NO. I ARCHITECTURE OF PUEBLO BONITO — JUDD 233 



CLIMATIC CHANGE 



Bryan was not alone in believing that the climate of Chaco Canyon 

 had changed toward the less humid sometime after the Bonitians 

 settled there. Presumably these agricultural folk were drawn to the 

 valley by its broad, flat acres, by an abundance of water just beneath 

 the surface, and by pine forests on bordering mesas. But they soon 

 exterminated the forests and they eventually learned that the flat acres 

 of the valley were altogether unproductive. They may have noted 

 also that floodwaters bearing upcanyon silt did not always arrive 

 when expected and that crops failed for lack of moisture and replen- 

 ished soil. 



It was possibly a period of subnormal precipitation at the end of 

 the 11th century rather than destruction of the Chaco forests that 

 initiated the erosion culminating in the third-cycle arroyo, Bryan's 

 "post-Bonito channel." Yet annual growth rings in many construc- 

 tional timbers from Pueblo Bonito are so uniform in thickness as to 

 indicate the pines had grown where moisture was fairly constant 

 year after year. This fact, plus the quantities of tule or bulrushes 

 {Scirpus actutus Muhl.) and coarse grasses utilized with those tim- 

 bers, led Bryan to the conviction that rainfall in Chaco Canyon for- 

 merly exceeded his 1925 10-inch estimate. He offered no geological 

 support for this conviction, but the botanical evidence alone led 

 N. H. Darton, another geologist and a guest of the Expedition 

 at its 1922 s)miposium, to share Bryan's belief in a climatic shift 

 from the dry toward the less dry. Bulrushes grow in wet places. 



Previously and for different reasons the theory of climatic change 

 throughout the Southwest had been advanced by Hewett, Henderson, 

 and Robbins (1913), Huntington (1914), Gregory (1916), and 

 others. But, as emphasized by Kidder (1924, p. 54), these advo- 

 cates did not always consider the fact that an Indian requires far 

 less potable water than a white man and is content to irrigate his 

 fields with the runoff following summertime showers. Together, 

 Darton and Bryan believed that as little as one additional inch of 

 precipitation annually would go far toward restoring the habitable- 

 ness of Chaco Canyon. Together, black alkali and the 12th-century 

 arroyo brought an end to local agriculture ; floodwaters subsequently 

 refilled that arroyo and buried the old Bonitian farmlands under an 

 additional 2-5 feet of upcanyon alluvium. 



