The Cheshire Basin is a major sedimentary basin which comprises the larger part of the county. This fault-bounded half-graben structure is bounded to the east and southeast by a series of faults of which the most significant is the Red Rock Fault. Numerous faults trend north-south through the basin, some of which help to define the series of hills which are known collectively as the Mid Cheshire Ridge. The basin was flooded on several occasions in the Permian and early Triassic periods resulting in the laying down of massive halite (salt) beds. These beds have been mined both by cavern working and hot water brine extraction for over 200 years, mainly in the area around Northwich. The collapse of some of these worked-out halite beds has given rise to some of Cheshire's noted features, the Cheshire Meres.