Aerial view of part of the Sevier Fold-Thrust Belt. Sun River Canyon, Montana. Looking north.
Each of the ridges on the right side of this photo is underlain by Mississippian-aged carbonate rock which has been thrust over Cretaceous rock by a series of north-trending thrust faults. As the Cretaceous rock is relatively easy to erode, it tends to form valleys. Towards the west (left), the thrusts cut down section, so the hangingwall rock is older.