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Friday, 18 December 2009

Glaciation of the Mendips

The Mendips -- glaciated or not?

I've always been intrigued that many geologists and geomorphologists have argued that the Mendips have never been glaciated. For some reason they want to maintain the pretence that ice coming in from the Bristol Channel skidded to a halt just a few kilometres inland from the present coast, somewhere around Court Hill. The main inland sites with good evidence of glacial deposits are Kennpier, Nightingale Valley, Weston-in-Gordano, and Yew Tree Farm, none of which is more than 5 km from the coast. These deposits are variable, and include flow tills and fluvioglacial gravels. So nobody can doubt that this area was glaciated by ice coming in from the west.

But then things get a bit silly. Because these deposits indicate ice-edge or ice-marginal conditions, some quite senior geomorphologists have argued that they show us the maximum inland extent of the ice. There is nothing to suggest that this was a true TERMINAL MORAINE, and indeed we know that the ice DID extend much further inland on at least one occasion. There are clear traces of glaciation at Bath University / Bathampton Down, Hampton Rocks Cutting, Newton St Loe, and Stidham Farm on the River Avon. So the ice extended at least 35 km to the east of Kennpier, Court Hill and the other sites mentioned above. Much of the Avon Valley must have been glaciated. To the south of the Mendips, the ice reached at least as far inland as Greylake. Maybe there were two lobes of ice, one pushing up the Avon Valley and the other pushing in from Bridgwater Bay into the Somerset Levels. But it is almost inevitable, given this scenario, that the Mendips were also glaciated; and indeed the deep gorges of the Mendips speak of huge volumes of glacial meltwater carving into the Carboniferous Limestone bedrock. Some authors have argued that the gorges were cut by water from snowmelt under periglacial conditions, but I just do not think that snowpatches, even over many thousands of years, could have provided the water volumes necessary. We need a full ice cover across the Mendips -- and that is indeed what the glacial modelling suggests.

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