RL. C. Hills—Extinet Glaciers of Colorado. 395 
the depth of erosion in the South Fork of the San Miguel. As 
before remarked, the course of this stream is diagonal to the 
direction of movement of the San Miguel portion of the ice 
sheet, which was approximately that of the main valley. The 
South Fork glacier cut down through not less than 800 feet of 
Cretaceous sandstones and shales, forming a cafion nearly half 
a mile wide bordered at intervals by. escarpments of sandstone. 
The erosion of this cafion must have taken place since the 
retreat of the ice sheet and before the retrograde movement of 
the local glacier had reached the junction of the South Fork 
with the main stream. However, the’ South Fork cafion does 
hot represent the average depth of erosion by local glaciers, 
but rather the maximum, for in most instances it has not ex- 
ceeded half this amount. 
Since the retreat of the local glaciers to the upper valleys 
the rivers have excavated chasms, or, what are usually termed 
“box cafions,” fifty to one hundred feet deep, according to the 
velocity of the current and character of the eroded rock. Evi- 
dence of this nature is shown in the Uncompahgre cafion near 
Ouray, in the cation of the Animas above Elbert, on the Dolo- 
res above Rico, and on the Lake Fork of the Gunnison above 
City. The depth of these chasms gradually decreases 
toward the heads of the streams, notwithstanding that the fall 
rising in the San Juan Mountains we find localities where the 
water is flowing but a few feet below the striated rock surface 
of the old glacier bed. 
Tt iS not unusual to find in these mountains limited aceumnu- 
lations of névé that never entirely disappear. There are two of 
these at the head of Henson Creek, near the point where the 
Animas Forks wagon road crosses the divide, at an elevation of 
13,000 feet, They are seldom less than fifty feet thick, from 
100 to 300 feet wide and from 400 to 600 feet in length. I 
‘Visited the smaller of the two on the 20th of September of the 
present year and found a stream of water, caused by the melt- 
Ing of a recent fall of snow, running the whole length of its 
trough-like surface. Scraping away some of the loose snow I 
discovered that the mass was solid ice, into which light was 
transmitted some distance. It seems moderately certain that 
the Glacial period of this portion of the Rocky Mountains ex- 
n Wousky up to the present time and that the névé accumu- 
lations found on the bead of Henson Creek, and elsewhere, are 
the remnants of the ice envelope, whic 
“ 
gradually increases, and around the sources of all the rivers . 
