The Lost River Fault runs along the base of the Lost River Range in central Idaho, at the foot of Borah Peak, the state's highest summit. It is a normal fault marking the eastern edge of the Basin and Range province, where the crust stretches and mountain-front blocks drop down along steep dipping planes.
Over hundreds of thousands of years, repeated slip on this fault has lifted the range and deepened the adjacent valley, producing the classic tilted-block topography of the region. Fresh scarps at the range front reveal the accumulation of many past ruptures.
The fault produced the 1983 Borah Peak earthquake (magnitude ~6.9), the largest historical quake in Idaho. The rupture created a spectacular fault scarp up to a few metres high along the mountain base and killed two schoolchildren struck by falling debris in the town of Challis.