Depositional cycles

Associated with coal are beds of ganister and fireclay which lie below the seams and represent the seat-earth in which the Coal Measure plants grew.  Overlying a coal seam is a series of shales, of varying thickness, which grade upwards into sandy shales and then sandstones. These are followed by the next coal seam, with its seat-earth and overlying shales and sandstones. The repetition of this sequence of rocks, over and over again, with minor variations is referred to as the Coal Measure ‘rhythm’ or ‘cyclothem’, each unit of which records the submergence of the vegetation of a coal swamp, and the eventual emergence (as the water shallowed) of sandy shoals on which the swamps grew again.

Washouts and splitting

Washouts occur where a stream or river channel had cut through the coal swamp, depositing sand etc in its bed.

Seam splitting occurs where parts of the coal swamp subsided to a greater extent than adjacent regions which were usually closer to a stable landmass. In the illustration above,  the southern end lay close to the Wales - Brabant land mass.

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05 May, 2009