GEOLOGY: ATWOOD AND MATHER 
179 
replaced, they would rise 3000 or 4000 feet above the present summits 
(see fig. 1). 
With the redoming of the area, which involved the warping or dom- 
ing of the summit peneplain, another cycle of erosion was begun. Val- 
leys were again formed, and in these valleys snows collected which in 
time formed glaciers that advanced to the lowlands bordering the range. 
These earHest Pleistocene glaciers retreated and disappeared. The 
range continued to be uplifted, and the streams were so rejuvenated 
that they cut great canyons below the broad troughs occupied by the 
Cerro glaciers. Again climatic changes favored the formation of ice 
among the summits, and that ice (the Durango glaciers) descended 
through the main canyons to the foothills and later retreated and dis- 
appeared. The canyons were still more deeply cut into the mountain 
mass, and then climatic conditions favorable for glaciation once more 
returned and the Wisconsin or third series of Pleistocene glaciers formed 
and descended through the great canyons, nearly as far as those of the 
Durango stage. These glaciers have now disappeared, and there is no 
true glacier ice remaining in the region today, but the streams are vigor- 
ously dissecting the mountain mass to still greater depths. The vigor 
of that work is illustrated in many a sharp, V-shaped notch cut below 
the depth of ice action. The debris taken from the mountin area is 
being distributed along the great valleys leading away from the range. 
The ice gouging of the three successive Pleistocene stages and the 
vigorous stream work during the interglacial intervals and since the 
last melting away of the ice suggest somewhat continuous mountain 
growth in this region during late geological time. 
The physiographic study of the San Juan region has associated with 
it much that is of human interest. As the region has passed through 
successive cycles of erosion the position of the ground-water table in 
the great mountain mass has necessarily changed. The base of the 
surface zone of oxidation has therefore shifted. With the deepening of 
erosion and the drawing off of ground waters larger portions of the ore 
bodies have come to be within the surface zone of oxidation. Secondary 
enrichment may, therefore, have proceeded to greater and greater 
depths as the erosion history has progressed. This suggestion has come 
from the study of the physiographic history of the San Juan Mountains, 
but as secondary enrichment seems to have been of minor importance 
here, the idea must be tested in other regions that have similarly passed 
through several cycles of erosion, and where the secondary enrichment 
of ore bodies has been recognized as of greater significance. 
Settlement was first made in the mountains by those seeking the 
