PHYSICAL GEOGRAPHY 



725 



Parapiti. This escarpment rises abruptly to an elevation of about a thou- 

 sand feet above the lowland and is broken at long intervals by such pic- 

 turesque gorges as that traversed by Eio Parapiti, approximately at right 

 angles to the trend of the hills. The crest of the hills above this cliff is 

 somewhat uneven and broken, but throughout nearly their entire length 

 the summit elevations are closely similar. To the west of this eastern- 

 most ridge there are typical hogbacks with steeply inclined dip slopes on 

 the west and rugged eroded cliffs on the east. Successive ridges toward 

 the west are lower in altitude and display more gentle dip slopes, until 

 finally the westernmost cuesta blends almost imperceptibly into the undu- 

 lations of the plain which separate this group of hills from the next ridges 

 on the west. 



In the vicinity of Eio Parapiti this lowland is about 10 miles in width 

 and extends to the east front of the Cuestas de Pipi, a group of hills situ- 

 ated a few miles west of Pipi on the Eio Parapiti. Their eastern margin 

 is a steep escarpment flung in a broad, sweeping curve from north to 

 south. It rises abruptly to elevations of about 1,500 feet above the inter- 

 montane lowland and is broken at only one point, where Eio Parapiti has 

 carved a steep-walled canyon which cuts across the line of hills from west 

 to east. The eastern rampart of these cuestas displays a close accordance 

 in summit elevation throughout the greater part of its length. This east- 

 ern ridge, with its steep east front, is paralleled on the west by a succes- 

 sion of four or five other ridges, separated from it and from each other by 

 youthful valleys. These are in reality hogbacks, for the western slope of 

 each conforms closely to the dip of the resistant sandstone strata of which 

 each is formed. The eastern faces are generally much steeper, and thus 

 the valleys between successive ridges are asymmetrical. The westernmost 

 hogback forms the eastern slope of the Guarui Valley and is shown in 

 figure 10. Eio Parapiti cuts through the successive ridges in a steep- 

 walled gorge, which is from 100 to 300 yards in width in the narrows 

 formed by each successive hogback, but broadens to a width of half a mile . 

 or so in the inter-hogback areas. Tributary streams, such as Wirendi and 

 Guarui creeks, flow roughly parallel to the trend of the hogbacks and 

 occupy valleys cut along the strike of the weaker strata. 



DRAINAGE SYSTEM 



The larger streams of this area all flow eastward with little apparent 

 regard for existing topographic features. The two largest, Eio Grande 

 and Eio Pilcomayo, have their sources far to the west, in the heart of 

 the Cordillera Oriental. Flowing east, they cut through the successive 

 sierras and cuestas in exceedingly rugged canyons and narrow gorges, 



