Pegmatite

Rocks of the southern Sapphire Mountains located east of Hamilton, Montana have had a bumpy ride. The rocks were first transported eastward horizontally as thrust sheets, during the Rocky Mountain Uplift in late Cretaceous time. Later, in Eocene time, the Sapphire tectonic block detached from a rising granitic batholith, known as the Bitterroot Core Complex, sliding farther east. Plutons, dikes and pegmatite-fracture-fillings further metamorphosed and deformed the rocks before they were exhumed and exposed by erosion. Ten kilometers of mountain erosion-unroofing (significant geologic time) allows us to see these large pegmatite crystals.