rocks of Mendip
Silurian | Devonian | Lower
Carboniferous | Upper
Carboniferous | Triassic | Lower
to Middle Jurassic
Silurian rocks (444 to 416 million years ago)
| The Silurian
rocks of the Mendips are formally known as the Coalbrookdale Formation, and
occur as a narrow elongated outcrop in the core of the eroded anticlinal
fold that forms Beacon Hill, north-east of Shepton Mallet. The rocks
comprise a sequence of fissile mudstones ('Wenlock Shales') around
600 m thick overlain by an interbedded succession of tuffs,
agglomerates and andesite lava flows. A vent agglomerate represents
a section through an ancient volcanic fissure, fortuitously exposed
at the surface because of the almost vertical dip of the strata.
Volcanic rocks are rare in the Silurian, and the Mendips are one
of the few places in the UK where they can be observed.
These siltstones and fissile mudstones contain the remains of brachiopods,
including Eocoelia angelini, suggesting a correlation with
the lower Wenlock part of the Silurian. They were once thought
to form the youngest part of the Silurian succession, before it
was realised that the beds had been inverted. In the Welsh borderland,
fossil brachiopod 'communities' in Wenlock-age sediments characterise
different ancient environments from deep to shallow marine settings.
Two of these same communities, the Isorthis clivosa community,
overlain by the Salopina conservatrix community (each named
after the dominant brachiopod), occur at successive levels in the
'Wenlock Shales' of the Mendips, and indicate a shallowing upward
trend from deeper shelf to restricted inshore conditions. This
shallowing probably reflects local uplift associated with the impending
eruption of the overlying volcanic succession.
Volcanic ash that has become consolidated into rock is known as tuff.
In the Mendip succession, tuffs are typically grey and grey-green
weathering, and can range from fine-grained to coarse-textured
units, the latter containing clasts up to 25 mm in diameter. At
some levels in the Mendip succession the tuffs are graded and cross-bedded,
suggesting that they were deposited in water.
Andesite is a type of lava. Today these lavas erupt from volcanoes
at continental margins where oceanic crust is being destroyed in
the process known as subduction (for example Mount St Helens in
North America). A similar setting might be inferred for the Silurian
of the Mendips. Several separate andesite lava flows occur, and
many of these contain small mineral-filled cavities known as amygdales.
The cavities formed by the escape of hot gases as the lava was
erupted, and subsequently they became the sites for mineral precipitation,
particularly calcite. The 'Main Andesite' beds, at least 390 m
above the base of the Silurian succession, comprise four major
flows that are quarried at the Moon's Hill Quarry [ST 662 461].
The tops of some of these flows are reddened and eroded, suggesting
that they were erupted under terrestrial conditions, although 'pillow'
structures seen in andesites towards the top of the Silurian succession
in the Mendips suggest a submarine eruption.
||Agglomerate and vent
Agglomerate is rock composed of broken lava fragments (larger than
20 mm in diameter) in a fine matrix, formed by explosive volcanic
eruption. In the Mendip succession, the clasts are typically up to
150 mm, and occasionally up to 200 mm in diameter, but are often
very weathered. The matrix is typically fine, grey or slightly purple weathering.
At least three agglomerates occur in the upper part of the Mendip
succession, and one of these has been interpreted as a vent agglomerate,
infilling a fissure through which lava was erupted. Vent agglomerates
normally infill steeply dipping volcanic vents and fissures and would
normally not form extensive outcrops at the surface. Here the steep
dip of the Silurian succession in the Mendips has allowed much of
this unusual deposit to be seen at the surface.
. . .