Early Proterozoic Geology of the Highland Mountains, Southwestern Montana, and Field Guide to the Basement Rocks that Compose the Highland Mountain Gneiss Dome

Route

Start point

Cardwell, MT

End point

Dougherty Butte, southern Highland Mountains

Roads

I-90

Total distance

63.6 miles

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Geology

Summary

The Highland Mountains are underlain by the largest of the northwesternmost exposures of basement crystalline rocks in southwestern Montana....Metasedimentary rocks in the Highland Mountains are in part lithologically similar to the Late Archean multilithologic sequence (in the Tobacco Root, Ruby, and the northern Madison and Gravelly Ranges), but are considerably thinner. In the Highland Mountains the individual beds of aluminous schist, marble, quartzite, and iron-formation extend for only short distances, but the assemblage as a whole is mappable. It nowhere exceeds 300 ft (100 m) in thickness and appears to pinch out to the north....these rocks in the Highlands, unlike those to the southeast, are overlain by more than 10,000 ft (3000 m) of aluminous biotite gneiss that may have been deposited as muds basinward from the Late Archean shelf edge.

Key Lithologic Features

Structures

Landforms

Other Features

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Reference

O'Neill, J.M., 1995, Early Proterozoic geology of the Highland Mountains, southwestern Montana, and field guide to the basement rocks that compose the Highland Mountain gneiss dome: Field guide to geologic excursions in Southwest Montana , Northwest Geology, v. 24, p. 85-97.

Availability

Order from the Tobacco Root Geological Society. Go to the List of available TRGS publications for ordering information.