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

The nature of the sedimentary deposits comprising the main fill of Chaco Canyon is clearly revealed in the walls of the present arroyo. Here 30 feet and more of successive strata have been laid bare and one has only to read the story they tell.

Before discussing the significance of these exposures and the methods used, some of the difficulties we encountered will be mentioned for the benefit of future workers. Study of arroyo banks involves the tiresome traversing of the stream bed, which is soft and sandy when dry and may be dangerously boggy just after a flood. The banks are a monotonous brown and in many places are covered with a film of mud washed down from above.

Like a coat of calcimine over a fresco, this film obscures the characteristics of the materials of the bank. The freshest exposures are usually on the outside of bends for here the stream cuts laterally with greatest activity. On the inside of bends, temporary bars or heaps of windblown sand accumulate and may cover the bank to half its height. Frequently there has been left adhering to the bank a remnant of such a bar formed at some past time in the process of cutting the arroyo. Occasionally these remnants form little terraces 3 to 20 feet wide and 4 to 6 feet lower than the top of the bank. They consist of Chaco River deposits similar to the rest of the valley fill and are so deceptive in appearance we came to call them "false banks." Sometimes the "false bank" contains sheep dung and recency of deposition is thus definitely established. Elsewhere one must rely upon the form of the bank or some local difference in bedding, in color, or in grain of the clays and sands to distinguish the false from the true bank.

In full sunlight the relatively small variations in color and texture of beds are almost invisible. Therefore each locality was scrutinized at least twice, once in the morning and once in the afternoon so that, as nearly as possible, every part of the exposure could be seen in the shade.

Detailed descriptions of the valley fill at each of the 23 sections examined would merely weary the reader. A number of such descriptions are offered hereinafter (pp. 51-58) for those interested primarily in the geological aspect of this study, but our present purpose will be served by a single example. Typical conditions are shown at a locality on the south bank of Chaco River, opposite Ruin No. 8 and approximately half a mile west of Pueblo Bonito (fig. 1). Our diagram (fig. 3) was constructed by measuring the beds on verticals spaced 10 feet apart and by sketching the form of the several bodies of material in the intervals.

On the left are shown the irregularly continuous beds of the main valley fill at section No. 5. A conspicuous feature here is a fireplace of nearly vertical stone slabs built when the surface was 5 feet lower than it is now (pl. 3, right). Archeologists consider this type of fireplace characteristic of the period of settlement known as Pueblo III. Nearby, both in the bank and on the surface, are potsherds of that age. A few feet to the west, however, and at a depth of 6 feet 3 inches, sherds of indeterminate age were found. A second hearth at a depth of 12 feet 8 inches, and charcoal at 16 feet 3 inches, are doubtless records of the presence of people earlier than Pueblo III. Near vertical 180 a potsherd of distinctive type attributable to pre-Pueblo (P. I) times was found in a loose block of earth. This block retained the shape of the bank so perfectly there is no doubt the sherd came from a depth of 11 feet 3 inches.

At the right of vertical 180 is represented the coarse, sandy filling of an ancient arroyo at section 16 (pl. 7, upper). In the gravel lenses of this old channel and at a maximum depth of 13 feet 10 inches, potsherds of the type characteristic of the latest phases of Pueblo Bonito were found. The sherd collection here was comparatively large because the gravel lenses could be followed for 50 feet at right angles to the section in the lateral tributary of the modern arroyo.

Generally speaking, material within this ancient arroyo is sandier and more crossbedded than that of the main valley fill. In other sections examined the laminations dip from both sides of the channel toward the middle and at or near the bottom there are one or more lenses of gravel. These lenses are, on the average, 3 to 12 inches thick and 2 to 4 feet long. The gravel is clayey and dirty and similar to gravel beds of the valley fill or those in the present stream bed. However, potsherds are relatively plentiful and most frequently of Pueblo III type. The unusual number of these late potsherds is in itself significant and, so far as my experience goes, an infallible indicator of the presence of the buried channel. Connecting channels of contemporary age are commonly filled with coarser material similar to that in the present tributary arroyos.

Typical conditions are thus recorded in the main valley fill between sections 5 and 16. The current arroyo has exposed its predecessor and bared evidences of human occupation in times past. To depths of 4 feet and more, rarely 6 feet, potsherds of Pueblo III type are fairly common. Below this horizon the sherds are of definitely earlier types or of indeterminate age. Here, as elsewhere, we detect no physical division between the lower and upper sedimentary beds, merely a separation of the early and late pottery fragments found in them.