46 II. E. Gregory — A Geologic Reconnaissance 



Pampa occupy smooth-floored, steep-sided depressions with the 

 flat longitudinal profiles characteristic of glaciated valleys. 

 Many of the notches in the southern highland rim are cols on 

 which ice tongues leading toward the Cuzco Basin united with 

 those directed toward the Apurimac. Ground moraine, largely 

 covered with debris washed from the valley sides, is exposed 

 here and there along certain streams which make their way 

 through swamps and gravelly flats. Below elevations of about 

 13,000 feet the glacial features disappear; the flat valley 

 floor is replaced by a deeply trenched ravine whose marginal 

 walls are intricately dissected. On interstream areas between 

 13,000 and 13,500 feet imperfectly drained flats and stretches 

 of swamp occur and incipient moraines border the highest 

 knobs — suggesting that ice or at least neve formed a continuous 

 sheet at these elevations. 



Glacial Gravels. 



Distribution and character. — The swing of the climatic pen- 

 dulum which introduced the ice age appears also to have 

 initiated a period of increased rainfall. Streams were thereby 

 increased in volume and their erosive power was greatly 

 augmented. The significance of this change of stream habit is 

 readily appreciated when we watch the action of streams fol- 

 lowing heavy showers and realize the havoc that would be 

 wrought even to-day by maintaining at flood stage the streams 

 on the steeply inclined slopes of the Cuzco Valley. The oppor- 

 tunity for collecting waste from the highlands is much less 

 favorable now than formerly, because large areas of the valley 

 sides consist of firm or slightly disintegrated rock. At the close 

 of the Pliocene epoch the slopes were probably deeply coated 

 with waste which had been accumulating for ages, and the 

 enlarged streams of early Pleistocene time found vast quan- 

 tities of material ready for transportation. The thoroughness 

 with which these streams performed their task of removing 

 superficial material is indicated by the wide belt of practically 

 bare rock now displayed along both sides of the valley axis. 



The materials stripped from rock on the upper slopes was 

 transferred by streams of glacial time to the floor of the Cuzco 

 Valley and to its bordering lower slopes. The amount car- 

 ried into the Cuzco Basin and added to the bottom deposits of 

 Lake Morkill is unknown, for later erosion has removed much 

 of the valley fill and cut deeply into the lacustrine sediments. 

 The deposits on the lower slopes, however, are exposed for 



