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

This Chaco Canyon study was begun as an isolated project. It was an attempt to relate recent geology to the life of prehistoric peoples in the area. The results proved so successful, however, that other studies were subsequently undertaken. The alternate periods of alluviation and erosion discovered in Chaco Canyon and related to the tree-ring dates of Douglass (1935) have been found in other localities. The periods of alluviation are, so far as evidence now exists, nearly synchronous over the whole Southwest. Thus there has been developed an alluvial chronology still imperfect but valuable as a measure of time in the dating of archeological events. It is presumably still more valuable as a measure of alternating periods favorable or unfavorable to floodwater farming, an important method of agriculture in the area. Still more important are the inferences on fluctuations in climate parallel with alternations in the regime of streams.

The report begins with a general consideration of the area and its climate, with such information as is available on the age of the present arroyo in Chaco Canyon, with rather detailed studies of geologic processes now current there, and a description of the alluvium of the valley floor. It then presents evidence that this alluvium is divisible into three parts: the terrace, the main valley fill, and the post-Bonito channel. The antiquity of these divisions and their correlation with similar alluvial formations elsewhere are also considered. The importance of floodwater farming in the Southwest and the effect of the recent epicycle of erosion on this type of agriculture are next set forth.

The cause of the alternation from alluviation to erosion in southwestern valleys is next discussed and the argument advanced that simultaneous alternations in the regimes of widely separated streams must be due to synchronous climatic changes. The concurrent effects of climatic change and change in stream regime throughout the known human history of the Southwest affords a clue to fluctuations in human culture otherwise unattainable.