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Monday, 8 September 2014

The shelly sands and gravels at Moel Tryfan

At Moel Tryfan, near the North Wales coast, there is a smallish mountain with a summit at 427m (not to be confused with the famous peak called Tryfan, to the NE of Snowdon) which has become one of the most famous Pleistocene sites in the British Isles.  The reason for its fame is the occurrence of "shelly drift" to the SE of the summit at an altitude of just over 400m.  The deposits were first described in 1831 (discovered because of the quarrying activity in the area) by Trimmer, and after that the site was visited and examined by many of the leading scientists of the nineteenth century, including  Darwin, Greenly, Ramsay, Buckland and Lyell.  There were furious arguments between the Diluvialists and the Glacialists over several decades, and it was not until 1900 that there was a consensus that the deposits had been laid down by ice and glacial meltwater in this very unlikely position.  It is now accepted that the shelly deposits were laid down by Irish Sea Ice flowing in from the coast about 10 km away and pushing uphill and inland -- probably during the early part of the Late Devensian glaciation. This dating now seems to be confirmed by radiocarbon and amino acid dating.  There must have been a huge amount of shearing in the ice as it pushed its way inland and uphill.  Later on, when the Welsh Ice cap was fully developed, the direction of ice flow was reversed, and the "Irish Sea" deposits were capped with Welsh till which is very different in appearance.

Here are a few images from this extraordinary site.

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