No damage is done to the surface by the mining of this lower bed of rock
salt. It is too deep for that. The subsidences are all connected with
the upper bed of salt. These upper beds used to be worked because the
lower beds were not known, and when they were neglected they fell in,
and in this way the large sheets of water of which I have spoken were
formed above the earth's crust.
[Illustration: _T. Birtles, photo._] [_Warrington._