What Forms When a Spit Grows Across a Bay?

When a spit—a narrow depositional landform of sand or shingle extending from the shore—grows across a bay entrance, it cuts off the bay from the open sea. The enclosed water body becomes a lagoon (or bar-built lagoon). If the spit fully closes the gap, a barrier bar or baymouth bar forms. The lagoon may become brackish or fresh, develop marshes, and eventually infill to become a coastal plain. Classic examples: Chesil Beach (England), Lagoa dos Patos (Brazil), and many Gulf Coast lagoons.