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Friday 18 May 2018

Devensian till at Lydstep

While out on o bookselling trip today I was inspired by the gorgeous summer weather to take a small diversion to Lydstep, where I wanted to check out an old record of mine regarding an ancient till deposit.  More of that in another post.

What I also discovered, on descending to the beach (which you can only do at low tide), was a splendid exposure of fresh till which has to be Devensian.  It's located in the stream gully where one has to scramble down from the grassy floor of the dry valley to the boulder-strewn beach.  The grid reference is SN 087976.

The exposure reveals at least 2m of fresh till, with a sandy and gravelly matrix and a wide assortment of striated, faceted and worn cobbles and smaller stones of many different lithologies -- including ORS, grey and buff sandstones and quartzites, blackish mudstones, shales, flints and quartz pebbles.   There's very little rounded material, suggesting that no raised beach or Pliocene pebble beds have been incorporated here.

The till is exposed almost up to the ground surface, and is capped only by the thin soil layer.  There is a lot of slumping on the exposure, but the till appears to be  underlain by a foxy red clay-rich deposit (at least 1 m thick) that appears to be relatively stone-free.  I could not determine whether this was a basal clay-rich till layer or a locally-derived gash breccia such as we see in many fissures along the limestone coast.

Anyway, this is one of the most coherent and easily accessible fresh till exposures on the South Pembrokeshire coast,  and there can be little doubt about its age, since it is completely uncemented (in an environment dominated by calcium carbonate) and has no scree or slope deposits on top of it.  It has to be Late Devensian in age.

As a matter of interest, this boulder rests on the beach just a few metres away, in a jumble of other large boulders.  It looks to me like gabbro -- similar to the gabbro that outcrops near St David's Head.  But that's just an educated guess......

Heavily abraded boulder of gabbro (?) not far from the till exposure at Lydstep.  It's one of the largest erratics seen thus far along the south Pembrokeshire coast.

As a matter of interest, the other posts describing Devensian till on the South Pembrokeshire coast are here:

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