|Artist's fantasy, from Harmsworth's Magazine|
This photo-feature shows the land and structural damage from major subsidence in the late 1880s. This was caused by the collapse of shallow rock-salt mines when, after the mines became uneconomical, salt extraction switched to the recovery of "bastard brine" (obtained by water injection, which dissolved what little structural support remained). This happened most spectacularly in the Great Subsidence of December 6th 1880:
Having been an eye-witness of the great subsidence of land at Northwich, on December 6th, I will endeavour to explain how it arose. The district, of which Northwich is the centre, has two beds of rock salt underneath it. The first one, about 40 yards from the surface, is on the average 25 yards thick. Below this there is a bed of much indurated clay, about 10 yards thick, and below this again the bed of lower rock salt, some 35 yards thick. From 1670 to 1780 all the rock salt mined was obtained from the "Top Rock," as it is locally called, and the miners left too few supporting pillars, and these not large enough, besides working the salt out so as to leave only a comparatively thin crust of salt as a roof. The great majority of the mines in the Top Rock salt have fallen wholly or partially. Since 1780 the rock salt has been "got" from the lower bed, and the pillars, especially of late years, have been left much larger, the roof at the same time being much thicker. Only from 5 to 6 yards of salt near the bottom of the bed has been worked. As a rule the mines in the bottom rock salt have stood firm, and where the owners have worked to their boundaries they have allowed the brine to run into the worked-out mines, thus converting them into reservoirs. The quantity of rock salt mined is small compared with the white salt manufactured. The white salt is made from a natural brine, which is found on the surface of the "Top Rock." It is found much cheaper to let the water do the mining and then pump up the salt in solution and drive off the water. The fresh water, as soon as it reaches the rock salt, eats it away till it gets fully saturated. This water running over the roofs of the old " Top Rock" mines has, in numbers of cases, eaten the whole of the salt away and opened a communication into the mine below. The overlying clays and earths, being deprived of their support, fall into the cavity thus opened, and a hole is made from the surface. On December 6th this was what occurred, and a hole or rift opened right across the course of the Wincham Brook, the water immediately rushing below. As the mines in both "Top" and "Bottom" rock were nearly exhausted of brine, the cavities to be filled, were enormous. Directly beneath where the fall occurred, and bordering on an old-abandoned Bottom Mine, was a rock salt mine being worked. The barrier between the two having been on two occasions penetrated, it now gave way, and opened a communication with 15 acres of mine having a worked-out depth of about 18 ft. Into this mine, down a funnel of 100 yards in length from the surface, the water rushed with great velocity, causing the lower portion of the brook to retrace its course and drain off a large body of water from the River Weaver and an adjoining lake called the Top of the Brook. This immense body of water, rushing into the underground cavities, drove out the air contained therein, and so violent was the compression of the air, that it forced its way through every portion of the contiguous district that was in the least rifted or weak, showing itself in violent ebullitions in all the neighbouring pits, and where the earth was fractured causing a number of miniature mud geysers of 10 to 12-ft. in height. Much property was seriously damaged, and a considerable piece of land covered with water. Five sets of salt works are stopped owing to the destruction of a road and the pipes conveying the brine from the pumping district to the works. The greatest sufferers by this subsidence had little to do in causing it, and this is one of the great anomalies of the system of obtaining salt. The property of numbers of persons in no way connected with the salt trade is seriously injured, and under the existing law no compensation can be obtained.
- "The Land Subsidence at Northwich" by Thomas Ward (speaking at an Ordinary Meeting, December 14, of the Manchester Literary and Philosophical Society)
A system for compensation for damaged/destroyed properties, by levying a tax on salt extraction in the area, took over a decade to be organised, against strong opposition from Salt Union Ltd; it was raised in Parliament as the Cheshire Salt Districts Compensation Bill of 1881, and finally passed as the Brine Pumping (Compensation for Subsidence) Act, 1891. Subsidence problems continued well into the 20th century, and a deal of Northwich's present-day folksiness derives from the readoption of historical timber-framed buildings that, unlike the brick ones they replaced, aren't completely dependent on the ground for their structural integrity. See That sinking feeling (Lynn Pegler, Geographical, January 2008) for an account that brings the story up to the present day.