50 GEOLOGICAL SURVEY OF THE TERRITORIES. 



Lave given the name of La Plata, 14,302 feet above the sea. A& we 

 look in this direction, bare, brown, granite masses, rising above timber- 

 line, meet our eyes, with here and there a few patches of snow to break 

 the monotony or contrast with the desolate somber hue of the granite 

 debris. Deep furrows extend down the sides of the mountains, the chan- 

 nels for untold ages of ice, snow, and water, the agents which have bro- 

 ken down these rocky masses and sculptured the forms which now so 

 much excite our admiration. We see also the smoothly-worn sides of 

 the mountain covered with a sort of enamel-like crust, as a mark of the 

 glacial power. On the sides of the peaks, at different elevations, are nu- 

 merous small green lakes, sometimes with a visible outlet and some- 

 times without, reservoirs of the melting snows. The pines are often 

 dead from the autumnal fires that have run through, adding to the deso- 

 lation of the scene ; these falling down in every direction render travel- 

 ing almost impossible. Sometimes no vegetation takes the place of the 

 pines after the fires have passed through them, but not unfrequently 

 the quaking-asp poplar, with its bright green leaves in summer and yel- 

 low in autumn, grow very densely, contrasting most charmingly with 

 the somber green of the living pines, and the somber brown or gray of 

 the dead. Down in the valley, and closed in on either side by mount- 

 ains and the morainal ridges or hills, are the two beautiful lakes, which 

 are laid down on maps as Twin Lakes, the basins of which were no 

 doubt formed by glacial action. If the reader will examine the map 

 accompanying the report he will see more clearly than we can describe in 

 words the location of the beautiful lakes, the morainal ridges and mount- 

 ains that hem them in ; the contour lines are not intended to indicate 

 elevation, but are used to show the surface forms. As there are really but 

 two forms, granite and morainal drift, but one color is needed, and with 

 this we have endeavored to separate the surface covered with the 

 morainal deposit'from the granite. The map will also show the eleva- 

 tion of the lakes above the sea, 9,182 feet; also the elevation at the 

 junction of the Lake Fork and the Arkansas, 9,096 feet. The depths of 

 the lakes are shown by actual soundings. The greatest depth of the 

 upper lake is 79 feet, and that of the lower 75 feet. It will be seen 

 that the greatest depth of the lower lake is near the upper end. We 

 may thus see by the depth of these lake-basins, as well as their shape 

 and morainal deposits around them, that the force that produced them 

 all moved slowly down from the mountain-range, and that the lake-basins 

 are scooped out of the solid granite rocks. From the lower lake to the 

 Arkansas River the morainal deposits are very thick ; the surface is 

 covered with bowlders more or less, and of greater or less size, some 

 small, others from 20 to 50 feet in diameter. Hundreds of mounds, 

 ridges, and curious depressions, of all shapes and sizes, impede the 

 traveler. The placer-mining has been very extensive here, and by this 

 means we arrive at the true character of this glacial drift. It is com- 

 posed mostly of rounded bowlders, but mingled with it is a kind of light- 

 colored clay and sand. The decomposition of the feldspar has produced a 

 kind of clay, which sometimes gathers into localities forming a consid- 

 erable thickness. 



The description of the glacial action in the valley of this branch of 

 the Arkansas will apply to the others, and presents a general view of the 

 detailed action in the entire valley. The history of this valley from the 

 beginning may be in some points obscure, but, as I have stated in a 

 former report, I regard it as largely due to erosion. In the process of 

 elevation a fissure or fissures must have been formed, and in these the - 

 process of erosion commenced, continuing through a vast period of time, 



