Geology
The rocks of the Elkhorns were formed about 74 to 81 million years ago (Late Cretaceous time) as a result of the Farallon tectonic plate subducting beneath western North America and allowing magma to rise to the surface. The Elkhorn Mountains Volcanics are extrusive rocks related to the plutonic granites of the Boulder Batholith. Volcanic flows, lahars, and ash falls from sources in the Elkhorn Mountains reach as far as Choteau, Montana, but the thickest deposits lie within a radius of about 60 miles (97 km) from the Elkhorns. The volcanics probably originally covered an area of about 10,000 square miles (26,000 km2). Mineral deposits associated with the Elkhorn Mountains Volcanics include those mined at Elkhorn, Montana, now a ghost town, and gold at the Golden Sunlight Mine near Whitehall, which is associated with a breccia pipe in the volcanics.
The mountain range seen today is related to regional structural uplift dating mostly to Oligocene time.
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