386 WALLACE W. ATWOOD AND KIRTLEY F. MATHER 
ice advance in a single mountain valley would presumably remove 
most if not all of the deposits of a preceding advance. In most 
cases the erosive action of the ice formed during the epoch of maxi- 
mum glaciation would effectually destroy all evidence of previous 
glacial epochs. In all cases of distinct glacial epochs which have as 
yet been worked out among the mountains the morainic materials 
of the earlier epochs were deposited beyond the reach of all succeed- 
ing ice advances. 
In each of the higher ranges of the North American Cordillera 
evidence of recent glaciation is abundant, but in only a few of the 
ranges have glacial studies been prosecuted with sufficient detail to 
demonstrate distinct glacial epochs. 
In the Bighorn Mountains of Wyoming evidences of two glacial 
epochs have been found.’ The earlier of these was slightly more 
extensive than the later and the time interval between the two must 
have been considerable. In addition to these two drift deposits 
certain large bowlders were noted as suggestive of a possibly still 
older and more extensive epoch of glaciation in that range. 
In the Sawatch range similar facts to demonstrate two glacial 
epochs have been ascertained.? 
In this region a deposit of huge bowlders has been reported 
beyond the earliest known glacial drift, and in such relations to the 
present topography that if proven to be of glacial origin they would 
demonstrate a third and much older glacial epoch. 
In the Uinta and Wasatch mountains morainic deposits of two 
well-defined epochs of glaciation have been identified,’ and in the 
Front range of the Rockies the drift deposits have likewise been 
referred to two glacial epochs. 
In the San Juan Mountains of southwestern Colorado the effects 
of glaciation have been noted by many observers. In the Telluride, 
La Plata, Silverton, Needle mountains, and Rico folios evidence of 
the recent glaciation is presented, and reference is made to certain 
« Salisbury and Blackwelder, Jour. Geol., XI (1903), 216-23. 
2 Capps and Leffingwell, Jour. Geol., XII (1904), 698-706; Capps, U.S. Geol. 
Survey Bull. 386 (1907); Westgate, Jour. Geol., XIII (1905), 285-312. 
3, W. W. Atwood, U.S. Geol. Survey, Prof. Paper 61 (1909). 
4S. H. Ball, U.S. Geol. Survey, Prof. Paper 63 (1908), 83-86; F. L. Ransome, U.S. 
Geol. Survey, Prof. Paper 75 (1911), 72-79. 
