208 /. Bowman — Physiography of the Central Andes. 



Their crumbling scarps are the last ascent one makes in reaching 

 the upper level of the plateau. From this altitude to the highest 

 altitude one attains on the peneplain remnants of this vicinity 

 (15,000 ft.) the slopes are relatively gentler. Nowhere does 

 one gain a more impressive notion of the extent and charac- 

 ter of these unclissected surfaces as from the 12,500 ft. level 

 east of the spring at Laguno Huasco, fig. 8. The photograph 

 (fig. 7) scarcely needs an interpretive text. In it one looks a 

 little north of west and observes from foreground to back- 

 ground about 25 miles of country, and perhaps an equal dis- 

 tance from left to right. The plateau surface frequently 

 referred to above is on the sky-line. The camera stands upon 

 a great alluvial fan tributary to the basin of Lake Huasco ; and 

 the basin itself is limited on the west by the bold and ragged 

 thousand-foot scarp that descends toward the observer in the 

 middle distance. The degree of baseleveling attained in this 

 region is brought out strikingly in this view as well as in fig. 6, 

 where, however, it occupies a warped attitude, assumed since 

 peneplanation. 



It would be singular indeed if the great altitude of the old 

 surface thus described had been acquired in a single period of 

 crustal deformation. The history of other regions raises the 

 expectation that successive uplifts, rather than a single pro- 

 found uplift, would occur, separated by periods of relative 

 quiet during which the drainage lines and the topography 

 would become organized with respect to the new base level. 

 This expectation is more definitely and abundantly met in the 

 eastern plateau than in the western, by reason of climatic influ- 

 ences to be defined later, but even in the western plateau we 

 have specific cases pointing to this conclusion. 



The fact of successive uplifts may perhaps be presented more 

 clearly after some consideration of the present attitude of the 

 deformed peneplain where block faulting has occurred. In 

 fig. 6 the gently warped, western slope of the plateau is repre- 

 sented as a practically smooth descent with but minor disloca- 

 tions. These dislocations have a considerable interpretive 

 value, as will appear in the further discussion, but they do not 

 destroy the general regularity of the flanking slope. In fig. 7 

 it has already been noted that while a portion of the peneplain 

 occupies a nearly flat position, its continuity is broken in the 

 middle distance by a thousand-foot slope and scarp, the west- 

 ern border of the basin of Lake Huasco. 



The basin quadrant which lies to the southeast of Lake 

 Huasco, fig. 8, is a huge fault block which gained its present 

 attitude after peneplanation. The scarp winch limits the basin 

 on the west is the locus of the fault, and the basin itself is the 

 product of a dislocation whereby the western edge of the block 



