Overthrust
Part of the Lewis Overthrust in the Rocky Mountain Front. This is in Montana. Gibson Reservoir (west of August, Montana) is visible juuuuuust to the right.
The Lewis Overthrust started during the formation of the Rocky Mountains 170 million years. Colliding tectonic plates created it: As the Pacific Plate and the Plate of Juan de Fuca put stress on the North American Plate, a huge rock wedge pushed eastward more than 50 miles (80 km). The wedge was several miles thick and several hundred miles long, and it slid eastward over softer rocks that were more than 1.4 billion years younger than the wedge.
The characteristic "crumpled" look of the overthrust came about as the wedge pushed forward, then relaxed and fell back as stresses west of the wedge eased. When they built up again, the wedge pushed another "wave" of rock and soil forward.
Spectacular formations such as Chief Mountain are the result of the Overthrust (named for Meriwether Lewis, who first scientifically documented its characteristics).