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.