36 FISHES OF WESTERN SOUTH AMERICA 
especially the central portion. This allowed time for a weathering down of the 
Andes to a peneplane and to a mature topography. That elevations were not 
great appears to be certain, for they were insufficient to cut off moisture-laden 
winds, and the weathering was more or less equal on the two flanks. 
A fourth phase during the Pliocene and Pleistocene consists of a second and 
rapid period of uplifting; this is a final chapter which continues on down to our 
time. (See Schuchert and Dunbar, page 411.) No other series of events would 
seem logically to explain the present topography, the apparent anomaly of a moun- 
tain system with immense areas of more or less level and matured relief at the 
top, with the most rugged and immature approaches from the flanks. 
The rapidity of the recent phases is well brought out by the fact that the Titi- 
caca-Poopo basin has not had sufficient time to find an outlet to the outside, and 
has become well base-leveled internally. A similarly elevated intercordilleran 
basin, the Pampa de Junin, has had a similar history, but for an unknown time has 
been pirated by the Rio Mantaro and drains to the outside. 
(2) GEOLOGICAL AGENCIES 
In establishing the present relief, it 1s probable that every known type of 
geological agent has somewhere had a part in the sculpture of the details. 
(a) Weathering 
a. Frost: Freezing rarely occurs in elevations below 12000 feet, commonly in 
many areas of 13500 feet; in many spots permanently above 16000 feet. Thus 
freezing and thawing are effective principally in the high puna. The critical points 
vary with precipitation, with latitude, with the slope (isolation). Bosworth 
has shown the heat of the sun to be a direct agent in the cleavage of pebbles, aside 
from freezing and thawing. 
b. Ice: Permanent ice fields furnish the principal sources of the water in certain 
streams, growing and diminishing with the season, with intermittent stream action. 
Many traces of direct ice erosion are evident in the higher elevations. 
ec. Rainfall: Mountains of such extreme height erected across the path of the 
moisture-laden Southern Trade Winds have inevitably resulted in the greatest 
inequality in precipitation. Two hundred inches per year are common for the 
eastern slopes. But it would not be an exaggeration to put the figure at an inch 
per two-hundred years in the rain-shadow of the western slopes. As a result the 
streams of the Pacific slope are of scant volume at any time, and often dry, very 
declivitous, short, and direct, while those of the Atlantic side have maintained 
throughout their history basins of great length and gorges of great depth to the 
interior of the ranges. 
d. Wind: On the western slopes the action of the wind may be read directly 
from the topography, in the absence of, and out of competition with, rainfall. 
First, this is effected directly by the wind-distribution of sand and earth, forming 
dunes, ete.; second, the wind-borne sand is a tool, responsible for the etching of the 
details of many areas, especially on the western scarp. 
