PHYSIOGRAPHIC STUDIES IN THE SAN JUAN DISTRICT 

 OF COLORADO 1 



WALLACE W. ATWOOD 



The University of Chicago 



The studies during the past field season were carried on near the 

 southern and southwestern margin of the San Juan Mountains 

 and over the adjoining plateau district. Investigations were 

 planned for the purpose of working out the complete physiographic 

 history of the district. The courses of the Pleistocene glaciers were 

 indicated on the maps and the deposits left by those glaciers 

 differentiated. In connection with these studies it was possible 

 to differentiate the moraines of two distinct glacial epochs in each 

 of the large canyons examined. Beyond the terminal moraines of 

 each epoch and extending for many miles down stream, terrace 

 remnants of valley trains were recognized. It was evident from 

 the position of the younger glacial moraines and younger outwash 

 valley trains that there had been a notable amount of valley 

 deepening in hard rocks during the interglacial epoch. This 

 suggests that the mountain area had been elevated by at least 

 several hundred feet relative to sea-level during the Pleistocene 

 period. The glacial features on the south slope of the range did 

 not differ from glacial features which have been fully described 

 by various writers who have become familiar with glacial phe- 

 nomena in the high mountains of the West. 



In examining the areas which rose above the upper limit of ice 

 action on the south slopes of the mountains, certain gravel-strewn 

 surfaces were found. The gravels were beautifully polished and 

 of very resistant material. They were composed chiefly of 

 quartzite, quartz, red jasper, flint, cherts, and greenstones. Much 

 of the material was less than half an inch in diameter, but some 

 of the pebbles ranged between one and two inches in their longer 



1 Published with the permission of the Director of the U.S. Geological Survey. 



449 



