22 



H. E. Gregory — A Geologic Reconnaissance 



by narrow ravines leading southward and the grass-floored, 

 open valle} T s tributary to the Huatanay. In the mountains 

 forming the Huatanay-ITrubainba divide these contrasted rela- 

 tions are even more noticeable. Pachatucsa (15,915 feet) is 

 a cuesta cut from the southern limb of a broad anticline in 

 the crest of which is sunk the canyon of the Urubamba. The 

 southern slope of Pachatucsa corresponds with the dip of its 

 component strata; its northern slope truncates the sandstone 

 beds exposed in the precipitous canyon wall of the Urubamba 



Fig. 10. 



Fig. 10. The crest of Sierra Pachatucsa. Elevation 15,915 feet. The 

 precipitous slope on the left descends to the Urubamba. The plateau in 

 the right background is formed of lava. 



(fig. 10). The culminating heights of Seneca and the adjoin- 

 ing masses to the east are pyramids and rectangular blocks 

 carved from steeply tilted strata; the twin summits of Cerro 

 Picol (14,607 feet) are likewise pyramidal masses bordered by 

 glacial cirques (see fig. 31). The jagged sky lines of Maho- 

 Pinta and of Huanaccaurai reveal the presence of rows of teeth 

 and sharp fins weathered from the upturned edges of strata of 

 resistant sandstone. 



The mountains coinciding in height with the level of the Inca 

 peneplain are subdued in form, but those above that level 

 present abrupt escarpments, sharply cut canyons, and a variety 



