736 ALLEN DAVID HOLE 
ice may be considered as possible sources of fragments; but they 
present no evidence of undercutting or of sapping. 
It will require much more extended observation to determine 
without question what the precise relation may be between the 
average elevation of an area of glaciation and the horizon of a 
formation prominently represented in the drift by large fragments. 
But the evidence, as far as observations in the valleys of this 
quadrangle ate concerned, points to the conclusion that an abun- 
dance of large bowlders of a certain formation in the drift from a 
given glaciated area indicates an outcrop of the formation in the 
bottom and sides of the middle and lower parts of the high valleys 
in which the glaciers in question were formed. As the San Juan 
formation lies at approximately 10,500 to 12,000 feet in elevation 
for its lower and upper limits, respectively, and as the Potosi 
rhyolite has its lower limit at about 12,500 feet elevation, we should 
have, on the basis of the foregoing conclusion, a position of the 
middle part of high glaciated valleys in the earlier drift epoch of not 
less than 1,000 feet above the level of the present cirque valleys. 
Or, in other words, that sufficient time has elapsed since the period 
of the earlier glaciation in this region to permit the removal by 
erosion in the high mountain tracts of not less than 1,000 feet of 
igneous rock. This interval of time manifestly must include (1) 
an interglacial interval, which presumably was of long duration, 
and (2) the period of more recent glaciation. 
POST-GLACIAL CHANGES _ 
The changes due to agencies acting in post-glacial time are 
chiefly the formation of alluvial cones and fans, alluvial and lacus- 
trine deposits along streams and in lakes or ponds, and talus slopes, 
and in the renewal of the process of downward cutting by streams 
in nearly all the valleys. Of the deposits named, the accumulations 
of talus are largest in amount, slopes of talus 1,000 feet or more in 
length occurring in a few places. But the total amount of all the 
post-glacial deposits is insignificant; they are, in general, recog- 
nizable only at the base of cliffs or steep slopes and at certain places 
in the bottoms of valleys. The amount of post-glacial erosion is 
also in all cases relatively very small; the materials of the drift 
