226 THE ANDES OF SOUTHERN PERU 



at Camana then as now. Likewise the Vitor at Quilca must have 

 cut straight across the Coast Range. An examination of the sur- 

 face leading down from the Coast Eange to the upper edge of 

 these valleys fully confirms this deduction. Flowing and well- 

 graded slopes descend to the brink of the inner valley in each 

 case, where they give way to the gorge walls that continue the 

 descent to the valley floor. 



Confirmatory evidence is found in the wide Majes Valley at 

 Cantas and Aplao. (See the Aplao Quadrangle for details.) 

 Though the observer is first impressed with the depth of the val- 

 ley, its width is more impressive still. It is also clear that two 

 periods of erosion are represented on its walls. Above Aplao the 

 valley walls swing off to the west in a great embayment quite in- 

 explicable on structural grounds; in fact the floor of the embay- 

 ment is developed across the structure, which is here more dis- 

 ordered than usual. The same is true below Cantas, as seen from 

 the trail, which drops over two scarps to get to the valley floor. 

 The upper, widely opened valley is correlated with the latter part 

 of the period in which were formed the mature terraces of the 

 coast and the mature slopes bordering the larger valleys where 

 they cross the Coast Eange. 



After its mature development the well-graded marine terrace 

 was upraised and dissected. The deepest and broadest incisions 

 in it were made where the largest streams crossed it. Shallower 

 and narrower valleys were formed where the smaller streams that 

 headed in the Coast Range flowed across it. Their depth and 

 breadth was in general proportional to the height of that part of 

 the Coast Range in which their headwaters lay and to the size of 

 their catchment basins. 



When the dissection of the terrace had progressed to the point 

 where about one-third of it had been destroyed, there came depres- 

 sion and the deposition of Pliocene or early Pleistocene sands, 

 gravels, and local clay beds. Everywhere the valleys were partly 

 or wholly filled and over broad stretches, as in the vicinity of 

 stream mouths and upon lower portions of the terrace, extensive 

 deposits were laid down. The largest deposits lie several hours' 



