196 THE ANDES OF SOUTHERN PERU 



eral hundred feet deep. Moreover, the glacial material is coarse 

 throughout, and was built up rapidly and dissected rapidly. In 

 most cases, furthermore, coarse material at the bottom of the gla- 

 cial series rests directly upon the rock of a narrow and ungraded 

 valley floor. From these and allied facts it is concluded that there 

 is no long time interval represented by the transitions from de- 

 grading to aggrading processes and back again. The early Pleisto- 

 cene, therefore, seems quite too short a period in which to produce 

 the bold forms and effect the deep erosion which marks the period 

 between the close of the mature cycle and the beginnings of deposi- 

 tion in the Pleistocene. 



The alternative conclusion is that the greater part of the canyon 

 cutting was effected in the late Tertiary, and that it continued into 

 the early Pleistocene until further erosion was halted by changed 

 climatic conditions and the augmented delivery of land waste to 

 all the streams. The final development of the well-graded high- 

 level slopes is, therefore, closely confined to a small portion of the 

 Tertiary. The closest estimate which the facts support appears 

 to be Miocene or early Pliocene. It is clear, however, that only the 

 culmination of the period can be definitely assigned. Erosion was 

 in full progress at the close of the Cretaceous and by middle 

 Tertiary had effected vast changes in the landscape. The Tertiary 

 strata are marked by coarse basal deposit and by thin and very 

 fine top deposits. Though their deformed condition indicates a 

 period of crustal disturbance, the Tertiary beds give no indica- 

 tion of wholesale transformations. They indicate chiefly tilting 

 and moderate and normal faulting. The previously developed ef- 

 fects of erosion were, therefore, not radically modified. The sur- 

 face was thus in large measure prepared by erosion in the early 

 Tertiary for its final condition of maturity reached during the 

 early Pliocene. 



It seems appropriate, in concluding this chapter, to summarize 

 in its main outlines the physiography of southern Peru, partly to 

 condense the extended discussion of the preceding paragraphs, 

 and partly to supply a background for the three chapters that 

 follow. The outstanding features are broad plateau areas sepa- 



