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Monday, 28 January 2013

The Devensian Cilgwyn Moraine

This is a false colour image of the Cilgwyn area -- courtesy Henry Patton -- which shows the small irregularities in the land surface because "false lighting" has been introduced from the NE.  On the top image you can see the mounds and terraces of fluvioglacial and glacial material; on the bottom image I have added a line to highlight where I think the edges of a glacier lobe might have been, at the peak of the Devensian Glaciation about 20,000 years ago.

The high ground on the right is Carningli -- a rocky summit which has signs of till and glaciated rock slabs on its northern and north-eastern flanks.  I'm still open to the idea that ice may have flowed right over Carningli during the Devensian -- but dating is very difficult.

At bottom left we see the inlet of the Gwaun Channel meltwater drainage system -- assumed by the authors of the big glacial lake paper about Glacial Lake Teifi to have been a spillway for Glacial Lake Brynberian.  See the previous posts for details.

I live slap in the middle of this area, and I have to say that I have not yet seen any trace of glacial lake deposits -- and am rather unconvinced about the spillway too.  But there are certainly pretty impressive terraces of sand and gravel that look to me like kame terraces -- along the right edge of the photo.

At the southernmost extent of the lobe, as shown on the lower photo, the morainic clutter is very impressive too, with a number of distinct mounds, vary large erratics scattered all over the place, and generally a very rough land surface which makes farming difficult -- very different from the areas where fluvioglacial materials have been dumped.

A lobe here would accord well with the idea of a highly irregular ice edge, with many lobes and embayments coinciding with areas of lower and more hilly land.

I'm still working on this, and will keep you informed...






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