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

Chaco River, a river only during occasional floods, entrenched itself at some past time, doubtless Pleistocene, in a broad plain that then existed in the San Juan Basin of northwestern New Mexico. In the nearly horizontal sandstones and shales that underlie San Juan Basin, the Chaco River flows, when it flows at all, alternately in broad valleys and narrow canyons. To one of these latter the name Chaco Canyon is applied almost exclusively, and here, in a stretch of about 12 miles, there are numerous ruins of prehistoric villages, the largest of which is Pueblo Bonito.

Chaco Canyon, after its excavation, was partly refilled with sand and silt during a period of alluviation common to most streams of the southwestern United States. On the flat floor of the canyon, resulting from this alluviation, the prehistoric peoples lived and left evidence of their long-time occupation in hearths, scattered charcoal, potsherds, and other relics. These remains extend to a depth of 21 feet below the present surface of the alluvium. An ancient type of dwelling known as a pit house has been found at a depth of 6 feet below the surface, but the typical Pueblo III type of construction has not been surely identified below 4 feet.

Alluviation in Chaco Canyon and generally throughout the Southwest has more recently been interrupted by the formation of an arroyo or steep-sided gully in which the floods of the stream are now wholly confined. The Chaco Canyon arroyo is presently 20 to 30 feet deep and from 150 to 400 feet wide, yet a military expedition of 1849 did not mention the gully, if it then existed. In 1877 an arroyo 16 feet deep and 60 to 100 feet wide was reported. Available evidence indicates that the arroyos of other streams were mostly formed in the decade 1880 to 1890 and that the process is still going on. The beginning of the Chaco arroyo appears to have been somewhat earlier and the date may, with some assurance, be placed in the decade 1860 to 1870.

A study of the deposits that make up the valley fill indicates that Chaco River never had a permanent low-water flow. No signs of irrigation ditches or other diversions of flowing water have been found in the alluvial deposits. It seems probable, therefore, that the prehistoric inhabitants of the canyon practiced floodwater farming, a form of agriculture still in use in the region. For this type of farming wide-spreading floods are a prerequisite, and the beginning of erosion, with formation of an arroyo that confines the floods and lowers the water table, puts an end to agriculture of this type.

The main body of the valley fill is of unknown depth. Only the upper 30 feet is exposed and of this the uppermost 21 feet contains relics of man. Pottery made by the ancient people varies in texture and design according to locality and age. Differences between the kinds of pottery typical of different stages in human culture are not wholly known, nor has a definite chronology of the stages been determined, but broad distinctions can be made between the older and younger civilizations.

Collections of potsherds can therefore be used as fossils in studying the stratigraphy of the valley alluvium. Generally, only a few potsherds are found at any one place and many of these are indeterminate, hence of no diagnostic value. Somewhat meager collections of sherds from depths of 6 to 21 feet have been examined by the expedition's archeologists who identify them as mostly a coarse ware characteristic of the Pit House culture. On the basis of these fragments, therefore, we may draw the inference that people of the Pit House period were the principal inhabitants of Chaco Canyon during the time required for deposition of those 15 feet of alluvium.

Potsherds collected from the zone of valley fill less than 6 feet below the surface are generally of Pueblo III type. This fact, together with ruins whose foundations are partly buried in alluvium, indicate that Pueblo III people occupied the valley during the period represented by the last 6 feet of alluviation.

In the bank of the arroyo near Pueblo del Arroyo there is exposed a buried channel which extends to a depth of 15 feet below the present surface. At this point the channel is a well-defined ancient arroyo that had been refilled and then buried under an additional 2 feet of sediment in the interval between abandonment of Pueblo Bonito and American Army penetration of Chaco Canyon in 1849. Potsherds removed from the gravel lenses of that buried channel included fragments of the latest Pueblo Bonito types. The channel, therefore, must have been refilled late in the occupancy of Pueblo Bonito or after its abandonment.

By means of test pits in which similar pottery was found, we traced this buried channel for about 1,000 feet across the plain fronting Pueblo Bonito and later discovered remnants of it both up and down the canyon. This buried channel clearly represents a period of dissection and arroyo formation for the full length of the valley and, assuming that the dissection occurred late in the occupancy of Pueblo Bonito, an adequate cause exists for abandonment of the canyon by aboriginal farmers whose floodwater fields were destroyed by confinement of the floods within this channel, and by concurrent events.

Our examination of the main valley fill suggests alternate dissection and alluviation of Chaco Canyon: three periods of dissection and two of alluviation. If this alternation represents a true cycle, we may expect the present arroyo to run its course and then be refilled and perhaps covered over. However plausible it may be to attribute formation of the present arroyo to destruction of the vegetative cover by overgrazing, the previous dissection and subsequent alluviation were in no way affected by domestic animals. It seems probable, therefore, that the ultimate cause of this periodic change in the regime of streams is climatic. A slightly increased rainfall would increase the vegetative cover and thereby both reduce the violence of floods and protect the soil from erosion. Any decrease in rainfall would produce a reversed effect. Although the deposits of Chaco Canyon contain no definite evidence of a more humid climate during the two periods of their deposition, it seems likely that an increased humidity did exist and was a factor in development of the distinctive Chaco culture. The subsequent change to more arid conditions was doubtless of less effect until it culminated information of the twelfth-century arroyo that unexpectedly became a dominant feature of this study.