516 ALLEN DAVID HOLE 
river, and extending one-fourth to one-half a mile up the valley 
from this point, are glacial deposits which are being treated by 
hydraulic process to recover the gold they contain. These deposits, 
as well as those extending eastward for half a mile from the small 
tributary referred to, and lying chiefly on the south side of the 
river, show in places layers of stratified silt, sand, and gravel; the 
greater part of the deposit, however, is unstratified. 
The drift in the vicinity of Keystone constitutes by far the 
largest accumulation of glacial débris to be found in the canyon of 
the San Miguel; judging from the comparatively small number of 
large San Juan bowlders found farther down the canyon, the 
Keystone drift is in the nature of a terminal moraine for the glacier 
which advanced from the east. On August 1, 1904, drift in the 
form of a ridge transverse to the stream at a point about four-fifths 
of a mile east of the mouth of Lake Fork was being washed down in 
the process of hydraulic mining; the work showed that the pre- 
glacial channel of the San Miguel River at this point was about 100 
yards farther north than at present, and approximately parallel to 
its present course, and that the pre-glacial channel had a depth of 
bed as much as 30 feet lower than the bottom of the present channel. 
It appears, therefore, that the pre-glacial channel was filled to such 
an extent as to displace the stream and cause it to flow at a higher 
level along the south wall of the valley where it has in post-glacial 
time eroded a new channel not more than ro to 20 feet in depth. 
This accumulation of drift in the vicinity of Keystone is believed 
to have been chiefly responsible for the existence of a glacial lake 
which extended eastward from Keystone to a point beyond the city 
of Telluride, a distance of more than four miles; and as the greater 
part of this drift for a quarter of a mile or more below the mouth of 
Remine Creek contains numerous large San Juan bowlders, the 
drift chiefly responsible for the existence of the lake must have been 
brought by glaciers from the east. The date of this glacial lake is 
therefore fixed for the time just following the retreat of the ice up 
the San Miguel valley after depositing the drift at and below the 
mouth of Remine Creek. The silting-up of this lake has produced 
a flat-bottomed, comparatively level valley, as shown in Fig. 3. 
The surface of this valley is now about 400 feet higher than the 
