Discover Thornbury and Explore the Fossils and Geodiversity of Brinkmarsh Quarry
- Wayne Munday
- 7 hours ago
- 3 min read
Sip back and discover Thornbury and explore the fossils and geodiversity of Brinkmarsh Quarry a designated Site of Special Scientific Interest (SSSI) and a long time disused and overgrown excavation of one of Britain’s most important sites from the Silurian Period. Brinkmarsh Quarry exposes the fossiliferous Brinkmarsh Beds a unit of the Wenlock Series first studied over 150 years ago. Dating from between 433.4 - 427.4 million years ago the Wenlock Series is named after the limestone escarpment of Wenlock Edge in Shropshire. The Wenlock Series forms part of the Tortworth Inlier an exposure of older Palaeozoic rocks ranging from the Cambrian to the Carboniferous periods found sitting within a much younger Mesozoic landscape. The Brinkmarsh Beds are a mix of interbedded sedimentary and volcanic rocks along with marine fossils and veins of the mineral Celestine. The fossil record of the Brinkmarsh Beds notably includes the Pycnactis Band known for its abundance of rugose coral Pycnactis mitratus and with other fossils such as crinoids, the tabulate coral Coenites juniperus, Favosites and the orthid brachiopod Resserella cf. basalis, articulate brachiopod Sphaerirhynchia and rare gastropods like Polemnita.
The Silurian Period is a key division of the Paleozoic Era. In the United Kingdom Silurian rocks are widely distributed across central Wales, the Welsh Borderland, Midlands, Mendip Hills the Lake District and the Pentland Hills of Scotland.
The Silurian Period was a time of relative stability and transition following the dramatic glaciations and mass extinctions of the Ordovician. Characterised by a warming climate and rising sea levels due to melting glaciers it saw the expansion of shallow seas and more stable environmental conditions. This period witnessed important biological developments including the emergence of jawed and freshwater fish, the growth of coral reefs, and the first colonisation of land by vascular plants. Though generally a time of recovery the Silurian Period also experienced smaller marine extinction events because of widespread oceanic oxygen depletion driven by a complex interplay of changing ocean chemistry, increased nutrient influx and glacial activity that led to anoxic and sulfidic conditions.
England and Wales lay south of the equator separated from Scotland by the Iapetus Ocean which gradually closed as Avalonia collided with Laurentia causing folding and faulting of marine sediments. Avalonia was the northernmost of several continental fragments originally derived from Gondwana and is defined as a composite terrane formed from the gradual assembly of distinct crustal blocks over time.
These Silurian Period sediments of mudstones, sandstones, impure limestone and volcanic ash forming tuffs of lightweight porous pyroclastic rock and andesite originated from being ejected from explosive volcanic eruptions and lava flows. This combination tells a story of a dynamic palaeoenvironment of volcanic activity, tectonic activity and a biodiverse shallow sea.
Brinkmarsh Quarry has a notable mineral concentration with the occurrence of Celestine or Celestite a strontium sulfate mineral (SrSO₄) whose name translates from Latin word "caelestis" meaning "heavenly" or "of the sky" and is known for its pale blue hue colour. Celestine commonly forms through three main processes: direct precipitation from the interaction of strontium-rich and sulphate-rich waters; replacement reactions, where it replaces minerals like gypsum or anhydrite in the presence of strontium-rich solutions; and precipitation from concentrated brines in evaporite deposits as water evaporates.
The formation of Celestine at Brinkmarsh Quarry is associated to evaporitic conditions from the presence of an enclosed marine or lake (lacustrine) environment. When trapped within an impermeable rock formations such as shales and clays the Celestine accumulates as vein fillings within the fractures or softer sediment layers often associated with calcite veins and features like slickensides seen in the Wenlock limestones. The slickensides are polished and striated rock surfaces formed by the frictional movement along fault planes with linear grooves called slickenlines indicating the direction of displacement. Common in fine-grained rocks like mudstone and shale they serve as key indicators of fault activity.
If you’re exploring the West of England its interesting to understand that Brinkmarsh Quarry as designated SSSI near to Thornbury is one of Britain’s most important Silurian fossil localities. This shallow, disused and overgrown quarry exposes Lower Palaeozoic rocks of the Wenlock Series within the Tortworth Inlier where ancient Silurian rock is preserved amidst a much younger surrounding landscape. The fossil-rich Brinkmarsh Beds reveals a diverse marine ecosystem featuring distinctive corals such as Pycnactis mitratus and Coenites juniperus, alongside abundant brachiopods, crinoids and rare gastropods. Brinkmarsh Quarry reveals that once a biodiverse Silurian Sea once shaped southern Britain nearly half a billion years ago.
