IX. ALLUVIATION AND AGRICULTURE^ 



That portion of Chaco Canyon in which we are primarily inter- 

 ested is a 15-mile-long section walled by massive beds of Cliff House 

 sandstone. Shales lie above and below the Cliff House, and the 

 disintegration of sandstone and shale has filled the canyon with clay 

 and sand to an estimated depth of 100 feet or more. Of this 

 valley fill approximately one-third lies exposed in the banks of the 

 present arroyo — a flood-carved gully that had its beginnings in the 

 middle 19th century (Bryan, 1954, p. 15). This exposed third pro- 

 vides an index to past geological conditions, but nowhere is there 

 evidence, at any time in the past, of a living stream in Chaco Canyon. 

 The entire valley fill, 100 feet of it, has been piled up, inch by inch, 

 by wind and rainwash. 



In consequence of his investigations Bryan concluded that the 

 present arroyo is part of a fourth cycle of alternating erosion and 

 sedimentation. We are not especially concerned here with the origin 

 of the canyon itself nor with the processes by which it was initially 

 filled with alluvium, then eroded again, and subsequently refilled. 

 But the arroyo identified with the third period of erosion may not be 

 ignored since it provides a very plausible reason for the abandon- 

 ment of Pueblo Bonito and other communal dwellings along its 

 course. 



The inhabitants of these multiple-storied settlements were farmers. 

 They cultivated fields of corn, beans, and pumpkins wherever soils 

 were suitable and moisture adequate. Agriculture was their liveli- 

 hood. The builders of the great Chaco Canyon pueblos were flood- 

 water farmers because they had no choice. Lacking a permanent 

 stream for irrigation purposes they necessarily depended upon the 

 runoff following midsummer showers. 



But summertime showers in Chaco Canyon are neither reliable nor 

 predictable. They may advance promisingly to within half a mile, 



^ The present chapter is based in large part upon a study of Chaco Canyon 

 geology made for the Pueblo Bonito Expeditions in 1924 and 1925 by the late 

 Dr. Kirk Bryan. Although that study was never carried to the point originally 

 planned, such data as were then available appeared posthumously in 1954 as 

 the second of the Expedition's scientific reports. Therein, on Anderson's 1922 

 topographic map of the canyon, Bryan illustrates the relationship of the present 

 arroyo to its 12th-century predecessor, his "buried" or "post-Bonito" channel. 



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