73° ALLEN DAVID HOLE 
earlier epoch or epochs is characterized by bowlders of Potosi 
rhyolite up to 18 feet in diameter; that of the more recent epoch 
by bowlders of diorite-monzonite up to 4 feet in diameter, or of the 
San Juan formation up to 15 feet or more. Striated bowlders are, 
in general, rare in the earlier drift; in the more recent drift they 
are of common occurrence, and locally are abundant. As to 
topographic position, the deposits of earlier age occur in the major- 
ity of cases on the tops of mesas or ridges ranging up to 500 feet or 
more above the upper limit of the adjacent deposits of the more 
recent epoch. Some of the earlier drift, however, lies on slopes and 
in valleys in contact with the upper limit of drift of the more recent 
epoch just as if the earlier drift sheet had been in part overridden 
by the more recent glaciers. Not only does the older drift usually 
lie at a higher elevation than the more recent, but it also lies outside 
the drift boundary of the latter. In one case the earlier drift is 
found in a valley in which the more recent drift is not represented; 
in other cases the older drift lies at distances ranging up to one and 
one-half miles beyond the edge of the more recent deposits. 
The difference in the amount of erosion to which the drift of the 
two epochs has been subject is shown both by the general field 
relations and by the topography of the deposits. The earlier drift 
deposits constitute in quantity a comparatively insignificant 
amount of material, distributed in isolated, small patches, at not 
less than twenty-five different places in the quadrangle. The more 
recent deposits, on the other hand, include prominent moraines and 
drift sheets which are in general continuous for each valley or 
drainage basin in which they occur. In topography, the earlier 
drift is characterized by a surface of greater regularity and smooth- 
ness, and by an absence of undrained depressions; the more recent 
drift, on the other hand, in a number of places shows a much more 
irregular surface with numerous kettle holes. 
EXTENT OF GLACIATION 
1. In the more recent epoch.—The number of glaciers occupying 
valleys in this quadrangle in the more recent epoch can be stated 
only in terms of number of terminal areas, and number of areas 
in which movement of ice originated. It must be remembered, 
