HENDERSON] GEOLOGY AND TOPOGRAPHY 33 



means very slowly from the standpoint of human histor>\ At th(^ 

 old pueblo in the open part of the Frijoles canyon down-cutting has 

 been almost at a standstill since the pueblo was built, possibly a 

 thousand years or more. The grade of the stream here is gentle and 

 the small volume of water has probably been so overloaded with 

 debris swept in from the walls of the canyon as to be able to do no 

 more than care for the load. A corner of the pueblo has been at 

 some time destroyed. This may have been done by a flood, by 

 undercutting in the ordinary meandering of the stream, which has 

 swung across the valley more than once, or by recent Mexican inhabi- 

 tants in the construction of a ditch along the edge of the terrace. 

 Half a mile or more above the pueblo, stream terraces and trees 

 furnish definite evidence on this subject, their positions on the banks 

 of the creek showing clearly that at most the channel could not have 

 been deepened more than five or six feet (probably less) since the 

 sprouting of the rock pmes, which are now 30 inches in diameter 

 and probably at least 300 or 400 years old. At that rate the stream 

 would have taken 30,000 to 50,000 years to cut the canyon. It is 

 quite likely, however, that cuttmg has been somewhat more rapid 

 (hiring part of the past, as it seems almost certahi that the volume 

 of water was larger during the time of the great lakes and glaciers 

 of the Southwest and the West. This is more fully discussed in the 

 accompanying report on climatology. 



Wind, frost, and the beating of rams have also taken a prominent 

 part in shaping the canyon. Without these forces the stream would 

 have cut a much narrower canyon and would have performed the 

 task in much less time. As soon as the stream made an incision in 

 the tufa, rains began to beat upon the banks of the gully, crumbling 

 the tufa, much of which is so friable that it may be picked to pieces 

 with the fingers. After one terrific shower the writer found a quarter 

 of an inch of debris from the cliff in a basin so placed as to catch the 

 water draming from a small projection. This process has slowly 

 widened the canyon and the resulting debris has hampered the stream 

 in its downward cutting by forcing it to take care of much more 

 material than that which it tore from the bed of the creek. Storm- 

 water, getting into the crevices and interstices of the tufa, has doubt- 

 less frozen and in its expansion has pried off fragments of the material. 

 Winds also have operated not only by carrying away the dust and 

 small particles of tufa loosened by frost and rain but even by pluck- 

 ing small fragments out of the walls of the canyon. It is not unusual 

 to see the wind blowing clouds of dust from the cliffs, and during a 

 brisk wind one may hear pebbles dropping constantly along the 

 canyon walls. Much of the fine material which fills the ancient ruins 

 is windblown dust. The small cavities which honeycomb the cliffs 

 have been formed by the wmd, which easily excavates the yielding 

 67519°— Bull. 54—13 3 



