Landslips.—The Cheshire Subsidences

Abstract

UNDER the guidance of Mr. Thos. Higgin, F.L.S., and your correspondent Mr. Ward, I have just been examining the subsidences that have been lately taking place in the neighbourhood of Northwich. To understand how they occur, it is necessary to know that there are two beds of rock salt in the Triassic marl. The upper bed, 25 yards thick, is from 40 to 60 yards below the surface; the lower, 35 yards thick, is separated from the upper by about 10 yards of hard marl. The greater bulk of the salt is obtained in the form of brine pumped up from the upper bed. The lower bed is to a smaller extent worked as a salt-mine. From these operations two classes of subsidences result: the one general and gradual, due to the removal in solution of the rock salt of the upper bed by percolation of water and pumping, by which the surface of the ground sinks in undulations; the other, sudden fallings in of the ground into the mines, forming crater-like pits. It is to these I wish to call attention. I was fortunate enough to see one before it had become, as they all do, partially filled with water. I should judge it to be about 70 feet deep, 150 feet diameter at the top, and 20 or 30 feet at the bottom, where a little water was lodging. The problem to account for is how such an inverted cone of marl capped with boulder-clay and drift-sand could apparently have disappeared through so small a hole? The explanation appears to be this: By percolation of water the roof of the mine begins locally to give way and fall into the mine, gradually working its way to the surface, where it first appears in the form of a hole about the size of a well. The vacuity will no doubt take a conical form, the base being at the roof of the mine; once the hole is formed, the surface-ground begins to slip and fall in around, gradually enlarging the orifice, the material disappearing into the mine below. This continues until the bottom is filled up and the sides of the “crater” attain the angle of repose. The whole thing will occur in a night. The subsidences certainly present a very remarkable appearance from the regularity of their circular or elliptical form and funnel crater-like shape. It is evident such subsidences could not happen except under special conditions, such as are provided by salt-mining and pumping in these Keuper marls.