Sinkholes (NCE 15 March) can occur in many strata.
When, during the Industrial Revolution, the Earl of Dudley mined large caverns of limestone at Dudley, he left large pillars to support the roof.
He took special care not to mine below the small hospital that served, among others, the miners wounded in accidents.
The roof between some pillars eventually collapsed and the void migrated upwards.
Some, reaching weaker strata near the surface, collapsed catastrophically leaving craters over 60m in diameter.
I have been in the workings and have seen photographs of these craters, including one holding a railway carriage.
Much of the mine workings have been successfully unfilled in recent years by the local authority.
John Constable (F), john.constable@which. net