Salt has been produced in the
Cheshire Area for over 2000 years. This was a as a result of Cheshire’s unusual
geology which resulted in the formation of large salt deposits located beneath
the ground.
Geology
The Cheshire Basin between the hills
of the Delamere Forest and the Staffordshire and Derbyshire border once formed
a large basin fed by the sea to the north. As the land levels to the north
rose, this area was cut off from the sea but was occasionally inundated with
sea water at high tides forming a large salt lake. Sedimentation and
evaporation led to deposits of salt forming between layers of soft sedimentary
rock known as marlstone.
The Cheshire salt-beds lie in the
red or Triassic Keuper marls in a kind of basin comparable with an elongated
saucer with its longest axis lying in a nearly north and south direction. The
best-known and most important beds of rock salt are about the centre of this
basin, in the neighbourhoods of Northwich and Winsford located below what is
now the level of the sea.
Two beds of rock salt exist in
Northwich an upper and lower bed. These beds spread from the Northwich town
centre in the Baron Quay’s area to the north-east of Marston in an area around
two miles in diameter. Each bed is from 25.6 to 27.4m (84 to 90 feet) in
thickness at Marston and Wincham, divided from each other by 9 to 10m (30 to 33
feet) of marl and marlstone. For example salt was located at Neumann’s Mine
between 60 feet and 144 feet below Ordnance Datum in the upper bed and 174 feet
and 258 feet in the lower bed. The bottom part of the lower bed was found uniformly
to be the best quality.
Brine Springs
The surface of the upper bed was
impermeable and an underground stream of water would flow across it. Where this
stream of water came close to the ground surface, at locations on the side of
the Weaver or Wheelock valleys the water would emerge as a brine spring. The
springs would look like most other springs, a small pool of water bubbling from
the ground within shallow valley locations. The difference was within the water
itself which would have been saline.
These became the first locations
for salt making in the Prehistoric and Roman periods. Brine springs denote the
locations of the Cheshire Salt towns of Northwich, Winsford, Middlewich and
Nantwich.
Salt Mining
In the 17th century the first of
a series of mines were begun in the Northwich region. A bed of rock salt was
discovered in 1670 by the Smith-Barry family when they were searching for coal
on the Marbury Estate, near Marston. This is believed to be the first mine sunk
for rock salt into the upper bed of salt at around 50 feet below the surface.
This resulted in the excavation of a number of ‘top rock’ mines around
Northwich. The early top rock mines resulted in subsidence in the area.
By 1779 investigation below the
upper bed of rock salt resulted in the discovery at the Marston Mine of a lower
bed of salt at a depth of around 150 feet. This was also mined for a number of
years until it began to be exhausted around 1850. During the 19th century the
mines of the Northwich district began to collapse and subsidence was
widespread.
Wild Brine Extraction
Wild brine extraction or brine
‘tapping’ had been carried on since the 17th century. This involved the sinking
of a shaft to natural brine streams below the ground that ran over the upper
bed of salt. This was then extracted and used to make salt by the open pan salt
process.
The exhaustion of the mined rock
salt supplies resulted in a rapid expansion of the wild brine extraction
techniques. This was aided by more efficient pumps that allowed the brine to be
pumped from both the upper and lower beds. The process of brine-tapping
involved the sinking of a shaft to the brine stream, a dangerous occupation. AK
Calvert describes the process;
In sinking to many of the springs, the supply of brine, when cut into,
was so copious that the sinkers had to flee for their lives, ascending the
shaft among the brine, and having no opportunity of seeing what was
underneath.
The brine was subsequently pumped
out of the ground to supply the salt works based at the surface. By the
late-19th century brine shafts and traditional open pan salt works dominated
the area around Northwich many controlled by the monopolistic Salt Union.
As the rock salt mines closed
they were inundated with water creating vast underground reservoirs of brine.
These were increasingly tapped for brine and it became increasingly common to
deliberately flood the mines to provide new sources of brine. The erosion of
the sides of the mine by the brine streams led to more and more mines
collapsing and subsidence was widespread in the Northwich area. This can be
seen by the large flashes that dominate the salt-making landscape.
No comments:
Post a Comment