Geological Survey 7th Annual Report Obsidian Cliff, Yellowstone National Park

Obsidian Cliff is at the northern end of Beaver Lake, in the Yellowstone National Park, about eleven miles south of Mammoth Hot Springs. It forms the eastern wall of a narrow cut in the plateau country through which Obsidian Creek flows at an elevation of 7,400 feet. The cliff extends for half a mile, rising from one hundred and fifty to two hundred feet above the creek and falling away gradually to the north; the upper half is a vertical face of rock, the lower portion a talus slope of the same material. Back of the cliff to the east the country rises in a series of rude benches to about four hundred feet above Beaver Lake; at this level, a little south of Obsidian Cliff, the edge of the plateau forms a small cliff fifty feet high, above a long, steep slope east of the lake. From here the top of the plateau rises in hillocks and basins eastward to an altitude of about eight thousand feet above sea-level. On the opposite side of Beaver Lake and Obsidian Creek abrupt hills slope up to the plateau country west. The whole region in this vicinity is thickly timbered with a small growth of pines.

The cliff presents a partial section of a surface flow of obsidian which poured down an ancient slope of rhyolite from the plateau lying to the east. The underlying rhyolite has a purplish-gray color and is readily distinguished from the black obsidian, as well as from the lithoidal portions of the obsidian flow, by the abundance of porphyritic crystals of quartz and feldspar which fill the older rock, the later flow being entirely free from them. The older rhyolite is not exposed beneath the obsidian along the creek, but is first met within the edge of the timber south of the end of the cliff, where a narrow drainage channel has cut into the slope. This rhyolite rises higher to the south and forms the long slope east of Beaver Lake, above which is the low cliff already mentioned. This cliff is of obsidian, which is exposed in a vertical section of more than fifty feet. Following the obsidian back from the face of this cliff up the hummocky surface it becomes filled with gas cavities and passes in to banded, pumiceous rock and finally into light-gray pumice. This covers the surface of the plateau for two and a half miles eastward to the valley of Solfatara Creek, which drains into the Gibbon River; 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 for a mile beyond the Lake of the Woods, and northward across the east and west drainage, which cuts off the higher portion of the plateau, a distance of some five miles.

What was the original thickness of this lava sheet it is not possible to say. The dense glass, or obsidian, forming the lower portion is from seventy-five to one hundred feet thick; the porous and pumiceous upper portion has suffered more or less erosion, which was in part the result of ice action, the evidence of glaciation being more marked along the lower western slope of the plateau than on the top of it. The surface of the plateau is mostly pumice, with little if any glacial débris scattered over it, but along the western slope the rock has been worn down to the massive obsidian, and the top of the cliff is covered with planed and striated glacial drift from a great variety of sources.

Half a mile southeast of Obsidian Cliff, on the plateau, about five hundred feet above the level of Beaver Lake, is a circular pit 100 feet deep, the mouth of it being 300 feet wide by 350 feet long; its sides stand at an angle of 35° and appear to be formed of pumiceous obsidian, the angular masses in the bottom being pumice. The rim of the pit does not rise above the level of the surrounding surface, and one comes upon it quite unexpectedly in the timber. The general appearance is that of a small crater which has been but slightly affected by glaciation.

Near by to the south is a short, narrow ridge of porous obsidian, rising two or three feet above the general surface. The ridge curves considerably and is cracked along its center. The only other surface features characteristic of a lava flow which are found in this vicinity are several small basins with obsidian rims; one is 20 feet long by 10 feet wide and about two feet deep, and within it at one end is a smaller basin, six by three feet.

The exact point at which this obsidian broke through the older rocks and reached the surface has not yet been discovered; but that forming Obsidian Cliff has evidently flowed down from the high plateau in a northwest direction into a pre-existing valley, the planes of flow in the lava clearly indicating that it has crept down the slope back of Obsidian Cliff and accumulated in the bottom of a channel between rhyolite hills. This old channel was just east of that occupied by Obsidian Creek, which has cut its way along the contact between the rhyolite to the west and the obsidian. This is shown by the fact that the general bedding or flow structure of the columnar portion along the face of the cliff dips slightly to the east away from the western body of rhyolite.