THE PERUVIAN LANDSCAPE 195 



ord while at the same time forming a basis for the scientific study 

 of the surface distribution of living forms. 



The geologic dates of origin of the principal topographic forms 

 of the Central Andes may be determined with a fair degree of 

 accuracy. Geologic studies in Peru and Bolivia have emphasized 

 the wide distribution of the Cretaceous formations. They consist 

 principally of thick limestones above and sandstones and con- 

 glomerates below, and thus represent extensive marine submer- 

 gence of the earth's crust in the Cretaceous where now there are 

 very lofty mountains. The Cretaceous deposits are everywhere 

 strongly deformed or uplifted to a great height, and all have been 

 deeply eroded. They were involved, together with other and much 

 older sediments, in the erosion cycle which resulted in the devel- 

 opment of the widely extended series of mature slopes already 

 described. From low scattered island elevations projecting above 

 sea level, as in the Cretaceous period, the Andes were transformed 

 by compression and uplift to a rugged mountain belt subjected 

 to deep and powerful erosion. The products of erosion were in 

 part swept into the adjacent seas, in part accumulated on the 

 floors of intermont basins, as in the great interior basins of Titi- 

 caca and Poopo. 



Since the early Tertiary strata are themselves deformed from 

 once simple and approximately horizontal structures and sub- 

 jected to moderate tilting and faulting, it follows that mountain- 

 making movements again affected the region during later Terti- 

 ary. They did not, however, produce extreme effects. They did 

 stimulate erosion and bring about a reorganization of all the 

 slopes with respect to the new levels. 



This agrees closely with a second line of evidence which rests 

 upon an independent basis. The alluvial fill which lies upon all the 

 canyon and valley floors is of glacial origin, as shown by its inter- 

 locking relations with morainal deposits at the valley heads. It is 

 now in process of dissection and since its deposition in the Pleis- 

 tocene had been eroded on the average about 200 feet. Clearly, 

 to form a 3,000-foot canyon in hard rock requires much more time 

 than to deposit and again partially to excavate an alluvial fill sev- 



