The Silurian rocks of South Gloucestershire
The Silurian period of geological time spanned the period from 439 million years ago to 409 million years ago. Two series are present in South Gloucestershire (youngest at the top):
- Wenlock Series
- Llandovery Series
Rocks of this age outcrop in the northern part of South Gloucestershire. Although the outcrop is small it is known from borehole evidence that the rocks extend over the whole area at deep levels.
The outcrop is called the Tortworth Inlier, a term used for an outcrop of older rocks surrounded by younger rocks. The outcrop extends from Milbury Heath in the south to Tortworth in the north. The western edge lies just west of the A38 and extends in a curve to the east of Damery, bounded by Micklewood and Tortworth village. There is also an isolated outcrop around Charfield.
The underlying geology does not have a pronounced effect on the landscape, which is gently undulating, with a vale in the area of softer shales. Ridges such as Falfield Windmill Hill reflect the harder limestone reefs.
The rocks were deposited as sandy and clay rich sediments in shallow warm seas south of the equator. From time to time lava flows were extruded across the seabed and the lowest rock exposed is one of these. It is a basaltic lava rich in heavy minerals, which have been altered from their original state. The lava bed, called the Lower Trap rests unconformably on much older rocks of Cambrian age, more than 500 million years ago. These are not easy to see and we know that 100 million years elapsed between the two ages.
After the extrusion of the Lower Trap more shallow water sediments such as sand silt and some lime-rich muds were deposited and eventually became sandstones, siltstones and some impure limestones. Fossils in the rocks enable them to be correlated with rocks found at Llandovery in Wales, so they are known as the Llandovery Series and the particular bed takes a local name, the Damery Beds.
On top of the Damery Beds is another lava flow with slightly different minerals from the one below. This is called the Upper Trap. It can be seen at Cullimore's Quarry (ST721926). This is a disused quarry on private land and so permission must be given by the landowner before any visit can be made.
More sandstones and shales lie above the Upper Trap. These are the Tortworth Beds which gradually grade into the topmost rocks of limestones, mudstones and sandstones of the Brinkmarsh Beds which belong to the Wenlock Series, identified as such because they have the same type of fossils as those at Wenlock Edge in Shropshire. The shallower seas in a tropical environment were the ideal sites for the development of coral reefs and the reef limestones can be seen in a few places.
Fossiliferous limestones and mudstones can be seen at Brinkmarsh quarry ST674913.This is on private land and permission must be sought from the owner at Brinkmarsh Farm, Brinkmarsh Lane.
More Silurian age rocks can be seen at Buckover Road cutting on the A38 near Thornbury ST667907 where the relationship between Silurian Rocks and the Devonian, Old Red Sandstone, can be seen. For further details see the section on the Old Red Sandstone.
Buildings in the area are frequently made of the Silurian rocks.
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