Water will flow through the ground if there are cracks in the rocks. If this water flows across a layer of halite (rock salt) it will dissolve the salt. If the salty water stays in the rock then things stay stable but people wanted the salt and so started to pump the salty water to the surface. With the salt water being sucked out, fresh water can move in to contact with the halite, dissolving more salt. As the process repeats the layer of rock salt starts to disappear leaving a hole underground. The hole gets bigger and bigger and back in the 19th century, when this method was most used, there was no way of knowing exactly where the hole was…until…
If you click on the link you will see what happened to parts of the town of Northwich.
Sinking of the land surface is known as subsidence and it tended to happen suddenly, without warning.
Ashton’s Flash (above) shows the same effect. Collapse of an underground cavern has caused the surface to sink leaving a depression. There are numerous examples of these, many of which are now full of water.
This is Neumann’s Flash. Flash is a Cheshire / Lancashire word for lake.
Collapse along the course of the River Weaver at Winsford resulted in the river widening considerably, forming flashes in three places. The lakes have been popular spots since the late 19th century, when day-trippers from Manchester came for boating. Bottom Flash still hosts the Winsford Sailing Club.
The mines beneath Northwich have now been made safe by filling them with cement so the town is stable again. Solution mining is still used at the factory at Middlewich, but it is very carefully controlled to avoid similar problems. Next week we will take a final look at this area to see how the landscape is recovering from its industrial past.
The rock salt mine at Winsford is not the only way that salt is removed from underneath Cheshire. The nearby town of Middlewich has a salt factory, but they get their supply of salt in a very different way.
Salt dissolves in water. If the salty water is collected and the water evaporated, then the salt crystals reform.
Hot water is pumped into the ground to dissolve the salt. It is then pumped out again bringing the dissolved salt with it. The water is separated from the salt by evaporation. There is a nice animation here that shows the process.
Sounds like a good idea doesn’t it. The salt is brought to the surface without the need to send any people underground. But this method needs to be controlled and monitored carefully. Find out why next week.
Underneath the Cheshire towns of Northwich, Middlewich and Nantwich lies a layer of rock known as Northwich Halite. Halite is rock salt.
Since before Roman times, salty spring water had been used as a source of salt.
Today salt is mined in the area at Meadowbank, near Winsford. This is the UK’s largest rock salt mine and one of only three in the UK.
Machinery is used to dig the salt out from a depth of more than 150m. As the halite layer is removed, sections are left to support the rocks above so that the mine doesn’t collapse. This has resulted in over 260km of tunnels underneath the area between Winsford and Northwich.
The mine is accessed via three vertical shafts.
One is for the miners. Another is used to bring the salt to the surface. The third is used for machinery and ventilation, with huge fans at the bottom, sucking air down so that it can be circulated through the mine.
Machines are taken down the shaft in pieces and reassembled underground. These work continuously, digging out the salt and transferring it to a conveyor belt. It is then crushed and graded by size before being brought to the surface.
The main use for rock salt is gritting our roads in winter. The salt is stockpiled on the surface so that it is ready to go out, keeping the gritters supplied throughout the icy weather.
The salt has a pink tinge due to sand mixed up within it. Colours range from clear to pink to dark brown depending on the other rocks present with the halite.
You can find out loads more about rock salt mining, including pictures and videos, from the Compass Minerals website.
If you live near a hill you will probably find one of these on the roadside. Take a look at the halite inside.
Salt is in daily use in most homes, but where does it come from?
One source of salt is from underground and extracting it and using it has formed the basis of much industry in Cheshire for many centuries.
“Wich” in a place name can refer to a salt works. On a road map find the “wich” towns of Cheshire – Northwich, Middlewich, Nantwich. This is the area we will be looking at.
The Romans were the first to really tap into the salt supplies at these locations. They discovered that the springs in the area were bringing salty water out of the ground; water which was saltier than the sea.
At Middlewich, which the Romans called Salinae, (from the Latin word salis meaning salt) the Celtic people already extracted salt, but the settlements of Nantwich and Northwich date from Roman times.
The Romans collected the salty water in large pans. These were then heated to evaporate the water and leave the salt deposit behind.
As the demand for salt grew other methods were developed to get hold of it. Join me next week to find out more.
Incidentally Nantwich’s salt spring still flows today. It feeds the outdoor swimming pool, which is open in the summer.
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