28 EEPORT UNITED STATES GEOLOGICAL SURVEY 



which the molten material was poured extended along what is now 

 the southern base of Sepulchre Mountain, extending thence across the 

 space occupied by Gardiner Kiver to the highest point of Mount Evarts, 

 and thence in an irregular line up the valley of the Yellowstone, but 

 always to the south of the present bed of the river. At that period 

 the valley of the Yellowstone did not exist, but in its place were the 

 foot-hills of the Great Yellowstone Range, rising gradually from the 

 brink of the lava lake, the borders of which I have partly defined. Why 

 the valley of the Yellowstone happened to be formed where it now is, 

 I shall not now attempt to say. I am convinced, however, that its 

 formation was subsequent to the period of rhyolite flows, and that at 

 the time it was laid out the waters of the Gardiner River Valley flowed 

 out to the north on the upper surface of the lava floor until the Yellow- 

 stone was reached. The latter has since cut a bed 2,000 feet in depth 

 along the northern border of the rhyolitic sheet, and the great valley 

 of the Lower Gardiner is but a notch cut in the edge of the sheet. At 

 three points, where the branches of the river descend, this cutting is 

 still actively going on, but at a much retarded rate, as every foot of 

 advancement to the south gives increased thickness to the lava cap and 

 diminished thickness to the friable shales beneath. The three branches 

 of the river are formed to the west, upon the upper surface of the floor 

 of rhyolite, and before their union reach its edge and descend by splen- 

 did cascades the steep faces into the valley beneath. There has doubt- 

 less been a time when they formed a junction before descending from 

 the plateau, and it is not improbable that one fork at least — the eastern 

 — reached the Yellowstone without forming a junction with the other 

 branches by passing down to the east of Mount Evarts. One cannot 

 pass along the trail which leads through the low pass from the East 

 Fork to the valley of Black tail-deer Creek, without being convinced that 

 at one time a stream occupied the depression. It is not impossible that 

 the Yellowstone has occupied this part of the valley at one time, as the 

 coarse river drift is everywhere found at and above this level from 

 Mount Evarts to the Great Falls, but it is probable that the East Fork 

 of Gardiner River occupied it much more recently. Such a change in 

 the course of this stream could have been brought about by the deep- 

 ening and expansion of the valley of the Middle Gardiner, which was 

 first to penetrate the harder strata of the surface- 

 Now, there are a number of interesting facts that bear upon the ero- 

 sion of these great valleys as to time. In the first place, the river drift, 

 both of the Yellowstone and Gardiner Rivers, is found everywhere upon 

 the upper surface of the rhj^olite plateau, and that drift contains granitic 

 materials which are not exposed on the south side of the river at as great 

 a height as the summit of that plateau by fully 1,000 feet. It therefore 

 had its origin either far up the river in the East Fork region or upon 

 the northern side, lower down. In either case the river must have been 

 at or above the level of the present plateau. It therefore seems plain 

 that the present river valleys, 2,000 feet in depth, have been eroded 

 since the period of rhyolite flows of the park district. Again, the very 

 noticeable series of terraces that occur on the west side of Gardiner 

 River, and on the northern side of the Yellowstone from Cinnabar Mount- 

 ain to Bear Gulch and above, reach to the level of the rhyolite plateau, 

 but apparently no higher. 



Another interesting fact that should be noticed is this, that the great 

 sheet of hot spring limestone capping the long ridge above Gardiner's 

 Springs is on the upper surface of the rhyolite, and that its debris is 

 mingled with the drift of the terraces. We have here, therefore, a very 



