The Early to Mid Jurassic, 200 – 161 million years
| At the dawn
of the Jurassic, Britain was between 30° and 40° north of the Equator,
with annual temperatures of 12–29°C. Across southern England there
was general expansion and deepening of marine conditions, the Mendips
forming part of a string of islands that stretched westwards into
The Early Jurassic seas teemed with life, particularly ammonites,
belemnites and marine reptiles. Usually the rocks in which the fossilised
remains of these creatures occur are dark mudstones or thin limestones,
but in the Mendips, beds of rubbly limestone occur (e.g. Viaduct
Quarry, near Shepton Mallet). This limestone formed in the current-swept
waters close to the shoreline of the Mendip islands, and contains
the fossilised remains of coarse-ribbed bivalves that were adapted
to life in turbulent waters.
Some parts of the Mendips were flooded by the Lower Jurassic sea,
such as the Milton area, near Wells. Here, in quieter water conditions,
thin limestones and shales were formed. A diverse assortment of fossils
also occurs, including bivalves, gastropods, crinoids and occasional
ammonites. Some beds contain the remains of fossilised ripples, showing
that the sea must have been quite shallow.
By the end of the Lower Jurassic it is likely that most of the Mendips
were finally submerged beneath the sea. Initially erosion occurred,
as powerful currents scoured the Carboniferous Limestone that formed
much of the former land-area of the Mendips. This rocky sea floor
became encrusted with oysters and bored into by marine organisms.
At Vallis Vale, near Frome, the erosion surface is remarkably flat,
and cuts across steeply dipping Carboniferous Limestone. The Middle
Jurassic rocks above this surface, known as the Inferior
Oolite, are 170 million years younger than the Carboniferous
Limestone beneath. This huge time break is known as an unconformity.
The sea covering the Mendips remained shallow in the early Middle
Jurassic, and periodic erosion continued to affect the deposition
of the limestones of the Inferior
Oolite, At Doulting, near Shepton Mallet, horizons of yellow-stained
pebbles, concentrations of oysters and borings made by the bivalve Lithophaga mark
levels where sediment accumulation was interrupted.
The geological history of the Mendips for the remainder of the Jurassic
and the following Cretaceous Period is poorly known, as any rocks
that formed during this time have since been removed by erosion.
It is assumed that the region was deeply buried beneath marine sediments,
with a brief return to land in the Early Cretaceous. The final exposure
of the Mendip Hills from beneath the mask of younger rocks has probably
only occurred within the last few million years.