The Classic U-Shaped Valley
Glacial valleys are best explained in contrast to their fluvial cousins: v-shaped valleys. Rivers carve v-shaped valleys because their erosional power is concentrated at the base of their channels, creating straighter valley walls. Glaciers, in contrast, apply approximately equal erosional forces to the entire valley within which they reside. This causes erosion both at the base of the valley and the sides to happen at relatively the same pace, resulting in curved valley walls.