360 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



formed Obsidian Cliff, 1 whose shining black columns of glass rise 100 feet 

 above the road. 



The cliff stretches for half a mile from the outlet of Beaver Lake 

 along the east side of Obsidian Creek, being 150 to 200 feet high near the 

 lake and becoming lower northward. The upper half is a vertical face of 

 rock, the base of which is obscured by de'bris of large blocks of the same 

 material. The obsidian sheet extends eastward up the rude benches to the 

 top of the plateau 400 feet above Beaver Lake. Along the west edge of 

 this table-land it forms a cliff about 50 feet high, which extends south to 

 the Lake of the Woods. 



Following the obsidian back from the face of this upper cliff, over the 

 hummocky surface of the plateau, the black glass becomes filled with gas 

 cavities and passes into banded pumiceous rock, and finally into light-gray 

 pumice. This covers the surface of the plateau for 2 J miles eastward, to 

 the valley of Solfatara Creek. Here again ■ the lava flow is exposed in a 

 cliff, the lower portion of which is black and red obsidian. Toward the 

 south the obsidian flow extends a mile beyond the Lake of the Woods, and 

 northward it crosses the east-west drainage that cuts off the higher portion 

 of the plateau a distance of some 5 miles. The original thickness of this 

 lava flow is not known, since the upper pumiceous portion has been eroded 

 to a variable extent. The denser obsidian portion is from 75 to 100 feet 

 thick. 



The point at which the obsidian broke through the older rocks has not 

 been discovered, but it is evident that the lava forming Obsidian Cliff 

 flowed down from the high plateau in a northwest direction into a preexist- 

 ing valley. The planes of flow in the lava clearly indicate that it crept 

 down the slope back of Obsidian Cliff and accumulated in the bottom of a 

 channel between rhyolite hills. 



The most noticeable feature of this body of obsidian is its columnar 

 structure, which is confined to the southern end of the cliff. It is shown 

 in PI. XXXIX. The glassy columns rise from a talus slope that extends 

 50 feet up the cliff. They are vertical prisms 50 or 60 feet high, and vary 

 in width from 2 to 4 feet near the south end of the cliff, the width of each 

 column being quite constant throughout its length. On the south face of 



; hidings, J. P., Obsidian Cliff, Yellowstone National Park: Seventh Ann. Eept. U. S. Geol. 

 Survey, 1888, pp. 249-295. 



