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Bardon Quarry
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East Midlands Branch met in Bardon Quarry car-park just after 10-0’clock on what promised to be a ‘perfect geology day’. Broken cloud and a warm but not burning sun made it comfortable for walking, and excellent for taking photographs. The forecast was good for the rest of the day, two white minibuses were available for when the walking got a bit tough, and John Carney was leading us – what more could we possibly want?
Our first stop was upwards to a viewpoint where we had a spectacular overview of the whole quarry, the working plant on the rim, and the surrounding countryside – even as far as another Precambrian outcrop on the horizon close to Nuneaton. The striking feature of the whole panorama is the contrast between the sombre, dark-grey of the hard Precambrian rock, and the bright red mudstone of the 300 million year younger Triassic deposits. What an unconformity - and it followed us around the whole day!
John introduced us to the quarry geology with excellent notes and diagrams, supplemented by explanatory additions as the day progressed. In essence the outline biography is as follows:-
1. Calc-alkaline volcanoes formed in an island-arc subduction zone off the Gondwana margin some 600-545 Ma. 2. The ‘roots’ of two of these volcanoes/vents may be found at both Whitwick and Bardon. 3. This landscape was buried, inter alia, by much later Coal Measures deposition. 4. Earth movements due to mountain building in the Devonian/Carboniferous created the Thringstone and other faults. 5. The Coal Measure deposits were stripped away at Charnwood. 6. The Precambrian volcanics were resurrected during the Triassic some 250 Ma, to be covered by aeolian deposition – possibly of loess. 7. More recent erosion has re-resurrected the Precambrian volcanics with its deep gullies of red Triassic mudstone infill – and that is what we see today!
We went in the minibuses to the north face of the quarry (Location 1), initially to about 25 m below the contact with the Triassic. Here we were shown the Peldar Porphyritic Dacite, a grey, phenocryst-rich rock full of very visible pink plagioclase feldspar and partly rounded quartz. John explained that we were possibly on the edge of a submarine dome where the dacite has partially mixed with the wet sedimentary volcanic layers deposited above it.
We moved downwards and southwards into the Bardon Breccia at Location 2, andesite here rather than dacite. This was the monomictic facies (same type fragments). The rock, especially in the bright sunlight, had a distinctive green colouration due to the presence of epidote and/or chlorite, a sure sign of low-grade metamorphism. There are some good examples of angular breccia inclusions within the rock on the quarry floor, but I didn’t see any ‘jigsaw breccia’ that is said to exist here.
On to Location 3 which was on the south face of the quarry. We were still in the Bardon Breccia but had now moved into the very distinctive hyaloclastite facies. It still has a green-grey matrix but contains broken crystals and angular debris of spherulitic andesite within it produced by rapid chilling and shattering when contacting ater saturated sediments. The glass component is very high, indicative of explosive eruptions. There are many good examples worthy of note. In particular I saw a black-rimmed spherulitic fragment (well worth photographing), plus good examples of strung-out glassy fragments (also worth photographing and no less spectacular). This is the margin zone of chilled andesite separating the fine-grained core from the hyaloclastite breccia. That we were within a fault zone too, was indicated by colourful slickensides trending in very different directions on either face of a rock outcrop.
Within a few dozen metres we had walked over the line of fault, (Location 4), that was quite probably linked to the end-Carboniferous Thringstone Fault. This ESE trending fault is a major feature that cuts right through the quarry (a nuisance to the quarrymen). The lithologies are highly foliated and we found numerous examples of shiny mica-rich phyllonite at our feet. There was also a particularly good example of drag-folding in the quarry wall a couple of metres above our heads (worth yet another photograph), plus numerous examples of vein quartz either as boudins, tension gashes, or both. As John pointed out, this all indicates a ductile form of deformation involving recrystallization and re-alignment of the minerals.
It was now lunch-time. I was certainly ready for a rest and something to eat as we had already seen and taken in so much. We found scattered shady spots and spread ourselves out. To conclude our break we had a very informative talk from one of our guides about the history of the quarry itself. Gold has been found on at least two occasions but it was not economically viable to extract and was ‘removed’ by further quarrying.
Moving uphill to the south-west from the fault-zone we passed into Location 5 and the volcaniclastic deep water sediments of the Bradgate Formation, so typical of Precambrian Charnwood Forest. Here we could clearly see the graded bedding with angular clasts incorporated. The deposits are from turbidity flows and include fine sandstones, siltstones and mudstones. We found no ripple-marks or cross-bedding, although examples of soft sediment deformation were evident. My eye was attracted to the highly irregular contact at the unconformity which was now at path level, and to the exposure of bright green malachite a couple of metres above the pathway that had been noticed by an eagle-eyed member.
Our final stop was at Location 6 near to the quarry top on the south side. Here we confronted the Triassic Mercia Mudstone, loose and crumbly apart from narrow beds of much harder and lighter coloured dolomitic siltstone. These could be traced for some distance throughout the lower depths of the Mercia Mudstone in the quarry. John explained that these were possibly laid down at a time of considerable subsidence due, most probably, to regional extension. Indeed it may have been part of the initial attempt at North Atlantic rifting. Looking at the bowing of these dolomitic beds within the quarry walls, it was certainly a thought worth pondering. And it was from this spot that we looked towards the top of Bardon Hill itself, the highest point in the county of Leicestershire. Quarrying has exposed two very majestic Precambrian palaeovalleys (or should that be palaeogullies?) that are infilled with Triassic mud. Erosional forces must indeed have been extremely violent!
A wonderful day! Many thanks to John Carney for leading us on yet another interesting and informative day. For me it brought back many memories from when we visited Ratchett Hill and Mount St Bernard Abbey a couple of years ago. Thanks also to the quarrymen and, not least, the organizers of the trip – it was very much appreciated.
Some of the rest of us carried on to Billa Barra Hill to see some more of the Cambrian rocks among the vegetation. This was an old quarry, now a nature reserve. Don
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