PRESENT GEOLOGIC PROCESSES
WEATHERING AND EROSION OF THE CLIFFS
The asymmetric form of Chaco Canyon is due to its east-west course and to the prevailing dip of the rocks, as explained on pages 10-11. The processes now at work on the cliffs differ from those of the past only because of two factors: (1) The relatively recent valley fill which covers the lower part of certain cliffs; (2) the possible differences between present and past climates. The first factor can have little effect on the nature of the processes; the second affects only the rate of erosion, as the probable changes in climate do not involve a change to a strictly humid climate. When past climates were wetter than the present but still relatively arid, cliff recession was doubtless accelerated; when the climate was drier, the process was slowed down.
Chaco cliffs can be divided into two sorts in two locations: The lower division of the Cliff House sandstone generally forms vertical cliffs but these differ in detail according to whether the base of the sandstone lies below or above the level of the alluvial plain. The upper division has domelike forms and generally produces low cliffs either stepped or rounded where the base is above the alluvium. In lateral canyons the base of this upper member falls below the level of the alluvium and here crenulated and rounded cliffs occur.
Erosion of the lower part of the Cliff House sandstone is largely due to differential sapping at its base. Rainwater entering the sandstone above emerges below. If the base is above the level of the flood plain, the water emerges at the top of the friable sandstones, coal, or shale of the somewhat variable underlying Menefee formation. This material is decomposed and carried away partly by this seepage water and partly by direct rainwash. As a result the cliff is undermined and blocks break off along characteristic joint planes. Perhaps because the edge of the cliff settles and these joints are open some distance back from the face, the cliffs of these localities are less sheer than those of the same rock where the base of the sandstone is below the level of the alluvial plain, as in the north wall of the canyon near Pueblo Bonito. Here water absorbed by the overlying sandstone emerges at or near the level of the alluvial plain. It dissolves the cement of the rock and appears as an efflorescence of a white salt. The sandstone becomes friable and grains are loosened from the surface. These grains fall by gravity, especially during windstorms, or are loosened and carried off by the sheet of water that covers the face of the rock in rains. Thus cavities or niches (Bryan, 1928c) are formed like the one shown in plate 3, left. With the formation of these cavities the rock splits on its characteristic vertical joints; as loosened blocks fall, the vertical face of the cliff is renewed. Narrow slabs several hundred feet long, partly loosened, are fairly common features, and one directly back of Pueblo Bonito has excited much interest because massive masonry below it shows that the prehistoric peoples attempted to brace the slab against falling. [It actually did fall, on January 22, 1941.N. M. J.]
The upper member of the Cliff House sandstone tends to weather in domal forms. Widely spaced vertical joints more or less at right angles to each other afford points of attack for the weather and the resulting, nearly cubical blocks are then rounded on the corners. The process of sapping takes place in this sandstone also. There is, however, a relatively irregular zone of slabby and shaly sandstone below, at which the water may emerge. On the double cliffs back of Pueblo Bonito the sapping takes place over such a thick and irregular zone that the upper cliff is discontinuous and in many places is replaced by a series of benches. In canyons and rincons that enter Chaco Canyon from the north, this contact passes below the level of the alluvium and the zone of seepage emergence is more or less confined. The rounded bosses are undercut. Where temporary waterfalls cascade over the cliffs during rains, niches are formed.3
Such a niche, in Rincon del Camino, is notable because water emerges under the overhang throughout the year and in sufficient quantity to constitute a spring. Other niches may have only enough seepage to support a few green bushes or there may be merely a damp place on the rock. It requires, however, no stretch of the imagination to perceive that with a slightly greater rainfall springs would exist in these and similar situations.
The rate at which cliffs recede according to the processes just reviewed is necessarily slow. That parts of some cliffs are newer than others is attested by their bright, unstained surfaces and the lack of talus. Other parts have relatively ancient, iron-stained surfaces and have shed no fragments since completion of the alluvial plain which laps their bases. Some cliff faces are marked by carvings or pictographs; others have had holes cut in them to support the roof timbers of abutting dwellings. Often blocks at the foot of a cliff are so like the cliff face in color they must be of equal age. In general it can be said that, except for fall of a few blocks here or a mass of debris there, the Chaco cliffs are essentially the same as they were when the canyon was inhabited by prehistoric peoples.
The blocks of sandstone that fall at the foot of a cliff also slowly weather and disappear. There are relatively few such occurrences in Chaco Canyon; rarely are the blocks numerous enough to form a heap or talus. The lack of talus may usually be explained as owing to burial of all blocks formed previous to alluviation of the valley floor. Exposed blocks are mostly recent falls and some of them are so little weathered that they can easily be correlated with scars they left on the cliffs. Others are much weathered and disintegrated. The principal process of weathering appears to be the solution of cement by rain water that percolates through the blocks and emerges on the side or near the base. Numerous fantastically shaped holes are thus produced, as illustrated in plate 4, lower. The movement of water through such rocks was particularly observed on August 3 and 4, 1924. A sharp shower occurred about 2 p.m. August 3 in which 0.14 inch of rain fell. After the shower several rocks of this type were examined. The exposed portions were wet, and part of the dust under the larger overhang was eroded by the splash of falling rain, but the cavities were dry. Next morning parts wet the day before were dry but the cavities were damp, and some were almost wet. Evidently within 12 hours water absorbed at the top had percolated through the rock. In rainstorms of greater duration water must pour out of such cavities in considerable volume and carry with it sand grains resulting from previous solution of the cement.
