Salt and the salt industry

by Albert Frederick Calvert

3. Deep troughs, much broken up, and with stepped or terraced sides.

With these three classes in mind, it is easy to follow the results of the action of the subterranean brine and associate the causes with the effects produced. At first the water flowed over the salt in irregular channels and reached the pumping centres by devious routes, but after a time it made defined courses for itself exactly as the rainfall carves out for itself channels on the surface of the earth. These underground streams of brine all gravitate towards the pumps, widening and deepening as the continually renewed water takes up its supply of salt. Where the earths overlying these brine “runs” are not too tenacious, they soon follow the hollow or trough formed on the surface of the salt bed, and a corresponding hollow or trough is formed on the surface of the ground. Where the hollow forms at an early stage, it rarely attains any considerable depth, for the sinking earths impede the course of the flowing brine stream and cause the fluid to spread and be diffused over a wider area. These subsidences are the shallow troughs, not stepped or terraced on the sides, and are best seen in streets and roads where the weight of the houses and the constant passage of traffic cause the earths to gradually follow the wasting surface of the salt. Where, at a considerable distance from the shafts, the water has not formed for itself a definite channel, it percolates over a wide area. The denudation in such cases is more generally spread, and a very extensive shallow trough or basin is formed. Again, where the pumping stations are close together, or in the same line, the various rivulets or streams of brine converge into one broad and deep channel, in which the denudation proceeds with great rapidity. The magnitude of these channels causes the super-incumbent ground to subside swiftly, forming deep troughs with stepped or terraced sides, where the earths have broken away in huge masses. Where the earth consists of strong marls and a kind of flagstone they are very tenacious and remain suspended for a considerable time over these deeper cavities. When they will bear no longer, a sudden fall occurs in one spot, and tens of thousands of tons of suspended earths fall into the trough below, forcing out the stream of brine at the weaker places and leaving a huge, crater-shaped hole on the surface, which fills with water.

REMARKABLE SUBSIDENCE IN NORTHWICH

In addition to the three classes of subsidences already mentioned, there is another which is the result of a combination of collapses of the surface earth caused by the rock-salt mining operations, and the denudation of subterranean strata caused by the pumping of brine. The pumping from the reservoirs formed by the flooding of the old mines does not empty these huge receptacles, as the place of the brine is continuously retaken by fresh water, which naturally gravitates to these centres and proceeds to dissolve and take up its quota of rock-salt. When a subsidence occurs on the site of these old workings it is of the most destructive nature, and as all the top-rock mines were in the neighbourhood of streams and brooks, the surface waters flow into the cavity until it is filled to the level of the earth and allows the streams to pursue their proper course. But as fast as the fresh water becomes saturated and is pumped to the surface, the overlying stream or brook lets in further supplies of fresh water to fill the vacuum, and the work of internal destruction is followed by further subsidences of the suspended earths.

The immense bodies of water in the neighbourhood of Northwich and Winsford, locally called “Flashes,” which cover a total area of many hundreds of acres, are the work of subsidences. The Flashes are not shallow swamps, but lakes varying in depth over many acres, from a few yards to 50 ft. The largest Flash, known as the Top of the Brook and resembling the letter L in shape, has a length in each arm of about half a mile, an average breadth of a quarter of a mile, and attains a depth of 150 ft. In an account of these subsidences, written in 1879, we read: “The whole of the surrounding district still sinks rapidly, and year by year the water covers more ground. The land subsides gradually here; but when we go a quarter of a mile to the north-east of the Top of the Brook, we come across a subsidence of a still more alarming character. Here the ground sinks bodily in immense masses to a great depth. A tiny brook or ditch that a child could skip across passed over flat fields some five years ago. Gradually the land began to sink, and cracks opened in the surface right across the course of the brook. The water went down the crevices. The land immediately sank more rapidly; huge cracks, wide enough for a man to slip down, formed, and very soon a district extending fully one thousand feet in length by as many in breadth, sank rapidly to a depth of forty or fifty feet in the centre, and was filled up to a certain height with water, which covered the hedges and trees. At times cracks opened in the bottom of this lake, and the whole of the water rushing rapidly below, caused still more extensive sinking.”

