PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT 273 



Valley below Bosalina, and again at the edge of the mountains at 

 the Pongo de Mainique. It is exhibited most impressively in the 

 Majes Valley, where the bordering slopes appear to be buried 

 knee-deep in waste, and where from any reasonable downward ex- 

 tension of rock walls of the valley there would appear to be at 

 least a half mile of it. It is doubtful and indeed improbable that 

 the entire fill of the Majes Valley is glacial, for during the Pliocene 

 or early Pleistocene there was a submergence which gave op- 

 portunity for the partial filling of the valley with non-glacial al- 

 luvium, upon which the glacial deposits were laid as upon a flat 

 and extensive floor that gives an exaggerated impression of their 

 depth. However, the head of the Majes Valley contains at least 

 six hundred feet and probably as much as eight hundred feet of 

 alluvium now in process of dissection, whose coarse texture 

 and position indicates an origin under glacial conditions. The 

 fact argues for the great thickness of the alluvial material of the 

 lower valley, even granting a floor of Pliocene or early Pleistocene 

 sediments. The best sections are to be found just below Chu- 

 quibamba and again about halfway between that city and Aplao, 

 whereas the best display of the still even-floored parts of the 

 valley are between Aplao and Cantas, where the braided river 

 still deposits coarse gravels upon its wide flood plain. 



