|Silurian | Upper
Devonian | Lower
Carboniferous | Triassic | Lower
to Middle Jurassic
to Middle Jurassic fossils (200 to 161 million years ago)
The Lias Group of the Mendips formed in relatively shallow water
conditions adjacent to emergent parts of the Mendip massif. Consequently
the fossil faunas are not always so diverse as in the better-known
Lias succession of Dorset and north Somerset. Bivalves such as Gryphaea, Pseudolimea, Plagiostoma, Pinna, Placunopsis,
Oxytoma and Liostrea are relatively common, and large
gastropods such as Pleurotomaria, also occur. The brachiopods Lobothyris and Spiriferina have
been collected from the Lias Group at Evercreech, with belemnites
and crinoids. Ammonites are rather rare, although large specimens
of Arietites bucklandi have been found near Shepton Mallet
and Caloceras is relatively common in the succession at
Milton, near Wells.
||The rare remains of the primitive mammal-like reptile Oligokyphus have
been found infilling fissures in the Carboniferous Limestone near
Shepton Mallet. The remains appear to occur in Lower Jurassic sediments,
and are thought originally to have accumulated around springs on
the Mendip islands, before being swept into the sea by flooded rivers
and entering the open fissures on the sea bed.
Stone (Lias Group)
The richly fossiliferous Downside Stone contains the broken-up remains
of bivalves (including coarse-ribbed forms), analagous to modern-day
shell banks, and suggests that this was a turbulent, shallow water,
near-shore setting. Common bivalves include Cercomya, Ctenostreon, Atreta, Liostrea, Plagiostoma,
Pseudopecten and Terquemia. Fossils that are more typical
of open-water environments are rare, but a few ammonites, such as Alsatites, Waehneroceras, Schlotheimia, Caloceras and Coroniceras have
In the Mendips, only the younger, upper part of the Inferior Oolite
is preserved, overlying a remarkably planar, oyster-encrusted erosion
surface that cuts across the Carboniferous Limestone and has been
bored in to by rock-dwelling bivalves such as Lithophaga.
The surface, cut by marine erosion at a time of rising sea level,
represents a rocky Middle Jurassic sea floor. At Vallis Vale, the
limestones above the erosion surface contain a variety of bivalves
(e.g. Pseudolimea, Pholadomya, Ctenostreon),
brachiopods (e.g. Acanthothyris, Stiphrothyris) and
echinoids (e.g. Clypeus, Pseudodiadema, Arcosalenia, Holectypus).
||At Doulting, the fauna of the Inferior Oolite includes
relatively common ammonites, such as Cadomites, Leptosphinctes and Orthogarantiana near
the base, and Parkinsonia, Morphoceras, Oxycerites and Zigzagiceras near
the top. A limestone succession at the top of the Inferior Oolite
is named the Anabacia Limestone after the abundance of the small
button coral Chomatoseris ('Anabacia') porpites.
Oolite Group, Fuller's Earth Formation
The youngest Jurassic rocks of the Mendips, represented by the Fuller's
Earth Formation, comprise an ammonite-rich limestone succession overlain
by oyster-rich mudstones. The limestone is named the Fullonicus Limestone
after the abundance of the ammonite Procerites fullonicus.
The limestone also contains brachiopods (e.g. Acanthothyris),
bivalves (e.g. Modiolus and rare Catinula) and gastropods.
The mudstones above the Fullonicus Limestone are named the Knorri
Beds, after their abundance of the oyster Catinula knorri.