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Mount St Bernard |
On April 1st we met up at Mount St Bernard Abbey for a trip led by John Carney from BGS, hoping for a warmer day than our last field trip to Miller’s Dale. We were not to be disappointed, as we were fortunate enough to have a clear sunny day to appreciate the landscape and rocks in the Leicestershire countryside.
John kindly gave us all an excellent hand-out, which not only went into great detail of the geological history of the area, but also contained some excellent illustrations and colour photographs of the rocks we had come to see. He started the trip by giving us an overview of the geographical situation of this area in late Precambrian times, when it was a volcanic arc (Avalonian Volcanic Arc) above a subduction zone just off the coast of Gondwana, approximately 30° south.
After Cliff had kindly volunteered my services to write up the details of the trip, we set off to the first location, Charnwood Lodge Nature Reserve, where we looked at the “Bomb Rocks”. John asked us all to look carefully at the composition of these rocks, and to discuss amongst ourselves our thoughts. He had given us a sheet of pictures of other bomb rocks, and we were looking closely to see if we could compare the shapes of the fragments. Although there were some similarities in some areas, it was clear that something wasn’t quite right, as the poorly-sorted fragments had many angular edges and sharp corners, not the smooth aerodynamic shapes you would expect from bomb rocks. It seems that despite their name these rocks are actually volcanic breccia and the small areas of similarity we had seen to bomb rocks was due to later deformation due to tectonic movements, flattening and elongating the blocks. The rocks are approximately 58-63% fine-grained andesite, typical of island arcs. As we moved further on, the make-up of the rocks changed, being more closely-packed and angular, and there were no longer any boulder-sized pieces. This could be evidence of a later flow, or maybe just later in the same flow, where the pieces were better sorted. The matrix is like a coarse sand, and is fine-grained andesite. This area is a sedimentary basin around the volcano, and although there is no direct proof that these deposits were lain in water, the rocks above and below were waterlain, and so it is likely that these were too.
Climbing over a wall into a muddy field, we moved on to look at two exposures of Grimley Andesite, which were fine-grained, with no fragments. John told us, however, that in thin section the Grimley Andesite is identical to the volcanic breccias we had seen previously, and that he believes that they are from the feeders to the volcanoes, possibly volcanic domes, and he then discussed the similarities of what had happened here with the recent activity in Monserrat, where the volcanic domes have collapsed, throwing out blocky pyroclastic flows.
Moving further to the south-west, we arrived at the three knolls of Warren Hills. The north knoll is at the waning stage of the pyroclastic activity, there are no large boulders and the rocks are lithic -rich material rather than ash. Unfortunately we couldn’t get close to these rocks as they are behind barbed wire, but John explained that this was the top of the Charnwood Volcanic arrangement, and he said that the rocks had a nobbly texture, with fragments a few centimetres across. Once we arrived at the middle knoll we could see signs of bedding, even some apparent cross-bedding, and we were now in an area that was probably under the sea at the time of the volcanoes. We could see sedimentary breccia, which John told us were clasts of laminated siltstone, and said this area had been compared with the Sliding Stone Slump Breccia in the Bradgate Formation, suggesting that by this middle knoll we had crossed the boundary from the Charnwood Lodge Formation. At the south knoll we could see alternating volcanic sandstone (distal parts from pyroclastic flows) and mudstone (degraded ash), which produced a striking black and white stripe pattern on the rocks.
Hungry and tired we walked back to the car park to have lunch and a break. We started off again with a stroll around the abbey. Built in 1844 by Pugin, the abbey has many types of local stone, and was therefore very interesting for us to look at. The roof is made of local Swithland Slate, ¼” thick, which is Cambrian, and was used extensively in the 19th Century before the advent of the railways, which brought thinner Welsh slate, which cleaves better. The window surrounds are Lincolnshire oolitic limestone. The walls of the abbey include Peldar Dacite Breccia, from nearby Whitwick Quarry, which is highly porphyritic, and I imagine that passers-by wondered what on Earth we were doing as we stood close the walls examining it with our hand-lenses! John said that this was probably never at the surface, and was part of the cryptodome.
Hand-lenses were once again at the ready when we were examining the outcrop next to the Abbey. An eagle-eyed Cliff noticed cube-shaped holes in the St Bernard Tuff Member. This evidence of mineralisation, probably pyrite, was a revelation even to John, the expert in this location, and created quite a buzz in the group. The St Bernard Tuff Member is only exposed here, and we saw some evidence of bedding. It is a fine-grained, crystal-rich tuff, with volcanic ash, and there was some quartz veining. We then followed the path around a religious shrine in the abbey grounds, called Calvary, built into the volcaniclastic rocks. At the summit was coarse, crystal-rich volcaniclastic sandstone, and then back down at the bottom were further examples of Peldar Dacite Breccia slabs in the wall.
By this point the sun was shining brightly and hats and scarves were removed before we headed off to Whitwick Quarry, now not actively worked but not abandoned. Whitwick Quarry is where the porphyritic dacite we saw at the abbey came from. We weren’t able to get too close to the quarry, but could see into it from the top, behind the wire fence. At this stage we had an unexpected guest, the eminent fossil expert Dr Helen Boynton, who was out walking in the area. After asking John a few questions about the depth of the beds in the area she was soon off again to enjoy her walk. John then pointed out the 20-30m wide shear zone to the north of the quarry, which separates the Grimley Andesite from the Sharpley Porphyritic Dacite to the east. We then walked on to an exposure of the Sharpley Porphyritic Dacite before heading on to Ratchet Hill where the breccia is similar to the “Bomb Rocks” we saw at the start of the day, although with more rounded blocks with more phenocrysts. Unfortunately this outcrop was on private land, so we were unable to get very close to it, having to look across a dry-stone wall about 5m away from the outcrop. Next time I must remember to bring binoculars as well as a hand lens to get close up views! Our last port of call was another outcrop of Grimley Andesite, before heading back to the car park.
It was a fascinating, if tiring, day, and everyone enjoyed themselves thoroughly. John did an excellent job of bringing the landscape and geology to life for us, and the hand-out was superb (and a lot of use when I was trying to interpret my notes of the day!). We were able to get an insight into how the rocks can tell the story of a location and relate it to recent events in a meaningful way. For us newer members the lesson of the day was that in geology “other explanations are possible”. I am hoping to use this as an excuse in my TMAs.
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