388 WALLACE W. ATWOOD AND KIRTLEY F,. MATHER 
progress of the field work. In the selection of the names an attempt 
has been made to recognize some of the glacial studies which have 
contributed to the solution of this problem. 
The earliest and oldest of the three known glacial epochs is 
designated the ‘‘San Juan glacial epoch”’; the intermediate epoch, 
formerly called the “earlier”’ or “older,” is here called the “ Bighorn 
glacial epoch”’; while the most recent and youngest of the three is 
referred to as the ‘‘ Uinta glacial epoch.” 
It is believed that it may also be of advantage to apply names 
to the interglacial intervals as well, and for that purpose the 
following names are tentatively proposed: for the interval between 
the San Juan and Bighorn glacial epochs the term ‘“‘ Uncompahgre 
interglacial interval” is suggested; and for the interval between . 
the Bighorn and Uinta glacial epochs we may use the term “‘ Animas 
interglacial interval.’’ These names have been applied because 
portions of the valleys of the two streams named are good examples 
of the canyon development accomplished during the interglacial 
times. 
The Pleistocene period in the western mountains would then 
be subdivided, beginning with the earliest of the known glaciations 
in that region, as follows: 
San Juan glacial epoch. 
Uncompahgre interglacial interval. 
Big Horn glacial epoch. 
Animas interglacial interval. 
Uinta glacial epoch. 
General features of the San Juan Mountains.—The San Juan 
Mountains embrace an area of about 3,000 square miles in south- 
western Colorado, extending westward from the San Luis Park to 
within fifty miles of the Utah boundary. North and south the 
mountainous area varies in width from 25 to 4o miles. The 
central portion of the region is a group of rugged peaks, many of 
which are over 14,000 feet in elevation, while the borders of the 
range slope downward to the plateaus of New Mexico and Utah. 
The central mountain mass is formed of igneous rock, both 
volcanic and intrusive, which during Tertiary times was built 
up into a great volcanic plateau several thousand square miles in 
extent. Beneath the volcanics, the sedimentary rocks of Paleozoic 
