230 THE ANDES OF SOUTHERN PERU 



on the eastern border of the Coast Eange 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. More- 

 over, the mature upper slopes and the steep lower slopes of the 

 large valleys make a pronounced topographic unconformity, show- 

 ing two cycles of valley development. 



6. Upon uplift of the coast and dissection of the marine ter- 

 races 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 Eange. 



7. Depression of the coastal terrace and its partial burial 

 meant the drowning of the lower Majes Valley and its partial fill- 

 ing 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 Fig. 155.) 



8. Uplift and dissection of both the terrace and its overlying 

 sediments would be accompanied by dissection of the former val- 

 ley 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 bow- 

 ing 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. 



It is known that the Pleistocene was a time of augmented 

 waste delivery. At the head of the broadly opened Majes Valley 

 there was deposited a huge mass of extremely coarse waste sev- 

 eral hundred feet deep and several miles long. Forward from it, 

 interstratified with its outer margin, and continuing the same al- 

 luvial grade, is a still greater mass of finer material which de- 

 scends to lower levels. The fine material is deposited on the floor 



