A sandbank is a geological feature that is formed when sediment accumulates in shallow water. It is usually composed of sand, gravel, or silt and can be found in both freshwater and marine environments. The term 'sandbank' can also refer to a BAR, SHELF, SHOAL, or HURST, all of which are related features. A BAR is a ridge of sand or gravel that is submerged beneath the surface of the water. A SHELF is a shallow area of the ocean floor that extends outward from the shoreline. A SHOAL is a shallow area of the ocean floor that is covered by water. Finally, a HURST is an elevated area of the ocean floor that is surrounded by deeper water.