A Study in Contrasts: Archean and Quaternary Geology of the Beartooth Highway, Montana and Wyoming
Start pointNo route given
End pointNo route given
RoadsBeartooth Highway (U.S. 212)
AccessThe road is paved and well maintained, but is usually closed by snow from middle September to mid-May. Land adjacent to the highway is National Forest.
Total distanceNo route given
SummaryNowhere in the U.S. are the oldest and the most recent aspects of geology as spectacularly displayed as along the Beartooth Highway. The Beartooth Mountains are a block of largely Archean bedrock uplifted along high-angle reverse faults of Laramide age. The Precambrian rocks (3400-700 Ma) contain one of the best records of the early history of the igneous and metamorphic basement of the middle Rocky Mountains. These rocks include granulite-facies supracrustal rocks proposed as products of continental collision, calc-alkaline volcanic and plutonic rocks generated along an Archean continental margin, and mafic dikes some of which were emplaced during continental rifting. This Precambrian record encompasses at least two extensive periods of crustal evolution and records more geologic history at one location than any other place in the Wyoming Province. Descriptions of two key areas showing the fundamental relations between the Precambrian rocks are included: the Quad Creek area and the Long Lake area.
The Beartooth Plateau has been extensively sculpted by glacial processes during the Pleistocene. The highway crosses a classic locality of "biscuit-board topography"--plateau remnants partially dissected by cirques--as well as deposits left by glaciers that etched the plateau. Features to be seen include glacio-fluvial terraces showing downstream effects of the glacial system and extensive areas of periglacial features that postdate glaciation. Glacial features are described from the Red Lodge area, Rock Creek Canyon, the hairpin turns, the Beartooth Plateau, and the Clark Fork basin.
Key Lithologic Features
- Archean metasupracrustal rocks, including mafic amphibolite, quartzite, tonalitic gneiss, granitic gneiss, ironstone, and pelites
- Long Lake Granite with inclusions of andesitic amphibolite and granodiorite
- mafic dikes
- glacial sediments
- shear zones
- glacial landforms galore: moraines, glacial terraces, linear erosional features, cirques, hanging valleys, "biscuit-board topography", periglacial terrain, U-shaped valleys, roches moutonnees, and glacial lake basins