Bar (O. Fr. barre, Late Lat. barra, origin unknown), in physical geography, a ridge of sand or silt crossing an estuary under water or raised by wave action above sea-level, forming an impediment to navigation. When a river enters a tidal sea its rate of flow is checked and the material it carries in suspension is deposited in a shifting bar crossing the channel from bank to bank. Where the channel is only partly closed, a spur of this character is called a "spit." A bar may be produced by tidal action only in an estuary or narrow gulf (as at Port Adelaide) where the tides sweep the loose sand backwards and forwards, depositing it where the motion of the water is checked. Nahant Bay, Mass., is bordered by the ridge of Lynn Beach, which separates it from Lynn Harbor, and ties Nahant to the mainland by a bar formed in this way.