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 depression 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’ ride south of Camaná, where locally they attain a thickness of several hundred feet. Their upper surface was well graded and they show a prolonged period of deposition in which the former coastal terrace was all but concealed.
Fig. 148—The Coast Range between Mollendo and Arequipa at the end of June, 1911. There is practically no grass and only a few dry shrubs. The fine network over the hill slopes is composed of interlacing cattle tracks. The cattle roam over these hills after the rains which come at long intervals. (See [page 141] for description of the rains and the transformations they effect. For example, in October, 1911, these hills were covered with grass.)
Fig. 149—The great marine terrace at Mollendo. See [Fig. 150] for profile.
The uplift of the coast terrace and its subsequent dissection bring the physical history down to the present. The uplift was not uniform; three notches in the terrace show more faintly upon the granite-gneiss where the buried rock terrace has been swept clean again, more strongly upon the softer superimposed sands. They lie below the 700-foot contour and are insignificant in appearance beside the slopes of the Coast Range or the ragged bluff of the present coast.