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

MATERIALS OF THE FILL

The alluvial fill of Chaco Canyon consists largely of sand, yet so much of its surface is covered by a layer of dark sandy clay (locally called "adobe"), that the true character of the fill is not evident except where exposed in the arroyo. Even there the degree of sandiness can be detected only by close inspection, for rains wash mud down over the vertical walls and this, like a film of plaster, conceals the sand and gives the impression that the whole bank is made of clay. The relative quantities of sand and clay will appear from the measured sections on pages 51 to 59; in the following paragraphs, each class of material is described separately.

Small discontinuous bodies and lenses of gravel occur sparingly. The gravels contain much fine sand and mud and are thus similar to the small bodies of gravel found in the present arroyo bed. Like the latter they are obviously the deposits of tumultuous and muddy floods. The pebbles are mostly partly rounded fragments of sandstone and of limy concretions derived from the Mesaverde and Pictured Cliffs sandstone, but there are also a few water-worn pebbles secondarily derived from the Tertiary rocks. These pebbles may vary in diameter from half an inch to 2 inches, but angular stones up to 6 or 8 inches in diameter are also found.

The sands are of several types not wholly distinct in character that grade into one another even in the same bed. Some sands have almost no bedding or lamination and consist of yellowish, rusty grains such as result from disintegration of the sandstone of the canyon walls. These accumulations seem to have been deposited by outwash and are parts of ancient fans similar to those now being formed at the foot of the cliffs. Lenses of clean white sand, laminated and crossbedded, were laid down by the stream of the main canyon, just as like lenses are being deposited today. Somewhat similar but generally finer grained sands may have very minute and irregular crossbedding that is a remnant of wind-made ripples. Such beds clearly have been subject to wind action but it seems likely that the sand was first deposited by water and subsequently reworked by the wind. The very same process may be seen today when sand recently deposited in the channel by a flood is reworked by wind shortly afterward.

Gray or brown silt in beds that are minutely though irregularly laminated make up a considerable part of the alluvium. Ripple marks, current marks, and mud cracks are common on the surface of the laminae; a few rusty streaks, some parallel and some at large angles to the laminae, are doubtless the impressions of stems and roots. In many places the silt beds are 2 to 4 feet thick, but in others silt occurs as thin layers in sand or clay. Obviously it was deposited in outer portions of the channel by the main stream or in nearby parts of the flooded area.

The clay of the valley fill locally known as adobe is mostly gray or brown and contains appreciable amounts of sand and silt. A more realistic name would be "sandy or silty clay" or "clayey fine sand." The usual clay bed is from 6 inches to 3 feet thick and is nearly uniform in color and texture. Lamination is not always apparent to the unaided eye but it probably is a constant feature since it has been detected under the microscope and doubtless would be readily visible were it not for vertical jointing and a tendency to "check."

Well-laminated, less sandy clays occur in discontinuous lenses a few feet long and varying from 2 to 12 inches in thickness. These lenses appear to have been deposited by floodwater that was ponded in abandoned and shallow channels. In contrast, the more abundant sandy clay is the result of sheetlike overflows of the main stream and of its tributaries. These overflows formed flats or meadows of alkali sacaton grass. The imperfect lamination and reticulated structure are probably to be accounted for by successive drying of the mud on exposure after each overflow and by the influence of grass roots. The dark color is due to finely divided organic matter incorporated in the clay. In such flats the rate of soil formation is rapid and the reticulated (chernozem) soil structure is quickly attained.

ARRANGEMENT OF THE MATERIALS

The walls of Chaco arroyo and its various branches afford ample exposure in which the several sorts of material just described are displayed. The dominant characteristic is lack of continuity. No bed, however uniform over a short distance, extends very far, nor does a like sequence of beds occur in the wall of the arroyo at any two places.

In general clay beds are the most continuous and some of them have a length of nearly a quarter mile. Such beds rest on, and in turn are covered by, essentially parallel beds of different composition from place to place. These changes in lithology imply that there are many minor unconformities between beds. In some places sharp and definite erosion surfaces cut clay beds which are overlain by sand deposited in the channel responsible for the erosion. Less definite evidences of erosion contemporary with deposition occur, but no one of these episodes of erosion within the formation has any time value, for such irregularities are to be expected in the deposits of ephemeral streams. Each large flood forms new channels of flow and destroys in part the work of its predecessor. In such localities the measure of erosion or sedimentation is the algebraic sum of the erosion and deposition of successive floods.

In several places a cross section of a much older channel is exposed in the wall of the arroyo. These exposures generally show a round-bottomed channel, filled with crossbedded sand to a depth of 2 to 10 feet, and the lenticular character of adjacent clay beds. Such channels are evidently interformational for they are shallow, occur at all levels, and are covered by beds that are conformable with others, adjacent and contemporaneous. Evidently these channels are of the discontinuous type described on pages 13-14 and are to be distinguished from the relatively recent, buried channel discussed on pages 32-37.

DEPTH OF THE ALLUVIUM

Tangible evidence on the total depth of the valley fill is wanting in Chaco Canyon. The well at the Society's Pueblo Bonito camp was dug about 10 feet below the level of the stream bed in the arroyo, here 30 feet deep, and thus shows at least 40 feet of alluvium. About 1900 a well said to have been 350 feet deep was drilled between Richard Wetherill's old home and the southwest corner of Pueblo Bonito and was promptly abandoned because the water was brackish. No record of the material passed through is available.

