Hi Carl,
I went to the Steam Museum when it was in Cardiff Bay in the early 90's. I went up to one of the guards and asked him why all the boats are sitting on the black dirt.
Alas the Industrial Museum is no more; bits of it being sent to other museums, the land that was nearly worthless when it was built became land that was worth a fortune! Even in 1990 after most of the collieries had closed a lot of the 'Black dirt' would have been coal even though the rivers had been filtered to remove it for many years. As a child the local beaches had tide marks of coal which was useful when sighting your deck chair above the water line.
Take care.
Tony.