In various places on the cliffs, especially on the bench between the upper and lower sandstones, there are patches of windblown sand. This sand, evidently derived from disintegration of the nearby sandstone, accumulates in places more or less sheltered from the wind. Some heaps are fixed in position by the growth of grass. That such heaps accumulate is proof that a much greater quantity of sand is moved by the wind and either blown off the cliff altogether or into position where it is carried away by rainwater. How much the movement of this sand scours the rock is difficult to evaluate. It seems likely that the scouring effect is small, for the rock at the surface is soft and crumbly. At the mouth of Escavada Wash, where the Farmington road left the Chaco prior to 1920, the wind has heaped up sand out of the wash to form a group of dunes that encroaches on cliffs similar to those upcanyon (Bryan, 1928c). Here the sandstone has the same domes, niches, pinnacles, and other characteristic details, yet the surface is hard and firm (pl. 4, upper). Tiny iron concretions stand above the surface like collar buttons. The surface is continuously scoured by sand and is in marked contrast, by reason of its firm texture, to the soft and crumbly surfaces seen elsewhere. Nevertheless, the total erosion is obviously less than it seems, for otherwise the domes, niches, etc., would disappear and be replaced by new details equally dependent for their shapes on the process of wind scour.
Near Pueblo Bonito, dust is easily raised on any windy day and windblown sand accumulates, as it has in the past, in every sheltered nook and cranny. Between the ruin and the cliff sand had collected to a depth of 4 to 6 feet between 1900, when the Hyde Exploring Expedition concluded its excavations, and 1921, when the National Geographic Society inaugurated its researches. Similarly, windblown sand near other ruins of the region evidences more wind work than appears at places of otherwise similar location.
These conditions are easily understood when the activity of man and his domestic animals is considered. Near his habitations man and his animals continually disturb the surface soil and thus make the work of wind easy. In addition the soil is made pulverant by abnormal quantities of organic matter consisting of the excrement of men and animals, the debris of crops gathered and brought in, the refuse of building materials and fuel, and litter of all sorts swept up and carried out of houses. These organic substances added to the surface soil serve to make it friable in the same way that manure improves the tilth of a field. The soil, when wet, is no longer a gummy mass that becomes pavement-hard on drying. It is less muddy when wet, and when dry is loose and friable and thus easily picked up and transported by the wind.
With only dogs and turkeys to help in the processes just described, prehistoric peoples probably did not create as much dusty ground as the same number would today. Yet their refuse mounds testify to an enormous quantity of rubbish discarded systematically and enable one to picture the proportion of such litter that must have been scattered about the inhabited areas. The quantity of windblown sand and wind-reworked material found buried in the refuse heaps of Pueblo Bonito evidences a considerable amount of wind work in prehistoric time that, on the grounds set forth above, may be considered as influenced by the habits of prehistoric man.
At the foot of the cliffs, and particularly at every indentation in them, alluvial fans are now forming. These are composed largely of buff and yellow sand derived from disintegration of the sandstone of the cliffs and talus blocks. During every rain, sheets and streams of water pour over the cliffs; the largest flows naturally occur at the indentations. Alluvial fans deposited by these streams are more or less proportional in size to the indentation and, by inference, to the area of surface drained. In a few places the sand of alluvial fans is picked up by the winds and formed into low heaps, but, as was pointed out above, currently there is more wind-moved material in and around the various ruins than at other places in Chaco Canyon.
The soil of these alluvial fans is loose and sandy and doubtless formed the best agricultural land in prehistoric time. During and after rains, waters that pour from recesses in the cliff could easily be directed over the fans. The problem of directing and spreading such runoff so it will wet without gullying the land is a difficult one which must have taxed the ingenuity of the prehistoric farmer. It is possible, but by no means certain, that part of these floodwaters were directed over the more clayey areas in the middle of the valley not only for the purpose of irrigation but also to mix water-borne sand with the less tractable clay.
Adobe flats still constitute a large part of the floor of Chaco Canyon and are added to each season by storm waters draining from adjacent areas. They are, however, no longer built up by the marginal waters of main stream floods or by deposition in temporary lakes. Progressive cutting of the arroyo has left these flats mere relics of past conditions, but the method by which they were formed can be interpreted from observations in such undissected tributaries as Mockingbird Canyon.
EROSION IN THE ARROYO
The initiation of dissection and the formation of arroyos has already been discussed (pp. 12-13). The present (1925) arroyo varies in width from 150 to 500 feet, and in depth from 10 feet near Escavada Wash to 30 feet at Pueblo Bonito. Upstream, however, the height of the bank again decreases to about 20 feet near Wejegi.
The vertical walls are formed by undermining the alluvium which then breaks off in blocks parallel to an obscure jointing. Undermining is largely due to the lateral cutting of floods in swirls and eddies on the outside of bends. It seems probable, however, that water absorbed in the alluvial plain seeps into the arroyo at the base of these banks, softening and helping to undermine them. The rate of such lateral cutting is rapid, and significant changes may occur in a single year (pl. 5, upper and lower).
Last Updated: --2008