The Silurian, 444 – 416 million years ago
| During the
Silurian the Mendip region was a volcanic island surrounded by a
shallow sea. Landmasses to the north and south were converging, and
the Iapetus Ocean that separated them was narrowing. At the end of
the Silurian the final closure of this ocean produced mountain ranges
across what is now Wales, Scotland and northern England.
The volcanic rocks that form the Silurian succession are a mixture
of andesite lava flows and compacted
volcanic ash (called 'tuff'). Agglomerates,
comprising coarse, brecciated broken up rocks composed mainly of
lava fragments, are testament to the explosive power of the eruptions.
Some of the lavas have pillow-like structures ('pillow lavas') showing
that they were erupted into water, but other flows have eroded and
reddened tops, indicating that part of the Mendip area was exposed
The shallow seas that bordered the Mendip region were rich in marine
life, and parts of the Welsh borderland preserve beautiful coral
reefs of Silurian age. However, the mudstones that
form the oldest part of the Silurian succession in the Mendips contain
a low diversity fauna of brachiopods and bivalves, suggestive of
a shallow-water, near-shore environment. Proximity to land and the
adverse effects of volcanic activity are possible causes for such
a poorly fossiliferous marine succession. Nevertheless, the remains
of the brachiopod Eocoelia angelini are a good indication
that these rocks belong to the upper part of the Silurian.
Artistic impression of the Mendip region in the Silurian. Ash
and lava are erupted from volcanoes into a shallow sea
In the Silurian the British Isles was just south of the Equator,
with Scotland forming part of a large northern landmass that was
converging with landmasses on the southern side of the Iapetus Ocean.
The Mendips were a shallow marine and volcanic area just off the
southern shore of Iapetus.