By editor
Ravalli County
The eastern face of the Bitterroot Mountains rises like a wall above the Bitterroot Valley. This is not a typical mountain range formed by compression; it is a metamorphic core complex formed by extreme extension. Roughly fifty million years ago, the crust here began to pull apart. The upper layers of rock slid eastward along a massive, low-angle fault known as a detachment fault. As the upper crust slid away, the deep, hot rocks of the middle crust were dragged to the surface.
Visitor Access
The Bitterroot front is largely public National Forest land, free and open year-round for hiking and geology viewing. While you cannot collect the mountain itself, you can hike the steep canyons and run your hands over the smooth, sheared mylonites—rocks that were stretched like taffy under the intense heat and pressure of the deep crust before being exhumed.