[1] The Mercia Mudstone Group is now divided into five formations recognised and mappable across its entire outcrop and subcrop.

Increasing knowledge of the sequences and the more recent development of seamless electronic mapping by the British Geological Survey (BGS) necessitated a reappraisal of these divisions.

A report published by BGS in 2008 recommended the abandonment of previous divisions and naming schemes in favour of a simpler approach which, having now been adopted, is set out below.

The group crops out widely across England, representing deposition within numerous Triassic basins, some of which are physically connected at depth.

Northwards the outcrop splits either side of the Pennines where deposition took place across the East Midlands Shelf of Nottinghamshire and through Yorkshire to the North Sea coast at Hartlepool.

A western arm includes the Stafford and Cheshire basins, West Lancashire and the Carlisle Basin—the latter are connected at depth beneath the Irish Sea.

It consists largely of green to grey mudstones and siltstones (which gave rise to the earlier name of this sequence, the Tea-green Marls) and varies from around 5 m to 67 m in thickness.

It is overlain by the Westbury Formation of the Penarth Group which represents widespread inundation of the Triassic basins as global sea levels rose.