42 FISHES OF WESTERN SOUTH AMERICA 
of the Eastern range cast a broad rainshadow upon the valleys giving rise to great 
variation in the amounts of precipitation received in various sections. 
Climbing southeastward by the Urubamba and its tributary the Vilcanota 
river, we come out upon the southern intercordilleran bridge, the Nudo de Vilca- 
nota, where, at La Raya, 14153 feet, we see the mingling of the waters of the 
Atlantic-bound Vilecanota with the headwaters of streams of the Titicaca basin. 
Here we enter the south-central region (F). 
The same impenetrable Western Cordillera here forms the barrier toward the 
Pacific, but from here well down into Chile the outlines of the sedimentary forma- 
tions are blotted out by volcanoes and volcanic formations, while the Eastern 
ranges, the Cordillera Real, have climbed to their highest elevations, and to the 
most elevated and most youthful portion of the entire Andean system. The snow- 
capped Sierra Nevadas fringing Lake Titicaca and attaining to heights of 20000— 
22000 feet, culminating in Sorata and Ilimani, the crowning glory of the continent, 
are granitic intrusions which have disrupted and upended the adjacent sedimentary 
formations of the region. Thus the Region F is unlike any other in being bounded 
by voleanic formations on the one hand and by igneous on the other. In this 
region the Central Cordillera had long been regarded as missing, but now its out- 
lines are thought to be discernible in the alluvial plains of the Poop6o basin as a 
series of low hills rising abruptly from the level pampa, partly lost by reason of 
their diminishing height, and partly by the burial of much of it through the proc- 
esses of aggradation. Elsewhere the erosive action of streams has kept pace with 
the uplift of the mountains, creating the gorges and water gaps draining to the 
Atlantic. But in this section, no stream has done so. In the southern direction, 
toward the streams of Argentina, the most logical outlet, the intercordilleran val- 
leys have been cut off, not by another bridge or Nudo, but by a series of volcanic 
formations. Hence the story of the Titicaca-Poop6é basin has been, despite its 
great altitude, one of aggradation. One stream alone, the Rio La Paz, has actu- 
ally penetrated the highest portion of the Cordillera Real and at this geological 
period is in the act of pirating the glacial waters of the western slopes which belong 
to the Titicaca drainage, escaping to the Atlantic side by way of an abrupt arroyo 
1300 feet deep in the detritus of the pampa and a canyon 7000 feet deep through 
the cordillera. 
The northern section of this area with its moderate precipitation drains through 
a number of small rivers entering the lake from north, west, and east. It is a 
terrain of moderate relief above the 12500 feet of the lake, rolling and mature. 
Titicaca itself occupies a constricted middle portion of its basin, and is bordered by 
alternating hills and aggraded valleys becoming marshy on much of the lake shore 
at the north and west. Its great depth with its contours unlike the surrounding 
aggraded terrain encourage the belief that it may have been formed by block 
faulting. In various places I have observed the sedimentary formations near its 
shores, much distorted and displaced, often standing on edge, and rising many 
feet above the pampa. This is especially noteworthy at the northeast and south- 
west corners. 