One of the most extraordinary subsidences, which was described in Chambers’s Journal, occurred in Dunkirk, on the outskirts of Northwich, in December, 1880. The earliest intimation of impending disturbance on an unusual scale was a rumbling subterranean noise, the violent bubbling of the water in all the surrounding pools, and the uprushing of air and foul gas through rifts which its passage tore in the ground. It was quickly discovered that Wincham Brook, a channel of water nearly 20 ft. in width, had broken into the earth about 1,000 ft. from its entrance into the Top of the Brook, and the uprush of air from the old mines, was caused by the force of the descending waters. A series of alarming, but comparatively small, subterranean displacements caused extensive rifts in the ground about Ashton Salt-works, and these were followed by a sudden explosion in a neighbouring pool, which ejected a geyser of mud and water some 30 ft. into the air. In the ruin that ensued, stacks of timber, an engine and boiler, a salt pan, and other material disappeared into the gaping earth, and a massive chimney stack, some 90 ft. high and 9 ft. square at the base, tilted towards the centre of subsidence and collapsed with a terrible crash. Scarcely had this subsidence ceased, says the writer in Chambers’s Journal, “when an enormous sinking of the whole of Ashton’s Old Rock Pit Hole and the surrounding land, for an area of over five hundred feet in diameter, took place, leaving two very deep holes. The land was riven and cracked all round, and fell in steps of two feet. Over ten thousand tons of water went down into the subterranean cavities. A huge brine cistern was riven in two, and the brine all lost; and two large brick kilns cut completely in halves, and the bricks scattered about. The whole surface of the Weaver and the Top of the Brook was lowered fully a foot over one hundred and sixty acres in about four hours; and if we add to this the whole of the water of the Wincham Brook for twelve hours, we shall find, on a careful computation, that not less than six hundred thousand tons of water rushed below.”

From the time of the “Great Subsidence,” as this event is described, the sinking has been continuous throughout the locality. In some places meadows have been converted into swamps, roads have sunk fully 30 ft. below their original level, and small brooks have become lakes of many acres in extent; sunken and distorted fences, roads, and streams are common objects of the country-side, the tenure of pastoral lands is precarious, and property is valueless for building purposes; and nothing but its inexhaustible reserves of brine saves the district from abandonment as a place accursed.

The shallow, gradual, almost imperceptible subsidences which occurred in the neighbourhood of the towns of Northwich and Winsford were at first infrequent and of comparative unimportance, but as time went on the damage to property increased so rapidly that, in 1860, the house-owners of Northwich combined in an unsuccessful attempt to obtain legal redress. By 1880, many parts of the towns were rendered unfit for habitation. In Northwich alone, nearly 400 houses and other property to the value of over £100,000 were more or less seriously affected, while water-mains, sewers, and gas-pipes were being continually repaired; houses were condemned, pulled down and rebuilt, and bridges had to be raised. The rents of many lots of property were absorbed in keeping them in repair, and in some districts property had been raised and rebuilt three times in eleven years. “The area of the mischief is extending yearly,” wrote Mr. Thomas Ward in 1881, “and a larger proportion of property is becoming affected, and more and more land is sinking beneath the water and increasing the area of the already existing extensive lakes. Very few, except those conversant with the district, have the slightest knowledge of the amount of suffering caused to property owners by this subsiding of the land.”

For over half a century the appearance of Northwich, with its undulating streets, its ramshackle, dilapidated houses, its fissured walls, and its system of shoring and bolting-up of property to postpone as long as possible its inevitable condemnation and demolition, has presented a tragico-comic spectacle. “If a stranger were to be set down some morning in the town of Northwich,” wrote a Times correspondent, “without any previous knowledge of its peculiarities, he would be struck with a startling and novel spectacle. He would see buildings of every sort, from the humble, two-storeyed cottage of the artisan to the solidly built church or chapel, standing many degrees out of the perpendicular, and suggestive, all of them, were it not for the props and iron stays with which they are secured, of some recent convulsion of nature. In main thoroughfares and back streets alike there are houses whose sloping floors and cracked walls would lend considerable colour to such an effort of the imagination. The inhabitants seem to take this tumble-down state of their dwellings quite as a matter of course. They have, in fact, to make the best of a condition of things from which there is absolutely no escape. The effects described are produced, not indeed by any sudden catastrophe, but by a slow, though equally effective process of subsidence, which may be detected in continuous operation over nearly the whole area of the Cheshire salt field, and which will continue to operate so long as the earth yields its vast stores of salt for human consumption.”