The property of sediments and rocks that allows the movement of water through them. Permeability is related to the size of openings and fissures in solid rock; the nature of the particles in unconsolidated organic and inorganic sediments, and the extent to which the void spaces are interconnected. High yielding aquifers usually have some rock layers or fractures with high permeability. Permeable materials, such as gravel and sand, allow water to move quickly through them, whereas impermeable material, such as clay, don’t allow water to flow freely.