STRATIGRAPHY 563 



has shown to be Permian, and the lithology of the sandstones suggests 

 beds to which Permian age has been ascribed in the vicinity of Lake 

 Titicaca, to the west. A few fossils were found in it, but, with the 

 exception of some unidentifiable fragments, they were all lingulas, value- 

 less for determining its stratigraphic position. 



The sandstones of the Bermejo are massive and heavy bedded. One 

 bed at least 300 feet thick, with no sign of either false or true bedding- 

 dividing it, was seen (see figure 4). For the most part the weathered 

 surfaces are reddish brown, but grays and yellows are also present. The 

 fresh surfaces are commonly yellow brown to gray, mottled with brown 

 spots of limonite or manganese. The sandstones are composed of color- 

 less, translucent quartz grains in a limy cement. Individual grains are 

 well rounded, but are irregular in size, although for the most part very 

 fine. Ripple-marks, rill-marks, mud cracks, worn borings, et cetera, 

 were not observed. Many of the sandstones contained a sprinkling of 

 pebbles, many of which were faceted by ice action, but no beds with 

 enough pebbles to earn the title of conglomerate were seen where the 

 Bermejo was crossed on this traverse. 



These conspicuous sandstone beds have induced a topography indi- 

 vidual to the Bermejo in this region — high cliffs with arched alcoves, 

 peaks bounded by sheer rock, vertical walls crowning ridges that fall 

 away in long, steep slopes, sharp-crested twisting divides and smooth- 

 sloped hogback ridges prevail (see figures 4 and 5). The type of 

 topography is not entirely due to lithology, but the influence of these 

 resistant beds is everywhere apparent. 



Probably about half the thickness of the formation is shale or indu- 

 rate clay. These clays are commonly of a delicate lavender shade, par- 

 ticularly in the upper two-thirds of the formation. Near its base, dark 

 gray and black replace the lavender, although other characteristics of 

 the beds are quite similar. The beds are rarely laminated and weather 

 much like massive sandstones. Under the hammer they break into 

 irregular chunks, apparently obeying no laws of cleavage. Their tough- 

 ness results in low cliff faces along streams or extremely steep slopes 

 along valey sides, a though these features are frequently modified by 

 landslides, showing that the clays soften quite easily in water. Scattered 

 through the clays are grains, pebbles, and cobbles of blue and gray 

 quartzite and granitic rocks that are commonly beautifully faceted by 

 ice action. There are also beds of true tillite 2 composed of angular rock 

 fragments in argillaceous cement. Some of these rock fragments are 



2 A detailed description of these tillites will be given in a report on the glacial and 

 fluvio-glaciaJ deposits of the eastern Andes of Bolivia, now in preparation. 



