Excursion Report: Cloud Hill Quarry, Breedon

 July 12th 2007

Leader: Keith Ambrose (British Geological Survey, Keyworth)

On Thursday 12th July the Section enjoyed an evening visit to Cloud Hill Quarry, Breedon, led by Keith Ambrose of the British Geological Survey. The weather was kind, and fine sunshine prevailed as we assembled at 6.00pm. After the customary meetings and hellos at the quarry offices the 17-strong party was ferried in stages into the bowels of the quarry by Keith, using a heavy duty truck. The first thing to strike us was the sheer size of the quarry, unsuspected from outside, and the height of the walls. Then our eyes were drawn to the magnificent unconformity between the limestone and the overlying contrasting red mudstones of the Triassic Mercia Mudstone Group. The structure too was of great interest, with intensive folding, resulting in vertical beds in parts and possibly some overturning. Many conglomeratic levels were noted. Keith began by giving an introductory talk about the quarry and what we would see during the evening. Cloud Hill is part of a series of Carboniferous Limestone inliers situated between Shepshed and Melbourne and has been extensively quarried, currently by Ennstone. The lowest excavations are now below sea level. Our first stop exposed the oldest strata in the quarry, comprising thinly-bedded dolostones of the Milldale Limestone Formation, with clay or shaly mudstone partings The beds are moderately fossiliferous and Cloud Hill is the type area for the brachiopod, Levitisia (Productus) humerosus, specimens of which were visible. Stylolites were also discernable in some beds.

Moving along the face to the second locality we saw a sequence of dark, only partly dolomitised beds containing crinoid debris, and in the corner of the quarry a massive bed of limestone has been interpreted as a reef or mud-mound (we had interesting discussions during the evening about the origins of these anomalous features). We examined some large blocks piled up near to the edge of the quarry face. Some of these contained curious concentric features, (HJ - a bit like when you first add cocoa powder to a cake mix), of banded calcite, called stromotactis, which are complex crystalline developments within cavities in the rock. We also saw many dolomitised fossils, chiefly crinoids, some of which were unusually complete.

(Clockwise from top left): Access road dropping into the quarry past vertical beds; limestones displaying stromatactis fabric; haematite mineralisation in overlying Triassic beds; the Cloud Hill party. All photos © Andrew Swift

At the third locality we had hoped to observe a significant intra-Carboniferous unconformity (the Main Breedon Unconformity) representing a time gap of some 10 million years, but it was hidden behind quarry rubble! However we were able to see tight folding and discontinuities which have been interpreted as soft sediment slumping. We then moved along the east face of the quarry where irregular white masses of chert could be seen. There was some evidence of karstification, and fissures were observed in the upper levels of the quarry into which much younger mudstones and siltstones had been emplaced. These areas were also sites of important mineralisation. The mineralisation, however, was most impressive at the final stop, where we viewed beds of the Ticknall Limestone Formation. At this location we were able to collect kidney haematite, galena, malachite, bournite and chalcopyrite. The large brachiopod Gigantoproductus was also present and indicated an age of ?Late Asbian to Brigantian. Many members collected fine examples of crystals, some of which showed secondary mineralisation, crystals on crystals! It is thought there were two main phases of mineralisation, the first during the Late Carboniferous/Early Permian and the second phase during the Triassic. The copper mineralisation is generally attributed to the Triassic period.

The meeting closed at 9:00 pm after a fine evening of both weather and geology. We were particularly grateful to the owners of the quarry for allowing access, and also of course to Keith for his time and expertise. Both he and the quarry manager (who also made himself available for running us about the quarry and gave much useful information) were presented with bottles of wine as a token of our appreciation. A small group went into Breedon after the excursion to discuss the events of the evening over a convivial drink.

Helen Jones & Andrew Swift