Penarth Group

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 or Rhaetic Beds.^[1]

Penarth Group
Stratigraphic range: Rhaetian PreꞒ Ꞓ O S D C P T J K Pg N ↓
Cut face of Cotham Marble, from the Lilstock Formation, showing the stromatolitic structures resembling a landscape
Type	Group
Sub-units	Westbury Formation, Lilstock Formation
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

Stratigraphy

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 Lilstock in west Somerset.^[2][1] The Westbury Formation is named from Westbury-on-Severn in Gloucestershire.^[3]

Ireland

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.^[4]