GLACIAL FEATURES 307 



Proof of the postglacial origin of the volcanoes studied in this 

 connection is afforded: (1) by the relation of the flows and the 

 ash and cinder beds about the bases of the cones to the glacial 

 topography; (2) by the complete absence of glacial phenomena be- 

 low the present snowline. Ascending a marginal valley (Fig. 202), 

 one comes to its head, where two tributaries, with hanging rela- 

 tions to the main valley, come down from a maze of lesser valleys 

 and irregular slopes. Glacial features of a familiar sort are every- 

 where in evidence until we come to the valley heads. Cirques, re- 

 versed grades, lakes, and striae are on every hand. But at alti- 

 tudes above 17,200 feet, recent volcanic deposits have over large 

 areas entirely obscured the older glacial topography. The glacier 

 which occupied the valley of Fig. 202 was more than one-quarter 

 of a mile wide, the visible portion of its valley is now over six 

 miles long, but the extreme head of its left-hand tributary is so 

 concealed by volcanic material that the original length of the gla- 

 cier cannot be determined. It was at least ten miles long. From 

 this point southward to the border of the Maritime Cordillera no 

 evidence of past glaciation was observed, save at Solimana and 

 Coropuna, where slight changes in the positions of the glaciers 

 have resulted in the development of terminal moraines a little be- 

 low the present limits of the ice. 



From the wide distribution of glacial features along the north- 

 eastern border of the Maritime Cordillera and the general absence 

 of such features in the higher country farther south, it is con- 

 cluded that the last stages of volcanic activity were completed in 

 postglacial time. It is equally certain, however, that the earlier 

 and greater part of the volcanic material was ejected before 

 glaciation set in, as shown by the great depth of the canyons (over 

 5,000 feet) cut into the lava flows, as contrasted with the rela- 

 tively slight filling of coarse material which was accumulated on 

 their floors in the glacial period and is now in process of dissec- 

 tion. Physiographic studies throughout the Central Andes demon- 

 strate both the general distribution of this fill and its glacial 

 origin. 



So recent are some of the smaller peaks set upon the lava 



