408 WALLACE W. ATWOOD AND KIRTLEY F. MATHER 
Cimmarron Ridge, and on the west of Horsefly Peak, the San Juan 
drift lies above and beyond fairly extensive bench surfaces covered 
with bowlder-gravels. These surfaces slope gently toward the 
Uncompahgre River. Their extreme evenness, as well as the size 
and assortment of the bowlders capping them, attest to their origin 
as graded surfaces of erosion upon which a bowlder deposit was 
made. These bowlder-covered surfaces are below, and appear to be 
cut into, the drift deposits and therefore are younger than the drift. 
It is believed that these surfaces are remnants of the graded surfaces 
developed during the bowlder-mesa stage of mountain dissection. 
If this is true the glaciation of the San Juan epoch was one of 
the incidents which took place during dissection of the late Tertiary 
peneplain previous to the development of the bowlder-mesas. 
On the south side of the range the Bighorn moraines and out- 
wash deposits are found adjacent to the graded surfaces of the 
Oxford stage near the Pine, Florida, and Animas valleys. The 
relations there indicate that the Oxford stage of mountain dissection 
was soon followed by the glaciation of the Bighorn epoch. During 
the Uncompahgre interglacial epoch the graded surfaces of both 
the bowlder-mesa and Oxford stages were developed. The Animas 
interglacial epoch was not of sufficient duration for the formation of 
great erosional features, but was brought to a close by the Uinta gla- 
ciation before the streams had again reached a temporary base level. 
Comparison of the glaciation in the San Juans with that of the 
adjacent mountains.—A. comparison of. the glacial deposits now 
known to exist in the San Juan mountains with those reported from 
the neighboring mountain ranges of Colorado and Utah, as referred 
to in the opening paragraphs of this paper, shows that the two 
glaciations of those ranges, known as the “earlier” and “later,” 
are to be correlated with the Bighorn and Uinta epochs as above 
defined. In the mountain ranges from which two glacial epochs 
have been reported it is noted that the moraines of each epoch are 
limited to the immediate valleys and that the earlier is only slightly 
more extensive than the later. They bear the same relation to 
each other and to the topographic features of each region as that 
which characterizes the Bighorn and Uinta deposits of the San 
Juan mountains. 
