Rock salt occurs in the Lower and Upper Keuper Saliferous Beds in Cheshire, the former being 190 m while the latter is 404 m in maximum thickness. Although most of the Triassic rocks in Cheshire are covered by thick superficial deposits, natural brine runs occur at the surface and salt was recovered from these springs as long ago as Roman times. Indeed, most salt was obtained in this way until the end of the seventeenth century. In 1670 salt was discovered at Marbury near Northwich, and the first mine was sunk in 1682. In the succeeding eighteenth and nineteenth centuries, the salt industry expanded enormously and many mines and brine pits were sunk. The mines frequently had a short life and poor mining practices often led to roof collapse and ingress of water. When water entered into a collapsed mine it attacked the pillars and walls and thereby penetrated into adjacent mines. As adjacent mines collapsed subsidence spread until sets of flooded mines formed large underground reservoirs. Associated “rock pit holes” occurred at the surface.
In terms of salt production, wild brine pumping became increasingly more important and overtook mining in the nineteenth century. The most successful wild brine pumping occurred on the major pre-existing natural brine runs. Such abstraction accelerated the formation of solution channels and active subsidence was concentrated at the heads and sides of brine runs. Serious subsidences could occur at considerable distances from the point of abstraction (up to 8 km). Flashes, that is, water-filled linear hollows, are probably the most notable features produced by such subsidence.
The most disastrous subsidences took place in the second half of the nineteenth century and were due to the development of bastard brine pumping, that is, pumping from abandoned mine workings. These subsidences occurred suddenly and were on a large scale, producing dramatic loss of property and even life. Bastard brine pumping also disturbed brine levels significantly which, in turn, gave rise to sudden local collapses.
Because of the unpredictable nature of subsidence due to wild brine pumping, responsibility could not be attributed to any particular wild brine operator. Hence, two Compensation for Subsidence Acts (1891 and 1952) were passed which imposed levies on the operators in order to provide compensation funds for subsidence damage. The latter act primarily extended the compensation district.
The number of wild brine pumpers declined throughout the twentieth century, and after 1968 the County Council withheld permission to extend pumping outside the demarcated areas and no permission was given to sink new wild brine wells. It was their intention to phase out wild brine pumping by the 1980's. This has very nearly been accomplished, controlled solution mining now producing 99 percent of the brine in Cheshire. It is alleged that this method of production has given rise to no subsidence.