148 STUDIES IN GEOLOGY, No. 4 



(Figure I). The trail descends to the northeast down the 

 valley of the Rio Palca, which river eventually reaches the 

 Amazon via the Mamore and the Madeira. Slightly over a 

 mile below Palca the trail leaves the valley and ascends to 

 the pampa of Pisllypampa, which owes its origin to a sym- 

 metrical terminal moraine that blocked the outlet of the valley 

 and formed the basin, in fact this whole region is beautifully 

 glaciated above 11,000 feet. 



Pisllypampa is about two miles wide by three miles in 

 length, rising rather rapidly to the bordering peaks, espec- 

 ially to the southeast. It lies at an altitude of 11,800 feet, 

 and is about 12 miles northeast of Cochabamba in an air- 

 line, although it is just twice that distance by the trail, and 

 is a hard day's journey riding up, although only about half 

 as long returning. 



The country rock is a yellowish sandstone, probably of 

 Silurian age. East of the pampa it dips steeply to the north- 

 east. West of the pampa it dips moderately to the south- 

 west, and in the Palca valley just south of Palca it is nearly 

 horizontal. The Tertiary outcrops in a limited area along a 

 small quebrada in the western part of the pampa, presum- 

 ably abutting against the Silurian bedrock within a quarter 

 of a mile. It strikes North 30 degrees West, and dips to the 

 southwest from 18 to 25 degrees. There are no traces of 

 Tertiary deposits along the more easterly and larger stream. 



It seems obvious that the Tertiary deposits were once 

 more extensive, and represent wind blown, flood plain and 

 small lake deposits, the major portion of which were subse- 

 quently removed by glacial erosion. If then the contained 

 flora can furnish consistent evidence regarding the age of 

 the deposits and the physical conditions at the time of 

 entombment, it will give a conclusive answer to the ques- 

 tion as to when the uplift occurred and its probable amount. 

 Before discussing this evidence it would be well to describe 

 the deposits somewhat more fully. 



