The cross section of an ice-carved valley is U-shaped. U-shaped valleys, also called parabolic valleys, often began their histories as V-shaped river valleys whose sloping sides were steepened and whose narrow ﬂoors were broadened by glaciers that bored out their contours. As valley glaciers advance down a river valley, they blade the bottom ﬂat, destroying and polishing rock walls as they do so and pushing the debris into high fenders called moraines. Gradually, the vertical walls and wide, slightly rounded bottom of a U-shaped valley are formed. When a valley glacier meets the sea, and then melts back during an interglacial, rising waters can ﬁll the valley, creating a fjord. U-shaped valleys are also called glaciated valleys, and classic examples exist in Glacier National Park in Montana and throughout the Rocky Mountains.