PHYSICAL GEOGRAPHY 721 



range with summits 2,000 feet above the adjacent lowlands and with a 

 magnificent cliff rising sheer along the eastern margin. South of Cangapi 

 this fault front is partially concealed by a series of eastward-dipping 

 cuestas which near Vitiacua rise well toward the crest of the .sierra. 



THE CHACO 



The Bolivian Chaco stretches eastward from the front of the Andes to 

 the Paraguay Eiver. Although the region of the Chaco is ordinarily re- 

 ferred to as a lowland plain, its monotony is broken by several groups of 

 hills and many isolated elevations. Most of the area is unexplored, but 

 that portion adjacent to the main routes of travel eastward and south- 

 ward from Santa Cruz is fairly well known. 



Between Santa Cruz and the Eio Grande there is a flat plain averaging 

 1,400 feet in elevation, marked by low hillocks 20 to 100 feet in height, 

 which once were sand-dunes, the migration of which has been stopped by 

 the growth of vegetation or by changing climatic conditions. The trees 

 are largely confined to these low mounds, and, as there is little surface 

 drainage, the intervening swales are generally converted in rainy weather 

 into shallow ponds and marshes. There is nothing, either in topography 

 or drainage, to indicate the character of the underlying rocks. A few 

 miles east of the trail leading southward in front of the Cordillera there 

 are occasional live dunes, still restless beneath the shifting breezes of the 

 plain. 



The margin of the Chaco, within 3 miles of the mountain front, is 

 much more broken and irregular in its topography. Hills 400 feet in 

 height are separated by steep-sided canyons, and nearly everywhere there 

 is a dense growth of trees, underbrush, and vines. The interstream areas 

 are broad and not greatly dissected by gullies. In many localities they 

 display the undulating topography of ancient dune country. Elsewhere 

 they seem to be the dissected remnants of a piedmont alluvial plain. The 

 stream-cuts are all developed in comparatively recent materials, wind- 

 blown sand or stream alluvium, and in no place do they reveal the older 

 underlying beds. 



South of the Eio Grande the east front of the Sierra de Charagua over- 

 looks the western margin of the Chaco, which is there generally covered 

 with a rather scanty forest. Close to the Cordillera the interstream areas 

 are generally dotted with low hills, which have a definite linear trend 

 parallel to that of the sierra. These are for the most part miniature hog- 

 backs formed of strata a little more resistant than the others and inclined 

 gently toward the east, away from the mountains. Similar conditions 

 maintain along the front of the Sierra de Aguarague. The low undulat- 



