THE 
f@ORNAL OF GEOLOGY 
(OLN EAOUGUST, 1072 
THE EVIDENCE OF THREE DISTINCT GLACIAL EPOCHS 
TN Crib PEBISTOCENE HISTORY OF THE SAN 
JUAN MOUNTAINS, COLORADO’ 
WALLACE W. ATWOOD 
AND 
KIRTLEY F. MATHER 
Each of the six advances of ice from the Labrador and Keewatin 
centers during Pleistocene times was presumably accompanied by 
a similar formation and advance of glaciers in the Cordilleran region 
of North America. Each of the five interglacial intervals which 
have been recognized in the Mississippi Valley must have been 
accompanied by a shrinkage of the ice in the mountains which may 
or may not have resulted in complete deglaciation of the mountain 
areas. It is also evident that certain mountain groups, depending 
largely upon the latitude and altitude, may have been affected by 
less than the full number of ice formations and advances. Examples 
have been pointed out among the Uinta Mountains where certain 
basins that were glaciated during the earlier of two epochs were not 
occupied by ice during the later glacial epoch. 
The difficulty of recognizing and differentiating glacial deposits 
of distinct epochs in the mountains is somewhat greater than that 
of identifying deposits of successive continental ice sheets. Each 
* Published with the permission of the Director of the United States Geological 
Survey. 
2W. W. Atwood, U.S. Geol. Survey, Prof. Paper 61 (1909), 13, 14, 22, 58. 
Vol. XX, No. 5 385 
