PHYSICAL GEOGRAPHY 719 



The Sierra de Aguarague is traversed by the canyons of two rather 

 large streams: Eio Vitiacua in latitude 20° 50' and Eio Pilcomaya, near 

 Villamontes, in latitude 21° 15'. It is also dissected by the ravines and 

 gorges of many smaller streams which head within the narrow mountain 

 range. The canyons of all these streams are very narrow and steep- 

 walled; that of Eio Vitiacua is shown in figure 7. At one point in this 

 canyon the walls close in so narrowly that there is not room for a trail 

 during the rainy season and progress within the canyon is barred. This 

 point is known locally as El Chorro and is shown in the photograph which 

 forms figure 8. Here the trail leaves the stream bed and climbs up a 

 steep tributary valley to La Loma, a saddle in the mountain ridge 600 feet 

 above the canyon floor, beyond which the descent to the floor of the main 

 canyon is equally as abrupt as was the ascent. Most of the larger streams 

 in the Sierra de Aguarague, like those in the other ranges of the region 

 under discussion, flow transversely to the longer axis of the range. Some, 

 like the Eio Vitiacua, show a rather unusual adjustment to the structure, 

 so that they flow for distances of a quarter mile or so along the strike of 

 the beds, then turn abruptly and continue at right angles to the strike 

 for a short distance, only to turn once more at right angles to flow along 

 the strike in the opposite direction to that previously followed. Thus 

 what presumably were once graceful meander curves are now a series of 

 angular patterns along which the stream zigzags across the range. 



■ The Quebrada de Los Monos, just south of Eio Pilcomayo, is unique 

 among all the stream valleys which we observed, although there are prob- 

 ably three or four other similar valleys in the Sierra de Aguarague. This 

 quebrada runs along the strike of the beds for many miles, instead of 

 cutting across them from west to east. It is a quarter to a half mile in 

 width and is carved in comparatively soft shales exposed along the axis 

 of the Aguarague anticline. After flowing northward for 5 or 6 miles in 

 this broad valley, the stream turns abruptly toward the east and plunges 

 into a steep-walled gorge, through which it traverses the eastern half of 

 the range. Beyond the portals of this gorge (see figure 9) the brook 

 meanders across the plain at the foot of the sierra and eventually is trib- 

 utary to Eio Pilcomayo. 



Sierra de Vitiacua. — Between the north end of the Sierra de Asmara o'ue 

 and the south half of the Sierra de Mandiyuti there is a third range, the 

 Sierra de Vitiacua, which parallels the Aguarague range and is separated 

 from it by a lowland 4 or 5 miles in width. This range rises near Ibo as 

 a line of low hills, true cuestas formed by the resistant sandstones of a 

 tilted fault block, with a northward trending escarpment on the east. 

 Toward the south these hills increase in height to form the mountain 



