Mountains on the Move

Northwest of Sula, the Bitterroot Mountains expose granite of the Idaho batholith, a series of igneous intrusions emplaced between about 80 and 53 million years ago as magma forced into older rocks and crystallized more than ten miles below the surface. As this magma rose, it uplifted overlying rocks and shed an enormous block that slid eastward over about seven million years, forming the Sapphire Range on the east side of the Bitterroot Valley; granite along US Highway 93 near Sula is part of that displaced block. About 50 million years ago, magma again rose through the crust and produced large volcanic eruptions in the southern Bitterroot Range southwest of Sula. Later, glaciers covered much of the Bitterroot Valley, carved U-shaped side drainages, and, during the glacial ages, repeatedly dammed the Clark Fork River near the Idaho-Montana border to create Glacial Lake Missoula, whose highest stand reached 4,200 feet above sea level and left a shoreline only a few miles downstream of Sula; the last glaciation ended about 15,000 years ago. The Sapphire Range later took its name from sapphire deposits mined in the early 20th century, with more than 40 tons sold for watch bearings before synthetic sapphires collapsed the market after World War II.

Montana · United States