Newsletter No. 202
The Black Country Geological Society
Fossil Bivalve, Lower Trigonia Grit
West of Site 3 was Whiteway Cutting, excavated by the Gloucestershire Geoconservation Trust to look at the underlying geology, where exposures of Lower Trigonia Grit (base of Aston Limestone Formation) overlie Snowshill clay and the Harford Member Sands. The Snowshill Clay is grey and rich in microfossils, suggesting lagoonal conditions. The Lower Trigonia Grit is orange/brown and contains abundant brachiopod, echinoid, belemnite and worm burrow fossils and fish/shark teeth, indicative of quiet shallow marine conditions. This unit is hard weathering and used for dry stone walling. On the eastern side of the cutting these units have been downthrown and smeared against the Lower Freestone by one of many roughly east to west trending faults that cross the
Cotswolds, and the fault plane was clearly visible with evidence of slickensides and scratches.
The trail continued due south to Site 4, Sand Mine Quarry, where exposures of large, rounded calcareous pillow shaped masses known as ‘doggers’ occur at the boundary of the Harford Member and underlying Snowshill Clay. Doggers are a post-depositional feature, formed by calcium carbonate precipitation within the sands, through which groundwater has percolated from overlying limestone strata. Similar features also occur within the Lower Jurassic clays. The sands of the Harford Member are pure, fine quartz, and were ideal for glaze in the Staffordshire Pottery Industry during the 18th and 19th Centuries. The sand was carried downhill by donkey to local canals and then transported up to the Potteries.
Roadstone Quarry, Site 5, exposes the lower and middle parts of the Aston Limestone Formation - the Notgrove Member and underlying Gryphite Member, which overlie the Lower Trigonia Grit. The Gryphite Member is a yellow and grey shelly limestone containing the fossils of Gryphaea, hence its name, and worm borings associated with relatively quiet conditions, ideal for colonial growth. The Notgrove Member is a yellow, fine, crystalline, bedded Limestone with few fossils, indicative of more energetic conditions and sifting by wave activity.
Rolling Bank Quarry, Site 6, exposes the youngest rocks of the trail from the Rolling Bank and overlying Clypeus Members. Two faults, as seen at the Whitbury Cutting, dropped these rocks against the older Lower Freestone strata, forming a graben. The Rolling Bank Member comprises the Phillipsiana Beds and underlying Bourgetia Beds, which are only exposed at this site. The Phillipsiana Beds comprise orange, yellow and grey, thinly layered, crystalline limestone occasionally interbedded with thin red brown mudstone bands/lenses and contain fossil oysters and worm borings. They were probably deposited by a local small channel feeding shallow marine conditions. The Bourgetia Beds are an orange thickly bedded clastic, fossiliferous limestone. Both units were used in the past for roadstone and possibly dry stone walling. An absence of limekilns in the area suggests that they were never used for industrial purposes. The overlying Clypeus Member is represented by the Upper Trigonia Grit, an orange and yellow, fine to coarse, crystalline shelly limestone, sitting unconformably over the Phillipsiana Beds. This rock was also used for dry stone walling.
After lunch we followed an extension of the trail in a southwards loop along the base of the escarpment to Site 7, Castle Rock and Site 8, Cleeve Cloud, before returning to the top of the escarpment and back to Wickfield Quarry. Backscars, rotated blocks and landslip lobes line the escarpment. Exposures of Cleeve Cloud and Crickley Members (Birdlip Limestone Formation) are seen along the escarpment and represent the oldest rocks seen on the trail. The Castle Rock backscar associated with the fault seen earlier, had downthrown fine grained oolitic marls against the Lower Freestone and Crickley Members. ►
Landslips on Cleeve Common Escarpment