Penarth Group

Penarth Group Stratigraphic range: Rhaetian
Cut face of Cotham Marble, from the Cotham Formation, showing the stromatolitic structures resembling a landscape
Type	Group
Unit of	New Red Sandstone Supergroup
Underlies	Lias Group
Overlies	Mercia Mudstone Group
Thickness	from 0 to more than 12m
Lithology
Primary	mudstone
Other	limestone, sandstone
Location
Country	Great Britain, Ireland
Extent	widespread
Type section
Named for	Penarth

The Penarth Group is a Rhaetian age (Triassic) lithostratigraphic group (a sequence of rock strata) which is widespread in Britain. It is named from the seaside town of Penarth near Cardiff in south Wales where strata of this age are exposed in coastal cliffs southwards to Lavernock Point. This sequence of rocks was previously known as the Rhaetic.

It includes the Lilstock Formation and the underlying Westbury Formation. The Langport and Cotham Members, grey limestones of marine origin with associated mudstones, are recognised within the Lilstock Formation, itself named from in west Somerset. The Westbury Formation is named from Westbury-on-Severn in Gloucestershire.

In 1999, the discovery of an ichthyosaur from Langport Member mudstones exposed at Waterloo Bay, Larne provided the most complete example of this in Northern Ireland.