PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT 267 



race up to five miles in width. The degree of dissection is varia- 

 ble, and depends upon the relation of the through-flowing streams 

 to the Coast Range. The Vitor and the Majes have cut down 

 through the Coast Range, and locally removed the terrace ; smaller 

 streams rising on the flanks of the Coast Range either die out 

 near the foot of the range or cross it in deep and narrow valleys. 

 The present drainage on the seaward slopes of the Coast Range 

 is entirely ineffective in reaching the sea, as was seen in 1911, the 

 wettest season known on the coast in years and one of the wettest 

 probably ever observed on this coast by man. 



In consequence of their deposition on a terrace that ranges in 

 elevation from zero to 1,500 feet above sea level, the deposits of 

 the coast are very irregularly disposed. But in consequence of 

 their great bulk they have a rather smooth upper surface, grada- 

 tion having been carried to the point where the irregularities of 

 the dissected terrace were smoothed out. Their general uniform- 

 ity is broken where streams cross them, or where streams crossed 

 them during the wetter Pleistocene. Their elevation, several hun- 

 dred feet above sea level, is responsible for the deep dissection 

 of their coastal margin, where great cliffs have been cut. 



PLEISTOCENE 



The broad regional uplift of the Peruvian Andes in late Ter- 

 tiary and in Pleistocene times carried their summits above the 

 level of perpetual snow. It is still an open question whether or not 

 uplift was sufficiently great in the early Pleistocene to be in- 

 fluenced by the first glaciations of that period. As yet, there are 

 evidences of only two glacial invasions, and both are considered 

 late events on account of the freshness of their deposits and the 

 related topographic forms. The coarse deposits — nearly 500 feet 

 thick — that form the top of the desert section described above 

 clearly indicate a wetter climate than prevailed during the 

 deposition of the several hundred feet of wind-blown deposits be- 

 neath them. But if our interpretation be correct these deposits 

 are of late Tertiary age, and their character and position are 

 taken to indicate climatic changes in the Tertiary. They may 