FIG. 3.—Diagrammatic sketch of the valley fill between sections 5 and 16. (click on image for a PDF version)

PLATE 6. Upper: Annually since 1877 Chaco floods gnawed at this small Pueblo III ruin until the last vestige of its disappeared in 1948. Pueblo del Arroyo stands at the right, beyond the sheds. (Photograph by Charles Martin, 1920.) Lower: A pueblo I pit house with roof 6 feet below the valley surface was revealed by caving of the arroyo bank during the 1921 flood season. (Photograph by Neil M. Judd, 1922.)

PLATE 7. Upper: The buried channel as exposed at section 16. Here Late Bonitian potsherds were found at a depth of 13 feet 10 inches. (Photograph by O. C. Havens, 1925.) Lower: The author stands below a cross section of the post-Bonito channel at section 4, near the southeast corner of Pueblo del Arroyo. (Photograph by O. C. Havens, 1924.)

PLATE 8. Upper: As it did in the days of Pueblo Bonito, the Rincon del Camino comes in from the north to join the main Chaco arroyo. (Photograph by O. C. Havens, 1924.) Lower: Lateral erosion caused by drainage from The Gap. View looking north across the Chaco toward Pueblo del Arroyo in left middle distance. (Photograph by Neil M. Judd, 1929.)

PLATE 9. Upper: About half a mile west of Ruin No. 9 a disarticulated skeleton was found in the arroyo bank 6 feet below the surface. Lower: Hearth at depth of 12 feet 8 inches, section 9, at mouth of Rincon del Camino. (Photographs by O. C. Havens, 1925.)

In other, somewhat similar canyons and valleys, the alluvium is known to be relatively shallow. As recorded by Gregory (1916, p. 160), wells at Leupp, in the Little Colorado Valley, northeastern Arizona, and along Oraibi Wash, farther north, show a depth to bedrock ranging from 60 to 80 feet. In the Puerco Valley, 50 to 70 feet of alluvium is found. At Gallup, near the Arizona line in New Mexico, there is 175 feet of alluvium above bedrock.

From these data one may assume that the alluvium in Chaco Canyon probably does not exceed 100 feet in depth. Thus there is exposed only about one-third, or 30 feet, of the total. The generalizations of this paper are based on an examination of this exposed third but we may be sure an equally thorough examination of the lower and unexposed portion would reveal equally interesting and significant information.

HUMAN RELICS IN THE VALLEY FILL

During investigation of the sediments exposed in the walls of Chaco arroyo, relics of man's activity were observed repeatedly. These relics consisted mostly of hearths, charcoal, stones, bone fragments, and potsherds. Remnants of masonry structures were also found, although infrequently. Most of the hearths are not constructed fireplaces but, rather, surfaces more or less blackened or reddened by heat and overlain with charcoal. They are merely places where fires had been built and obviously represent no more than temporary camps. Some may have been used for only a day; others, to judge from the depth of the burnt ground and the amount of charcoal, could have been used for a period of weeks. Some hearths have been buried by sand or clay laid down so gently by running water that the charcoal was not eroded. They are thus good evidences (1) that the contemporary surface at that place was occupied by people during the period of alluviation and (2) that alluviation was accomplished in part by gently moving sheets of water which did not destroy previously existing surfaces.

Scattered charcoal, on the other hand, is not conclusive evidence of the presence of man since it may have been incorporated in the sediments by erosion of areas where the vegetation was burned by fires of natural origin. However, as previously pointed out, the character of the sediments indicates that the region has always been an arid one. In such a region fires of natural origin are, because of the scarcity of burnable vegetation, likely to be small in extent and thus produce small amounts of charcoal. Certainly charcoal is not common in the stream beds of the present time, either in this area or in the Southwest generally. It seems more likely, therefore, that occurrences of scattered charcoal in the Chaco fill are due to the erosion of hearths or refuse dumps. Since charcoal is soft and friable, it cannot be carried far and indicates human occupation at no great distance upstream.

Isolated stones in the fine-grained sediments occur in many places, but, however others may have reached their present positions, those found near hearths were probably brought in by man. Their presence thus tends to confirm such evidence of onetime human occupation as worked flints, stone and bone artifacts, and fragments of pottery.

Bone fragments are not positive proof of man's presence unless showing signs of human workmanship but, like scattered stones, may be confirmatory when in association with man-made objects.

Potsherds are resistant to erosion and provide durable and unquestioned evidence of man's presence in the past. They have such large surfaces in proportion to their weight that they may be carried in currents unable to transport small stones and thus often occur in fine grained deposits. Generally, however, they are associated with gravels. Some potsherds, found singly in fine-grained beds, may have been dropped on the surface and simply buried by the mud of the next flood. Deposition by floods on uneroded surfaces was doubtless common, as shown by the lack of erosion of charcoal in hearths. Potsherds also have a genuine stratigraphic value because pottery made by prehistoric peoples at different periods differs in texture and decoration and can be identified with some certainty. Thus, if collected systematically, potsherds can be used as fossils are used to identify the age of sediments.