The stratigraphy of the Penarth Group (Late Triassic) of the East Devon coast

The cliff and foreshore exposures in the Devon part of the Dorset and East Devon Coast World Heritage Site expose an unbroken
late Triassic to early Jurassic succession. The change from the terrestrial, red-bed facies of the Triassic Mercia Mudstone Group to
the fully marine conditions of the Jurassic Lias Group takes place via the Penarth Group, a succession of mudstones, siltstones and
limestones deposited in lagoonal and sheltered shallow-marine environments of varying salinities. The Penarth Group as
currently defined is divided into the Westbury Formation overlain by the Lilstock Formation, based on type sections in the Severn
Estuary area. The lithology and sedimentology of the Westbury Formation strata exposed on the east Devon coast are closely
comparable with those of the type area, but those of the Lilstock Formation are not. It is therefore proposed on lithological and
historical grounds that this formation should be replaced by a Cotham Formation overlain by a White Lias Formation. This would
reinstate, without any change in their original definitions, two of the oldest formally defined stratigraphical names in the British
Phanerozoic. All three formations are lithologically distinctive, and are separated from their neighbours by erosion surfaces that
represent non-sequences. Those at the bases of the Westbury and Cotham formations are overlain by pebble beds rich in
vertebrate remains (‘bone beds’). The Cotham Formation is a highly condensed succession comprised of thinly interbedded
mudstones and limestones with ripple trains, stromatolites, desiccated surfaces and slumped beds, the last of which have been
attributed to earthquakes or a bolide impact. The limestones of the White Lias Formation exposed on the east Devon coast are
sedimentologically complex with channels, slumps and desiccated surfaces. The position of the Triassic-Jurassic boundary is
currently under review. Possible positions include the base of the Cotham Formation, a horizon within the formation, the base of
the White Lias, the base of the overlying Blue Lias Formation or a horizon within the Blue Lias Formation.