Valleys made by water are, in cross section, shaped like a giant V. Water, depending on its velocity, cuts down into rock, engorges itself in rock, and keeps cutting. The Grand Canyon of the Yellowstone is an example of a V-shaped valley. There, water spills out, tumbles, pools in rock basins, spills again, eventually carving more depressions into rock. When these basins overﬂow, the cutting through earth and rock begins again; water coils in a sinuous braid, drops again, and cuts deeper. Some rivers tend to ﬂow in a straight line; thus the sediments they carry quickly erode the rocks below and carve a classic V-shape. (This pattern is, however, sometimes interrupted by climate change or tectonic activity.) Other rivers ﬂow more slowly, cutting from side to side, creating the common S-curves characteristic of these rivers.