404. WALLACE W. ATWOOD AND KIRTLEY F, MATHER 
The position of these drift deposits, on and near the divides 
between the present drainage channels, and in front of the present 
mountain spurs, is In sharp contrast to that of the deposits referred 
to the Uinta and Bighorn glacial epochs. The latter are restricted 
to the present valleys and are much nearer the present mountain 
front. It is therefore evident that these outlying and somewhat 
scattered bodies of drift must be the work of the earlier and more 
widespread body of ice which has been referred to as the ice of the 
San Juan glacial epoch. 
The great age of the deposits made during the San Juan epoch 
is, ina general way, attested by their location on the present divides. 
Hundreds of square miles of surface, between the mountains and 
the remnants of San Juan drift, have been entirely cleared of glacial 
débris, and notable canyons now separate the mountains from the 
isolated hills capped with San Juan glacial bowlders. Furthermore, 
some of the bowlders in the San Juan drift are from formations 
which formerly covered the neighboring mountains but have since 
been removed from all but a few summits. It is quite certain that 
these formations were much more widespread at the time of the 
formation of the San Juan ice and, therefore, that the relief in the 
mountains was much less than at present. 
On the southern side of the range in the Piedra Valley drift of 
this oldest epoch which came from the mountains at the head of 
Middle Fork and the main Piedra River is now separated from the 
mountains by the 1,000-foot canyon of the Huerto River which must 
be entirely of post-San Juan age. The deposits capping Horsefly 
Peak and West Baldy are now separated from the mountain front 
by the drainage of Dallas and Leopard creeks which have lowered 
their channels far below the surface upon which the ice of this epoch 
rested. South Baldy and other elevations to the west that are 
capped with San Juan drift are distinctly separated from the main 
mountain front by erosion depressions. 
In general, erosion since the San Juan glacial epoch has been so 
complete and widespread that only those deposits which were 
situated where conditions were most favorable for their preservation 
have escaped removal. This is well illustrated by the conditions 
at Horsefly Peak. Although the drift there has been responsible 
