180 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



Mount Hancock its mode of weathering is well brought out by the many 

 abrupt walls of rough, jagged surface and numerous deep trenches along 

 the mountain side, which make travel very difficult and well nigh impossible. 

 The breccia, while closely resembling the imposing mass of Two 

 Ocean Plateau, and probably allied to it in age, is a distinct body, having 

 its source along lines of displacement west of the Big Game Ridge uplift 

 and the continuation northward of the Bobcat Ridge fault, It was the result 

 of a powerful local eruption of coarse agglomerate, uniform in its composi- 

 tion, violent in its explosive action, and oidy dimmed in interest by its close 

 proximity to the larger masses of the Absaroka Range. It consists mainly 

 of pyroxene-andesite and basaltic fragmental material. 



Near the source of Coulter Creek broad sheets of purple lithoidal 

 rhyolite occur, resting directly upon these basic breccias. On the west side 

 of the stream there occurs an isolated body of rhyolite, lying on the breccia 

 and completely surrounded by it, This exposure is interesting, as it is 

 evidently a remnant left by erosion, and probably at one time was con- 

 nected with the larger fields to the east. High up the mountains Coulter 

 Creek bifurcates and the two branches encircle a broad table of rhyolite 

 which lies in the middle of the mountain valley. It is not known on what 

 this rhyolite rests; the base of it is being deeply buried beneath an accumu- 

 lation of glacial drift and coarse quartz gravel, derived from the Eocene 

 conglomerate of Pinyon Peak. 



Bobcat Ridge. — Bobcat Ridge is one of the marked physical features of 

 this part of the country, standing out prominently above the surrounding 

 rido-es bv reason of its great elevation. The ridge trends northwest and 

 southeast, and measures about 6 miles in length. It is a singularly narrow 

 ridge, having an average elevation of over 9,000 feet, with a number of 

 peaks scattered along the summit, which attain altitudes of 9,500 feet or 

 more. At its northern end it is connected with the Wildcat Peak mass and 

 the high country around the head of Coulter Creek. At its southern end it 

 falls away in long dreary spurs toward the valley below, lying just outside 

 the limits of the area shown on the map. Both the east and west sides of 

 this ridge are abrupt and present a rather dreary, monotonous aspect, with 

 little standing forest, as the mountain sides over large areas are covered 

 with fallen and dead timber. 



Geologically Bobcat Ridge has not been studied much in detail. A 



