3. Such a high degree of topographic organization requires the dissection in the late stages of the erosion cycle of at least the inner or eastern border of the piedmont deposits of the desert, largely accumulated during the early stages of the cycle.
4. Since the graded slopes of the Coast Range on the one side descend to a former shore whose elevation is now but 1,500 feet above sea level, and since only ten to twenty miles inland on the other side of the range, the same kind of slope extends beneath Tertiary deposits 4,000 feet above sea level, it appears that aggradation of the outer (or western) part of the Tertiary deposits on the eastern border of the Coast Range continued down to the end of the cycle of erosion, though
5. There must have been an outlet to the sea, since, as we have already seen, the water supply of the Tertiary was greater than that of today and the present streams reach the sea. Moreover, the mature upper slopes and the steep lower slopes of the large valleys make a pronounced topographic unconformity, showing two cycles of valley development.
6. Upon uplift of the coast and dissection of the marine terraces at the foot of the Coast Range, the streams cut deep trenches on the floors of their former valleys ([Fig. 152]) and removed (a) large portions of the coast terrace, and (b) large portions of the Tertiary deposits east of the Coast Range.
7. Depression of the coastal terrace and its partial burial meant the drowning of the lower Majes Valley and its partial filling with marine and later with terrestrial deposits. It also brought about the partial filling by stream aggradation of the middle portion of the valley, causing the valley fill to abut sharply against the steep valley walls. (See [155] .)
8. Uplift and dissection of both the terrace and its overlying sediments would be accompanied by dissection of the former valley fill, provided that the waste supply was not increased and that the uplift was regional and approximately equal throughout—not a bowing up of the coast on the one hand, or an excessive bowing up of the mountains on the other. But the waste supply has not remained constant, and the uplift has been greater in the Cordillera than on the coast. Let us proceed to the proof of these two conclusions, since upon them depends the interpretation of the later physical history of the coastal valleys.
