THE PERUVIAN LANDSCAPE 193 



above the valley floors, though rarely more than 3,000 feet above 

 them. In the more central tracts, far from the main streams and 

 their associated canyons, dissection in the present erosion cycle 

 has not yet been initiated, the mature slopes are still intact, and 

 a topographic unconformity has not yet been developed. The 

 higher slopes are faced with rock and topped with slowly moving 

 waste. Ascent of the spur end is by steep zigzag trails ; once the 

 top is gained the trail runs along the gentler slopes without spe- 

 cial difficulties. 



It is worth noting at this point that the surface of erosion still 

 older than the mature slopes herewith described appears not to 

 have been developed along the seventy-third meridian of Peru, or 

 if developed at one time, fragments of it no longer remain. The 

 last well-developed remnant is southwest of Cuzco, Fig. 130. I 

 have elsewhere described the character and geographic distribu- 

 tion of this oldest recognizable surface of the Central Andes. 1 

 Southern Peru and Bolivia and northern Chile display its features 

 in what seems an unmistakable manner. The best locality yet found 

 is in the Desaguadero Valley between Ancoaqui and Concordia. 

 There one may see thousands of feet of strongly inclined sedi- 

 ments of varying resistance beveled by a well-developed surface 

 of erosion whose preserval is owing to a moderate rainfall and to 

 location in an interior basin. 2 



The highest surface of a region, if formed during a prolonged 

 period of erosion, becomes a surface of reference in the determina- 

 tion of the character and amount of later crustal deformations, 

 having somewhat the same functions as a key bed in stratigraphic 

 geology. Indeed, concrete physiographic facts may be the only 

 basis for arguments as to both epeirogenic and orogenic move- 

 ments. The following considerations may show in condensed form 

 the relative value of physiographic evidence : 



1. If movements in the earth's crust are predominantly down- 



1 The Physiography of the Central Andes, Am. Journ. Sci., Vol. 40, 1909, pp. 197-217 

 and 373-402. 



2 Results of an Expedition to the Central Andes, Bull. Am. Geog. Soc, Vol. 46, 1914. 

 Figs. 28 and 29. 



