A subsurface layer of rock permeable by water. Although gravel, sand, sandstone and limestone are the best conveyers of water, the bulk of the earth's rock is composed of clay, shale and crystalline. A saturated permeable material (often sand, gravel, sandstone or limestone) that contains or carries groundwater. An underground, water-bearing layer of earth, porous rock, sand, or gravel, through which water can seep or be held in natural storage. Aquifers generally hold sufficient water to be used as a water supply.