The region along the headwaters of the Yellowstone, Shoshone, Greybull, Wood, Wind, and Snake rivers in northwestern Wyoming is perhaps the least known and certainly the least appreciated mountain area in the state. Unlike the other mountain uplifts in the state, the Absaroka Range is composed entirely of volcanic rocks. The summit of the Absaroka Range is essentially a plateau coinciding with the top of a vast pile of nearly horizontal sheets of rock derived from volcanic vents only a short distance away. This broad rolling upland above timberline, at about 10,000 to 12,000 feet above sea level, is reminiscent of the moors of Scotland. Looking across a landscape of bogs, small lakes, and numerous small streams, there is no sense of height and few peaks. Only when one looks down into a major valley does the great vertical relief become apparent, and only then can the mountainous character of the terrain be appreciated. Access to this high plateau is practically impossible except by horseback or on foot, but the lower parts of the range can be observed from the highway over Togwotee Pass northwest of Dubois, from the North Fork and South Fork roads west of Cody, and from the Chief Joseph and Beartooth highways.
The Absaroka Range is a large (9,000-square-mile) volcanic field that contains up to 5,000 feet of Eocene andesitic, basaltic, and dacitic volcaniclastic rocks, along with some air-fall tuffs and related intrusive rocks. They lap onto and in some places bury parts of the Beartooth and Gallatin ranges in the north and the Wind River, Washakie, and Owl Creek ranges in the south. Quaternary rocks of the Yellowstone volcanic area bound the area to the west and the Bighorn Basin bounds the area to the east where the volcanic rocks cover and obscure the underlying sedimentary sequence. Volcanic rocks of the Absaroka Range are not related to the much younger Yellowstone volcanic rocks, except that the Yellowstone hot spot has obscured and possibly melted the former western extent of the Absaroka volcanics.
A series of igneous intrusions cuts across the Absaroka volcanics along a northwest-trending line, roughly along the crest of the range. A striking example is the Washakie Needles in the southern Absarokas near the headwaters of the South Fork of Owl Creek. These intrusions, consisting of stocks, plugs, laccoliths, and radial dike swarms, were associated with four primary vent areas for the stratovolcanoes and shield volcanoes that sourced much of the volcanic materials. These vent areas are also sites for important mineral deposits, including the Silver Crown, Stinkingwater (Sunlight), Eagle Creek, and Kirwin mining districts.
During deposition of the thick pile of volcanic debris and the 45 million years since, the region has experienced the ever-present forces of erosion, which cut deep valleys into the range, leveled off the highest peaks, and spread volcanic debris into the surrounding areas. Erosion into the crudely stratified and poorly consolidated volcanic debris was very rapid—the Absarokas are the remnants of this erosion and provide a classic example of this type of mountain development. Like other high mountainous areas in Wyoming, much of the Absaroka Range contained permanent ice fields and/or active glaciers during the ice ages of the last million years. Because of the easily eroded nature of the Absaroka rocks, however, any evidence of glacial activity has been largely destroyed, resulting in very steep slopes, sharp, jagged outcrops, and few “u-shaped” valleys that indicate glaciers.