GLACIATION IN THE TELLURIDE QUADRANGLE 720 
valleys, as in the valley of the San Miguel River near Keystone 
(Fig. 4); sometimes they are on the tops of mesas 1,000 to 1,200 
feet above the bottoms of the adjacent valleys, as on the mesa 
lying between Bilk Creek and Lake Fork. 
4. Silted-up lakes and ponds.—Nearly level areas due to the 
silting-up of ponds or lakes occur at several points in glaciated 
valleys. The largest of these areas is in the valley of the San Miguel 
River (Fig. 3); it has a length of nearly five miles and a width of 
about half a mile. For the greater part of its course through this 
area the San Miguel River has a grade averaging about 30 feet per 
mile. In some parts of its course the grade is much less than this 
and the stream flows in wide meanders. At one point the generally 
level surface of this lacustrine plain is somewhat broken by low 
morainal hills; in other places it has been slightly modified by the 
accumulation of material in post-glacial time in the form of alluvial 
fans, a kind of modification to which practically all similar areas 
in the quadrangle are subject. 
5. Terraces and valley trains——Beyond the termini of the 
various glaciers, drift in the form of stratified deposits is usually 
found at intervals along the sides of the valleys, sometimes evident 
as narrow terraces or remnants of terraces. In the case of the San 
Miguel River these deposits extend beyond the boundary of the 
quadrangle, and are found at elevations up to too feet above the 
stream. The amount of débris left as valley trains is slight owing 
to the narrow, steep-sided valleys in which the streams flow. 
CHARACTERISTICS OF THE DEPOSITS OF DRIFT OF THE EARLIER 
AND THE LATER: EPOCHS 
The drift deposits of the earlier and the later epochs are alike in 
being made up of a heterogeneous mixture of rounded and sub- 
angular rock fragments of various kinds and various sizes, including 
some bowlders and pebbles with striations. 
The most important differences are with respect to (1) the kinds 
of rock present, (2) the topographic position of the deposits, and 
(3) the amount of erosion which has taken place since the with- 
drawal of the ice sheets to which the deposits of the two epochs, 
respectively, owe their origin. As to composition the drift of the 
