Sand is never stationary, constantly moving with the wind-driven water along the seashore, piling up as sandbars, hooks, necks, ﬁngers, and points. Currents that stream parallel to the shore build up land by carrying their sediments along the coast. When the outline of the coast changes abruptly, the current cannot change quickly enough to accommodate it; through the force of its inertia, the current continues in a straight line, depositing sand into deeper water, creating a spit. Spits cannot grow indeﬁnitely. Their creating currents are eventually overpowered by swifter currents. Provincetown, Massachusetts, is at the end of a long, dramatically curved spit—Cape Cod.