1880.] Twin Lakes and Teocalli Mountain, Central Colorado. 839 
were undoubtedly scooped or worn out of the granite rocks by 
glacial action. They afford a splendid example of what Prof. 
Ramsay, the eminent geologist of England, calls “ Rock Basins,” 
the origin of which he has so graphically explained in his volume’ 
on the “ Physical Geography and Geology of Great Britain.” In 
the Upper Arkansas valley there seems to have existed in glacial 
times, one immense glacier, rising to the height of 1000 to 1500 
feet on the mountain sides, and filling up the entire valley, with 
tongues or branches extending up the numerous side cafions. A 
description of this remarkable district may be condensed from 
the Report of the U. S. Geol. and Geog. Survey of the Territo- 
ries for 1873 and 1874. The Arkansas valley, from its head in 
Tennessee pass to the point where the river cuts through the 
Front or Colorado range and opens out into the plains, has been 
sworn out of the granite mass to a great extent. The origin of 
‘this valley is mostly due to erosion. From the crest of the Park 
‘range, on the east side of the Arkansas river, to that of the Wa-. 
satch on the west, the average distance in a straight line must be 
sat least ten or fifteen miles, and the average elevation above the 
water level of the river 1500 feet. It is probable that this great 
Space was, at no very ancient period, filled with one vast glacier, 
which doubtless performed the greater part of the grinding up of 
the rocks and the wearing out of the valley. The glacier-worn 
sides of the gorges, point strongly to that conclusion. 
But in this brief article we must confine ourselves mostly to 
the limited district, the valley of Lake Fork, in which the Twin 
lakes are located, the subject of the illustration. The valley of 
Lake creek is filled with the morainal deposits for which both 
sides of the Wasatch range of mountains are so remarkable. It 
would seem that the great glacial force moved here in a direction 
a little south of east, inasmuch as the mass of the detrital mat- 
ter is heaped up on the south side. The two lakes are about 
three hundred and fifty yards apart, with a small stream flowing 
from the upper into the lower, about twenty feet in width. The 
interval is made up of worn detrital matter, but over it and 
around both lakes, are mounds or oblong ridges of drift; and scat- 
tered over the surface, are masses of granite, coarse in texture, 
with crystals of feldspar, one and two inches in diameter, aggre- 
gated together. The rock has the appearance of a feldspathic 
breccia. The lower lake is about two and a-half miles in length | 
