PLEISTOCENE HISTORY OF SAN JUAN MOUNTAINS 389 
and Mesozoic age dip away from the central portion of the region; 
in a few of the deeper valleys and near the present borders of the 
volcanic area, the complex of pre-Cambrian sediments, schists, 
gneisses, and intrusive rocks upon which the Paleozoic and Mesozoic 
rocks rest, is exposed. About the margin of the range there are 
some clastic sediments of Tertiary age. 
The range has been maturely dissected by the headwaters of 
four great streams which have attacked it from every side. On the 
east the Rio Grande, on the south the San Juan, on the west the 
Dolores, on the north the Gunnison, each with its many tributaries, 
has penetrated to the very heart of the mountains. The Rio 
Grande flows to the Gulf of Mexico, while the other three, finding 
their way across the plateaus through deep canyons, join the 
Colorado River and thence empty into the Gulf of California. (See 
ige 1) 
The work of each of these stream systems has been greatly 
influenced during Pleistocene times by the action of glacial ice. 
Many of the peaks and divides were notably sculptured by ice action 
and the valleys in the mountains show the effect of the passage of 
the glaciers through them. On the surrounding mesas and plateaus 
there are some glacial deposits and in the valleys beyond the base 
of the range there are many terraces and benches which may be 
traced upstream to the terminal moraines just within the moun- 
tainous region. 
The more readily recognized deposits of the two younger epochs 
will be first described, so that the evidence bearing upon a third 
and still earlier epoch may be more clearly presented. 
The' Uinta glacial epoch in the San Juan Mountains.—During 
Uinta times ice collected at the heads of nearly all of the larger 
streams in the San Juan area and moved for a greater or less 
distance into the valleys below. In the heart of the range only the 
higher peaks and divides were above the glaciers, but as the ice 
moved outward from the central portion of the range it became 
concentrated in the larger valleys and left much of the region 
unglaciated. In each of the valleys thus far studied the glaciers of 
the Uinta epoch were strictly limited to the valleys and in no case 
did they extend far beyond the foothills of the range. 
