556 HEALD AND MATHER RECONNAISSANCE OF EASTERN ANDES 



stones, the streams traverse deep box canyons, the walls of which rise 

 abruptly from the edges of the swift torrents (see figure 3). However, 

 the existence of the peneplain is still evidenced by the uniformity in 

 elevation of individual ridges, the general absence of prominent peaks 

 regardless of the differing natures of the various geological formations, 

 the uniform decline in elevation from ridge-top to ridge-top progressively 

 toward the east, and by a few plateau-like remnants. Xowhere in the 

 area between Araui and Santa Cruz is there evidence of Pleistocene 

 tdaciation. 



Figure 2. — Coclia~bamba -Santa Cruz Geology 



The barren rounded heights of the Andes, 13,000 to 14,000 feet above the sea, carved in 

 Arani sandstones and shales a few miles south of Vacas, on the Santa Cruz trail. 



SANTA CRUZ PLAINS 



To the east of the mountains the country is quite flat and poorly 

 drained. Its elevation is between 1,200 and 1,800 feet above sealevel. 

 A few major streams cross the plains, but side streams flow only during 

 the rainy season, and in other times are represented merely by sand- 

 covered courses. Such rivers as the Piray and Surutu flow in shallow 

 valleys bordered by low hills, some three or four miles apart. Over 

 much of this flat area the rain water collects where it falls, with practi- 

 cally no run-off, and makes bogs and morasses. Stream channels are 

 almost invariably floored with treacherous quicksands. 



