Geology of Durnford Quarry
350 million years ago...
Durnford quarry extracts dark grey limestones that were formed around 350 million years ago during the Carboniferous period of geological time.
History of the geology of the quarry
The environment when the rocks were formed was very different to that of today. The area that is now Britain was situated near the tropics and much of southern Britain was covered by a shallow tropical sea, near the shore of a large landmass to the north. The environment would have been very similar to the current Bahamas; warm blue seas with large coral reefs teaming with marine life. High temperatures in the tropical sea caused huge volumes of calcium carbonate to crystallise out from the sea water. This ‘carbonate mud’ combined with biological material derived from marine organisms and reef material to form a great thickness of sediment rich in calcium carbonate that cemented to form the limestones at Durnford Quarry.
Pictured top right: Bedded Carboniferous limestones at Durnford quarry. The rocks dip down to the southeast at around 20° as a result of major earth movements which affected the rocks in southern Britain.
Limestone formation
Limestone continued to form in the shallow seas for around 40 million years, over which time a 600m thick sequence was laid down in the Bristol area. Deposition was eventually brought to a close by a major plate tectonic event (plate collision) which caused the area to become uplifted above sea level. These same events eventually caused the limestones in southern England to become folded in a series of large folds and displaced by faults. These earth movements and associated folding caused the development of much of the topography of the region as we know it today. The peaks of the folds form the highland of the Mendip Hills and the Ashton Court/Clifton area whilst the troughs formed low-lying basins that were infilled with later sedimentary deposits.
During the later stages of the earth movements, fluids rich in dissolved calcium carbonate and metal elements were forced through the fault and fracture systems within the limestones to deposit rich mineral veins. In Durnford quarry, veins have been uncovered by quarrying which yield exceptional specimens of calcite, fluorite and the rare manganese carbonate rhodochrosite, for which the quarry has become renowned. Pictured right is a specimen of Rhodochrosite with calcite, a rare mineral from Durnford quarry (thanks to the Southern Branch of the Russell Society for providing the image).
Britain's limestone formation
Britain has a great wealth of limestone rocks. Unlike the younger and softer Jurassic limestones present through the Cotswolds and the east of the country, the Carboniferous limestones present around Bristol and the Mendips are very hard and so provide an excellent source of crushed-rock aggregates for use in construction. To this end, around 65 per cent of the county’s limestone extraction is from Carboniferous rocks and with a wealth of limestone in the Bristol and Mendip areas, quarrying is and will continue to be a major industry in the region.
