PHYSIOGRAPHY 555 



Totora. From Arani we traveled by mule train across the successive 

 ranges of the Cordillera Oriental past Totora, Chilon, Pampa Grande, 

 and Samaipata to Santa Cruz, which was reached the evening of Decem- 

 ber 13 (see figure 1). The necessities of rapid travel rendered impos- 

 sible the detailed examination of many tempting exposures, but it is be- 

 lieved that the data found on this trip will nevertheless prove of some 

 value. The following week we made a hasty visit to the famous oil seep- 

 age in Espejos Canyon, in the foothills of the Andes, 30 miles southwest 

 from Santa Cruz. Finally, between December 25 and January 11, a num- 

 ber of days were spent in the vicinity of Eio Colorado; likewise in the 

 Andean foothills near the small hamlet known as Buenavista. 



Physiography 

 the area examined 



The area which we were thus permitted to examine ranges in altitude 

 from about 14,000 to about 1,000 feet above sealevel. The greater part 

 of it is within the boundaries of the Cordillera Oriental, or Eastern 

 Eange of the Andes, but the eastern portion extends out upon the Santa 

 Cruz Plains. Each of these two strongly contrasting regions forms one 

 of the major physiographic provinces of Bolivia. 



THE CORDILLERA ORIENTAL 



During late Tertiary ( ?) time this area was a low-featured peneplain, 

 which is now represented only by riclge-tops and a few undissected rem- 

 nants. The peneplain was tilted so that its western edge, near the town 

 of Arani, was at an altitude of more than 13,000 feet, while its eastern 

 margin, at the crest of the easternmost range of the Andes, was at an 

 altitude of less than 3,000 feet. The major streams of the region were 

 probably developed on this old peneplain. That they antecede the 

 present cycle is abundantly shown by their disregard of structural and 

 stratigraphic conditions. Apparently dissection of the peneplain started 

 near its eastern margin. The highlands on the west, near Arani, are 

 very little modified by stream erosion^ whereas most of the area to the 

 east has been maturely dissected. Stream valleys in the area between 

 Pocona and Samaipata are moderately broad, with flat alluvium-tilled 

 bottoms, and their sides, although precipitous in many places and every- 

 where steep, culminate in narrow divides, with but slight suggestion of 

 the plateau surface that must have preceded the present cycle (see figure 

 2). East and north of Samaipata, in the easternmost of the Andean 

 ridges, where the exposed strata are in large part massive red sand- 

 XXXVII — Bull. Geol. Soc. Am., Vol. 33, 1921 



