls that receded each year marked the head of their respective arroyos. Since 1925, extension of these gullies probably has destroyed the alluvial plains in the tributaries. That the main Chaco arroyo has increased since early expeditions to the canyon was recognized by Dodge (1910) and the evidence is here reviewed. Not only are its physical features recent but they resemble in every detail those of other arroyos in the Southwest whose date has been fixed with some assurance (Bryan, 1925a, 1928a). It seems reasonable to assume, therefore, that the trenching of Chaco Canyon took place at about the same time as the trenching of other valleys.

During a military expedition against the Navahos in 1849 under command of Col., afterward Gen., John M. Washington, Chaco Canyon was visited and a description of its ruins recorded by Lt. J. H. Simpson. According to this account (Simpson, 1850, p. 78), the "Rio Chaco" had, on August 27, 1849, a width of 8 feet and a depth of 1-1/2 feet at the expedition's camp near Una Vida. It is evident that this description applies to the muddy water then flowing. No mention is made of a gully although Simpson described the steep-walled arroyos of three other streams that were crossed on the way to Chaco Canyon.

Lt. C. C. Morrison (1879, p. 367) visited the Canyon in 1875 but does not mention an arroyo. Oscar Loew (1879) was there about the same time, but his description of the topography of the canyon is too vague to be of value.

William H. Jackson, whose pioneer archeological work in the Southwest is of great accuracy as to detail, spent five days in Chaco Canyon in 1877. Five or six miles above Pueblo Pintado the arroyo was so shallow that Navahos had formed "water pockets" (reservoirs) by obstructing the channel; nearer Pintado the arroyo was 10 or 12 feet deep (Jackson, 1878, p. 433). Between Pueblo Pintado and Wejegi the depth of the arroyo almost entirely cut off communication from one side of the canyon to the other. Numerous small cottonwoods grew along the bank. Near Una Vida Jackson noted that the arroyo was dry where Lt. Simpson had found running water in 1849 and explains that his own visit was made in the spring, when floods are rare, whereas Simpson was there in August when floods are more common (ibid., pp. 436-437). At Pueblo del Arroyo the arroyo was 16 feet deep and 40 to 60 feet wide, as stated and also shown by his sketch map (ibid., p. 443 and pl. 59); 250 yards below this ruin there were shallow pools of stagnant water and here Jackson camped. New grass among young willows and cottonwoods in the bed of the arroyo extended for half a mile up and down stream (ibid., p. 446). The rapidity with which this channel has developed may be judged from the fact that, at Pueblo del Arroyo where Jackson recorded a depth of 16 feet and a width of 60 feet, the arroyo is now (1925) 30 feet deep and 150 to 300 feet wide.

Mr. Judd has diligently collected local traditions on past conditions in Chaco Canyon. Jack Martin, a long-time freighter in the region, said that between 1890 and 1900 the arroyo was so shallow most freight wagons could be hauled across without doubling the teams. The favored crossing was just south of Pueblo del Arroyo and part of the dugway down the north bank is still recognizable. In 1925 Padilla, an old Navaho who lives 7 or 8 miles west of Pueblo Bonito, stated to Mr. Judd and also to me that in his boyhood the arroyo was an "arroyito" not more than breast deep (about 5 feet) and that along it grew cottonwoods and willows. Since his boyhood it has continually widened and deepened.

Later the same year Wello, a Navaho thought to be about 75 years old, told Mr. Judd that Chaco River had no channel when he was a boy; that there were cottonwood and willow trees on the flat opposite Pueblo Bonito and grass was knee high. Water was close to the surface of the ground. Padilla was present at this interview and agreed that these things were so. He may be 5 to 10 years younger than Wello and doubtless based his agreement on knowledge gained from older men, but he still insisted on the truth of his previous statement that, in his boyhood, the "arroyito" was only breast high. If we can accept 1860 as the period to which these elders referred then we have an approximate date for the beginning of the Chaco arroyo. It was 5 feet deep when Padilla was a boy; about 10 years earlier there was no channel at all, according to Wello. Simpson did not mention an arroyo because such a feature did not exist in 1849.

The original notes and maps of the townships surveyed under contract for the U. S. Land Office Survey in the early 1880's have been inspected. These records contain gross errors, and some of the township surveys appear to be entirely fictitious. Hence no useful information was obtained from them.

In attempting to determine a proper date for the beginning of arroyo cutting in Chaco Canyon, the problem is to decide whether the arroyo described by Jackson in 1877 is a portion of a through-going and complete arroyo system. It may have been merely a discontinuous arroyo with a head in the broad areas of the valley near Fajada Butte and a fan near the entrance of Escavada Wash downstream. Our information refers only to this part of the river and we have no data on conditions farther down the Chaco. It seems necessary, therefore, to assume that the arroyo of 1877 was the headward portion of a new system and that the present arroyo is its successor.

The year 1877 cannot be the beginning of this arroyo and allowance must be made for growth of the one described by Jackson. Some weight must also be given to statements of the two Navahos, Padilla and Wello. If Padilla is 70 years old he cannot remember farther back than 1860 to 1865. Wello, an older man, remembers, or remembers statements by others, that there was once no arroyo. Balancing these considerations, it seems that 1860 is as early a date as is possible since it affords 17 years for cutting the arroyo Jackson saw and is consistent with the stories of the two old Indians. The date must be considered as an arbitrary choice, however, and not very precise.

Elsewhere in the Southwest existing evidence indicates that the phenomenon of arroyo cutting began at slightly different times, stream to stream. A considerable period elapsed between initiation of a given channel and its completion throughout the length of its valley floor. The date of beginning is apparently earlier in southern Colorado, northern New Mexico, and northern Arizona than elsewhere, and may easily fall in the decade 1860 to 1870. On Rio Puerco, however, a nearby and similar stream, the date has been satisfactorily placed within the years 1885 to 1890, and in the Hopi country, arroyos were not cut until after 1900.

The effect of arroyo cutting on native vegetation and on habitability of the area affected is very considerable. In southern Arizona, meadows of coarse-top sacaton with groves of cottonwood and walnut have been drained by arroyos, and dense forests of mesquite have since sprung up. On the Rio Puerco, natural hay fields of fine-top sacaton and other grasses once naturally irrigated have dried up and the deep rooted chamiso, "wafer sage," has replaced the grasses. Areas near San Ignacio and San Francisco that were once irrigated by use of crude ditches or by natural flooding are now 30 feet above the stream bed. Since these areas can no longer be farmed, the towns have been abandoned.

Instances of similar change in native vegetation and of the abandonment of fields could be multiplied (see references given in Bryan, 1925a, 1928b). The present lack is a quantitative estimate of the decrease in vegetation and consequent lessened opportunity for man under the changed conditions. Primitive man, without domestic animals, would not suffer from decrease in forage or from loss of hay fields but, to the extent that he was dependent on floodwater farming, might have his very existence jeopardized by these changes. On the other hand entrenchment of Moenkopi Wash, near Tuba City, Ariz., has, according to Gregory (1915), increased the low water flow of the stream and thereby provided more water for irrigation during the critical period of plant growth. An old Hopi farming village here, after long abandonment, was reoccupied in 1880 and is now a thriving community of about 300 people.