U-shaped Valley

A U-shaped valley, also called a glacial trough, is a valley with steep, nearly straight sides and a flat or rounded floor, in cross section resembling the letter U rather than the narrower V profile cut by a river. It forms when a glacier moves down a valley originally carved by a river and scours away the rock through which it flows, a transformation from a V shaped to a U shaped profile that can take between ten thousand and one hundred thousand years. The glacier preferentially removes the rock offering the most resistance to the moving ice, shaping the valley into a smooth parabolic form that lets the ice flow with the least friction. Once the ice retreats, the trough is often left holding glacial till, and may fill with water to form a lake, or, where it reaches the sea, a fjord.