Washouts occur in a coal seam where a river or stream channel has cut through the accumulating vegetable debris which will eventually form the coal.  The route of the channel is usually filled with the sands and gravels which were laying on the river / stream bed and which now form elongated lenses of sandstone.

The washout sandstones could be extremely hard and presented major difficulties for both hand-got and mechanised faces.  If they were only a few feet wide they could be cut through, though this was a slow process and would usually incur replacement of the cutting picks on the machine.  If they were wider, then the rock would have to be drilled and blasted to break it up into manageable sized pieces before the machine could continue.  The sandstone lens frequently extended up into the roof rock above the seam and would leave a cavity after removal which would need timbering up.

The Top Hard seam contained a number of large washouts to the west of the colliery. The Piper seam had a number of smaller ones which were referred to as “rock rolls” and contained a particularly hard, fine-grained, massive white sandstone.

Permian geology to the west of Pleasley Colliery and washouts in the Top Hard seam