PHYSICAL GEOGRAPHY 713 



side overlooks in a similar way the intermontane depression which sepa- 

 rates this range from the next to the west. 



Throughout nearly its entire length the accordance in elevation of 

 higher ridges and peaks of the Sierra de Charagua is such that from a 

 distance the sky-line formed by this range is very even. Its summit has 

 an estimated altitude of 4,500 to 5,000 feet above the sea; the surface of 

 the lowland along its eastern foot is close to 2,700 feet in elevation, while 

 the intermontane depression on the west is generally between 3,200 and 

 3,600 feet. For long distances along both sides of this sierra there are 

 numerous dip slopes formed by resistant strata pitching away from the 

 axis of the mountains at high angles. These make rank after rank of 

 hogbacks, which in most localities decline in elevation regularly from the 

 innermost to the outermost (see figures 3 and 4). The steep hillsides 

 are carved by youthful drainage systems and trenched by narrow ravines 

 with unusually high gradients. The alternation of shale and sandstone 

 affords abundant opportunity for the development of landslides, and these 

 in most places are responsible for many details of the topography. 



The summit elevations of the Sierra de Charagua are rounded rather 

 than serrate. In places the mountain crest is- a horizontal line formed by 

 the truncation of the resistant inclined beds, but more commonly it is a 

 succession of rounded knobs with an occasional sharp peak. There are 

 no dip slopes on the very summits of the range, but rather the mountain 

 crests are the result of the erosion of the inclined beds and the truncation 

 of the wrinkled strata. There are occasionally gently sloping upland 

 surfaces, but these bevel the upturned sediments and are ordinarily sur- 

 rounded by slightly higher rounded knobs. 



The range is traversed by the gorges of four master streams with 

 courses approximately at right angles to the major axis of the range. 

 Named in order from north to south, these are : Tacuru Creek, Saipuru 

 Creek, Eio Charagua, and Kio Parapiti. In crossing the sierra, each 

 flows through a remarkable canyon, with narrow floor, rarely wider than 

 the stream bed and with steep rugged walls towering to elevations of 

 1,500 feet or so above the valley 'bottom (see figure 5). Tacuru Creek 

 rises among the hogbacks along the west margin of the sierra and flows 

 thence, in a slightly meandering course, through the central portion of 

 the range and past the hogbacks of the eastern side until it reaches the 

 wooded lowlands east of the mountains. Here its channel is choked by 

 gravels and boulders which cause it to spread out in numerous distrib- 

 utaries over a broad floodplain. The other three streams all have their 

 sources in or west of the intermontane depression which separates the 

 Sierra de Charagua from the next ranges to the west. Traversing this 

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



