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 geology of Chaco Canyon in relation to its prehistoric inhabitants was a subject that greatly interested Kirk Bryan. Born and schooled in New Mexico, he had seen hundreds of ruined Pueblo villages, mostly abandoned before advent of the Spaniards in 1540, and had given much thought to the reasons behind their desertion. Warfare may have been one cause but it obviously was not the only one.

A geologist with the United States Geological Survey and engaged primarily in a study of groundwater resources of the Southwest, Dr. Bryan seemed to us especially qualified to seek out the factors that had invited, and then repelled, colonization of Chaco Canyon in the days of Pueblo Bonito. He accepted with enthusiasm our invitation to undertake this study but was able to devote only two brief vacation periods to field work, in the midsummers of 1924 and 1925. His observations in Chaco Canyon, admittedly incomplete, prompted like inquiries in other valleys during the decade that followed.

In 1926 Dr. Bryan left the Geological Survey to accept a call from Harvard University, and thereafter academic commitments and summers in the field allowed him but little leisure. In consequence, he never found an opportunity to finish this report on his Chaco Canyon researches. A first draft, dated March 1925, and written before his second visit to the canyon, was repeatedly revised and expanded as his continuing investigations annually provided new data. He appears to have made no change in the text after 1940. For these several reasons some sections of the report lack references to the more recent literature.

Following Dr. Bryan's untimely death in the summer of 1950, his unfinished manuscript was forwarded to me by Mrs. Bryan. I have undertaken to arrange its several parts in conformity with his original table of contents and to eliminate repetitions of subject matter and phraseology. The various stratigraphic columns Bryan examined and the course he plotted for an arroyo more or less contemporary with the decline of Pueblo Bonito are shown on the accompanying map of Chaco Canyon. Stratigraphic sections 10 to 23 were studied in 1925, but we have descriptions for numbers 15 and 17 only, and a third, without number but adequately located in relation to the expedition's camp.

Test pit No. 3, about midway between camp and the west refuse mound, was among those I had caused to be dug in 1922 in connection with an analysis of Chaco Canyon soils. When it was deepened three years later at Bryan's request and was found to penetrate the buried channel he was then trying to isolate, a common impulse was to extend the exploratory trench we had previously dug through the west refuse mound and thus reveal the original surface between buried channel and the old village dump. Pit No. 4, dug expressly for Dr. Bryan, was so named because of its proximity to his section 4, where the buried channel stood exposed near the southeast corner of Pueblo del Arroyo. Thus test pits 3 and 4 and the extended west-mound trench enabled Bryan to plot the course of that prehistoric arroyo as it passed Pueblo Bonito, and led to his search for traces of it as far east as Pueblo Wejegi. The extent of this ancient channel, together with evidence of alternating periods of erosion and sedimentation, formed the basis for Bryan's growing conviction that a slight change in climate was the most likely cause for disruption and dispersal of the Chaco Canyon population in the early twelfth century. His conclusion is certain to exert a profound influence upon future interpretation of past history in the Southwest.

I gladly acknowledge our obligation to Mrs. Kirk Bryan and to two of Dr. Bryan's former students, Dr. John T. Hack and Dr. Luna B. Leopold, both of the United States Geological Survey, for their cooperation in the preparation of this report. Two members of my Pueblo Bonito staff, O. C. Havens and Lynn C. Hammond, and several of our Zuñi workmen assisted Dr. Bryan in Chaco Canyon. The illustrations are mostly from photographs by Mr. Havens.

It was originally intended that this paper appear as fourth in the series reporting the results of the National Geographic Society's Pueblo Bonito Expeditions. But the series was discontinued after the first number, "Dating Pueblo Bonito and Other Ruins of the Southwest," by Dr. A. E. Douglass (1935). Early in 1953 the Society made the present manuscript available to the Smithsonian Institution, which proposed to publish it under the Charles D. and Mary Vaux Walcott Research Fund.

The life and achievements of Dr. Kirk Bryan are briefly reviewed by Frederick Johnson in American Antiquity, vol. 13, No. 3, p. 253, January 1951.

NEIL M. JUDD.
Leader of the National Geographic
Society's Pueblo Bonito Expeditions.

Washington, D.C.
June 1953.

PLATE 1. Pueblo Bonito from the north cliff. The east and west refuse mounds like close beyond the ruin. At the left, the embankment of Wetherill's reservoir; at the right, midway between ruin and camp, a ring of dirt makers pit No. 3. (Photograph by O. C. Havens, 1924.)