48 GEOLOGICAL SUEVEY OF THE TERRITOEIES. 



only in detacbed portions that seem to illustrate a series of remarkable 

 faults. 



The Arkansas Valley, from its head in Tennessee Pass to the point 

 where the river cuts through Front range and opens out into the 

 plains, has been worn out of the granite mass to a great extent. 

 The valley is partly a fissure, but is mostly due to erosion. The drain- 

 age was undoubtedly started by the fissures produced by the great up- 

 lift, but, as broad and deep as it is, it is undoubtedly due mostly to 

 erosion, and by this illustration we may form some conception of the 

 work of this powerful agent in giving form to the surface of this mount- 

 ain-region. From the crest of the Park range across the Arkansas 

 Yalley to the crest of the Sawatch the distance will average from ten 

 to fifteen miles, in a straight line, and the average elevation above the 

 water-level of the Arkansas Eiver must be about 1,500 feet. Now, it is 

 probable that three-fourths of this vast space from the Tennessee Pass 

 to the Poncho Pass, near the head of San Luis Yalley, a distance of 

 one hundred miles, has been worn out by erosion, and the greater por- 

 tion of the material carried down the river and distributed over the 

 plains. It is probable, also, 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 mountains on either side of the valley extend- 

 ing nearly to the summits, the remarkable morainal deposits in the main 

 valley and on the sides of the gorges, point strongly to that conclusion. 

 We hope in the succeeding chapter to describe more in detail th'j phe- 

 nomena of ancient glacial action, which is so admirably shown on both 

 sides of the Sawatch range. 



The remarkably rounded and grassy appearance of these high mount- 

 ain-ranges in many instances is quite surprising, and we ask how so 

 great a thickness of superficial earth could have accumulated so far 

 above timber-line? Besides, this drift-like deposit is covered with 

 masses of rock of various sizes, more or less worn, mostly granitic, and 

 mingled with the finer materials are numerous bowlders, so that there 

 must have been some agent that acted quite generally in grinding down 

 the surface. All along the west side of the Park range the granite 

 rocks crop out, but from a point opposite Tennessee Pass to Bufialo 

 Peak this old glacial deposit covers a great portion of the surface. 

 When the underlying or basis rocks do crop out to any extent the ab- 

 rupt side faces west foward the Arkansas, and the gentler slope is 

 toward the east, so that even the granitic nucleus testifies to the anti- 

 clinal character of the range. This is very well shown on the west 

 side of Buffalo Peak, and southward where the granitic rocks rise in 

 high, conical peaks, with the abrupt, wall-like face to the west, and 

 sometimes even overhanging at the summit. The heavy snows that fall 

 on these mountains, melting in the spring, thoroughly saturate these 

 surface-deposits, and great masses become more or less movable, de- 

 pending upon the steepness of the slope. The degradation of the under- 

 lying rocks is constantly going on, and the movements of the great 

 masses of earth produce results much like those of a glacier. 



About five miles below the mining town of Granite the upper valley 

 of the Arkansas begins gradually to expand in width. Terraces have 

 been formed on either side, which show the former existence of a lake. 

 It is most probable that the lake- waters set high up the Arkansas Biver, 

 even to its source 5 but the greater portion of the waters were gathered 

 into the lower part. By Upper and Lower Arkansas I mean the portions 

 above or below the canon. In the lower portion of the Upper Arkan- 



