1. A narrow, wall-like ridge, 3-7 feet (1-2 meters) high, built up by waves along the seaward edge of a reef flat, and consisting of boulders, shingle, gravel, or reef rubble, commonly capped by dune sand.

2. A wall-like ridge of unconsolidated material formed along a beach by the action of strong waves and current.

3. A crescentic or ringlike deposit of pyroclastics around the top of a volcano.