of the Cuzco Valley, Peru. 37 



down-sunken rock basin was drained away and how much its 

 water was shallowed by accumulations on its floor are not 

 indicated clearly by existing geologic features. The narrow 

 gorge at Angostura, still partly filled with Pleistocene gravels, 

 was cut to nearly its present depth during preglacial time; 

 the elevation of its floor, 10,400 feet, is sufficiently low to have 

 permitted the drainage of all ponded water within the Cuzco 

 Basin, provided the present relative altitudes of basin and 

 border prevailed. It is probable that the close of the Pliocene 

 epoch found the Cuzco Basin with a few shallow ponds and 

 swamps united into a sluggish drainage system and surrounded 

 by expanses of sands and silts of a former lake floor. It is 

 possible, indeed, that the Cuzco Basin had reached a stage of 

 completely integrated drainage by the close of the Pliocene and 

 that no bodies of water were to be found in it. 



The advent of the Pleistocene was a matter of fundamental 

 importance to Lake Morkill, for during the glacial epoch 

 climatic conditions in the Andes were favorable for aggrada- 

 tion. Waste coating the upper slopes was carried into the 

 valleys in such quantities that the streams were unable to dis- 

 pose of it, and terraces and deltas and fans of gravel were 

 built on the borders of the depression. At Angostura the rock- 

 walled canyon of the ancient Huatanay appears to have been 

 completely filled with debris to the 11,100-foot contour line, 

 forming a dam that held the waters flowing into the Cuzco Basin 

 and converted the eroded valley floor into a sheet of water whose 

 maximum extension is represented by the boundary of the San 

 Sebastian formation (see map, PI. II). Lake Morkill was thus 

 given new life. Into this water body were carried the muds 

 and finest sands screened from the masses of waste accumulated 

 along its borders. In large part the material was siliceous, 

 furnished by disintegrated sandstone. Calcareous ooze 

 weathered from the Tucay limestone was also supplied. That 

 Lake Morkill fluctuated in volume in response to changing 

 climatic environment is indicated by alternating beds of 

 limestone, silt, peat,, and adobe exposed in nearly all sections 

 of the lake beds. From time to time wide zones of shore 

 were exposed for periods long enough to enable plants to send 

 their roots into the fine materials, for the sun to bake and 

 crack the muds, and for streams to spread gravels over portions 

 of the recovered lake bottoms. Lake Morkill probably existed 

 during all or nearly all of the glacial epoch. Older glacial 

 gravels appear to be interbedded with lake sediments; the 



