2 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Plata mountain and its acutely serrated spurs, the rolling or mou- 
tonnée surface of the broad cirque floor at the head of the hanging 
valley, and the truncated spurs on the sides of the two troughs. 
The larger trough truncates the large spurs in a liberal fashion, 
shown more clearly in figure 2, which was drawn from a knob on 
the north side of Lake creek trough, somewhat east of the hanging 
valley. A photographic view of the same spurs is given in the Plate 
(B), from the high spur of Mt. Elbert. The body of the spurs here 
appear to be in the main of preglacial origin, modified by ordinary 
erosion during the glacial period, for through most of their length 
1 
7 
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Fic. 1.—La Plata peak, with Crystal lake gulch hanging over the deep trough 
of Lake creek; looking southwest. 
they correspond in essential features to the spurs of the non-glaciated 
mountains thereabouts. Features of this kind will be called “normal.” 
While the normal spurs of non-glaciated mountains gradually de- 
scend into maturely open valleys, the spurs here sketched are trun- 
cated by the south wall of the broad Lake creek trough in the most 
significant fashion. 
The smaller trough of Crystal lake gulch truncates the smaller 
spurs which descend from a sharp ridge or arête on the west, and 
similar but less pronounced spurs on the eastern slope of the 
trough, as shown in figure 3. It should be said, however, that 
