Notices of Memoirs — Thos. Ward — Subsidence in Cheshire. 517 



strife, so that the region subject only to the older glaciation now 

 resembles a non-glaciated area. The glaciers and their bordering 

 lakes described above should therefore be considered as belonging to 

 the second or last glacial epoch. 



III. — The History and Cause of the Subsidences at Northwich 

 AND its Neighbourhood, in the Salt District of Cheshire. 

 By Thos. Ward, Esq. 



THE frequent occurrence of subsidences in the neighbourhood of 

 Northwich makes it desirable to learn their history and cause. 



Northwich overlies extensive beds of salt. These occupy about 

 three square miles. The first or ' top ' rock-salt lies at a depth of 

 about fifty yards from the surface, and is covered by Keuper marls, 

 and these by the drift sands and marls. Between the two beds of 

 salt there are 30 feet of indurated Keuper marl. The second, or 

 * bottom ' rock-salt, is over 30 yards in thickness. These beds of 

 salt occupy the lowest portion of an old Triassic salt lake. 



The first bed of rock-salt was discovered in 1670, the second in 

 1781. From about 1730, at which date the river Weaver and the 

 Witton brook were rendered navigable, until after 1781, all the 

 rock-salt mines were in the ' top ' bed, and the whole of these with 

 one exception have been destroyed, and in almost every case by 

 water, leaving funnel-shaped nearly circular holes. These are now 

 filled with water and are known as ' rock-pit ' holes. The rock-salt 

 mines are now in the lower bed and very rarely fall in. "When 

 worked to the boundary, water and brine, either or both, break in 

 or are let in, and the mines are utilized as huge reservoirs. 



The falling in of a rock-salt mine is a very rare occurrence, and 

 subsidences of this kind do not give rise to the reports which are 

 met with in the newspapers. The first reported destruction of 

 a mine was in 1750, and from that date to the end of the eighteenth 

 century every two or three years a mine collapsed. In the present 

 century, at considerable intervals of time, collapses of mines have 

 occurred, but these with scarcely an exception were old abandoned 

 'top ' mines. 



The subsidences which are so destructive in the town of North- 

 wich and the neighbourhood are entirely caused by the pumping of 

 brine for the manufacture of white salt. It was only about 1770 

 or shortly afterwards that the first sinking was noticed ; since that 

 date subsidence has gone on very rapidly, and much destruction of 

 property has resulted. Lai-ge lakes or 'flashes,' one of more than 

 100 acres in area, and of all depths up to 45 feet, have been and are 

 being formed. Prior to 1770 not more than 30,000 tons of salt 

 were sent down the Weaver navigation ; by the end of the century 

 it reached 100,000 tons, and in 1880 had increased to 1,087,000 tons. 

 The whole of this salt was taken off the surface of the first bed of 

 rock-salt by the solvent action of water. In fact, water is the 

 instrument used to mine and carry off the salt to the pumping 

 centre. The brine pumps set up a circulation of the salt water 

 or brine lying on the rock-salt, which flows to the pumping centres. 

 The brine thus removed is replaced by fresh water, which on its 



