The mineral form of sodium chloride (NaC1), or rock salt. Halite occurs, sometimes to considerable thicknesses, in many buried-rock successions, from which it has been extracted both by mining and by redissolving it in water pumped from and back to the surface. The existence of brine springs indicates that natural water movement occurs through buried halite sequences, presumably through voids that could be thought of as caves. Although distinctive halite (or salt) karst features are known in some arid areas, a range of features analogous to those found on karstic rocks such as limestone are unlikely to form, and less likely to be preserved, due to halite's relative weakness and very high solubility. In Britain expressions of salt karstification are limited to relatively subdued surface features. The "flashes" of the Cheshire area, are hollows, sometimes transformed into water-filled meres, formed by subsidence of overlying rocks and superficial deposits where salt has been dissolved from buried halite beds of Triassic age .