Geology of the Absaroka Mountains
The Absaroka Mountains Today:

The Absaroka Mountains are named after the Crow Indians (Absaroka being the Indian name for Crow) who inhabited much of south-central Montana prior to the white man's entrance into the area.
They are characterized by rugged, exposed peaks (Mt. Cowan is the highest in this range at 11,206 feet) and forested valleys.

Geologically, the Absarokas are dominated by stratified volcanic rocks rich in petrified wood and geodes. The volcanic eruptions that are thought to have formed the rock in this range were rather "quiet" - oozing lava out over the landscape rather than exploding. Events such as landslides and floods may have carried rubble down slopes to create a chaotic mixture of rocks, ash, and mud that hardened into a rock called breccia. This volcanic breccia has the appearance of pebbles of various sizes welded together in a brown matrix. Other rocks formed from andesite lava flows, basalt flows, airfall ash, mudflow conglomerate, stream gravel, and sand which combine to form the Absaroka Volcanic Supergroup. The range Absarokas2.jpegis riddled with volcanic dikes and some, like the Chineese Wall along the North Fork of the Shoshone, stand out against their weathered surroundings. Outcrops of bright limesone stand out against the brown volcanic background. Example of these outcrops include Sheep and Logan Mountains west of Cody. In addition to volcanic breccia and limestone, other rocks such as basalt, rhyolite, andesite, granite, sandstone can be found.

Of special interest in this range is the amount of petrified wood and fossilized trees. Many of these fossilized trees were actually preserved upright while others were transported by streams or mudslides in a horizontal position. On the western slopes of the Absarokas, in the Lamar Valley of Yellowstone National Park, entire fossilized forests can be observed. Not only were trees preserved, but leaves, needles, cones, and pollen grains have been preserved and identified revealing over 200 plant species. Included in these are: spruce, fir, pine, breadfruit, avocado, dogwood, maple, oak, hickory, and redwood.

Theoretical Formation:

It is thought that the formations described above resulted from volcanic activity from 50 to 40 million years ago. Following this period of volcanic activity and mountain building only uplift and weathering/erosion occured until the next volcanic cycle which started around 2.5 million years ago and included three "supervolcanoes" and produced the Yellowstone Plateau volcanic field. Several smaller volcanic episodes happened between the three big eruptions and in the time since then. Many of these produced the rhyolitic flows that blanket the park today. One of these smaller eruptions, for example, produced the caldera that is filled in by the West Thumb of Yellowstone Lake today.

Weathering and erosion, particularly due to glaciation and abrasion by rivers has helped carve the landscape we see today. The Pinedale glaciation, for example, appears to have covered most of the park in ice and came as far east as Pahaska. The Northfork of the Shoshone River then carved away the landscape from there eastward to Buffalo Bill Reservoir and the canyon just west of Cody.

1. Yellowstone: A Visitor's Guide. Stackpole Books. 1992
3. Fritz, William J. "Roadside Geology of the Yellowstone Country" Mountain Press Pub. 1985
4. Good, John M & Kenneth L. Pierce. "Recent & Ongoing Geology of Grand Teton & Yellowstone National Parks". Grand Teton Natural History Assn. 1996