PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT 269 



in the valleys, and (3) lacustrine deposits formed on the floors of 

 temporary lakes in inclosed basins. Among these the most varia- 

 ble in form and composition are the true glacier-laid deposits at 

 the valley heads. The most extensive are the fluvial deposits ac- 

 cumulated as valley fill throughout the entire Andean realm. 

 Though important enough in some respects the lacustrine deposits 

 are of small extent and of rather local significance. Practically 

 none of them fall within the field of the present expedition ; hence 

 we shall describe only the first two classes. 



The most important glacial deposits were accumulated in the 

 eastern part of the Andes as a result of greater precipitation, a 

 lower snowline, and catchment basins of larger area. In the 

 Cordillera Vilcapampa glaciers once existed up to twelve and fif- 

 teen miles in length, and those several miles long were numerous 

 both here and throughout the higher portions of the entire Cordil- 

 lera, save in the belt of most intense volcanic action, which coin- 

 cides with the driest part of the Andes, where the glaciers were 

 either very short or wanting altogether. 



Since vigorous glacial action results in general in the cleaning 

 out of the valley heads, no deposits of consequence occur in these 

 locations. Down valley, however, glacial deposits occur in the 

 form of terminal moraines of recession and ground moraines. 

 The general nature of these deposits is now so well known that 

 detailed description seems quite unnecessary except in the case 

 of unusual features. 



It is noteworthy that the moraines decrease in size up valley 

 since each valley had been largely cleaned out by ice action before 

 the retreat of the glacier began. Each lowermost terminal 

 moraine is fronted by a great mass of unsorted coarse bowldery 

 material forming a fill in places several hundred feet thick, as be- 

 low Choquetira and in the Vilcapampa Valley between Vilca- 

 bamba and Puquiura. This bowldery fill is quite distinct from the 

 long, gently inclined, and stratified valley train below it, or the 

 marked ridge-like moraine above it. It is in places a good half 

 mile in length. Its origin is believed to be due to an overriding 

 action beyond the last terminal moraine at a time when the ice 



