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Monday, 23 April 2018

Strange till near Stackpole Head

A couple of days ago, on this very beautiful stretch of coast, on the cliffs of Stackpole Warren, between Mowingward and Raming Hole, I encountered several exposures of rather interesting till.  It's something of a miracle that I am still here to tell the tale, because accessible clifftop exposures are few and far between on the top of these vertical limestone cliffs.  Taking samples is very difficult in most cases, and even getting photos requires rather a good head for heights.  It's a good job I don't suffer from vertigo......

Anyway, to the till.  Here are some pics:

All being well, you can click to enlarge each of the above.  The most striking characteristic is the presence of abundant quartz pebbles and cobbles.  Some of them are broken, but many are intact -- having survived glacial transport from not very far away.  But where did the pebbles come from?  At Flimston, a few kms to the west, there are some claypits in which the famous Flimston Oligocene clays are exposed. Organic remains confirm the age.  They are up to 15 m thick, and appear to have been laid down in lacustrine conditions;  that's interesting in itself, given that the bedrock is highly fractured Carboniferous Limestone.  But associated with the clays (probably above them) are deposits of beautifully rounded quartz pebbles and boulders -- assumed by geologists to be Pliocene river or beach deposits laid down in association with the formation of the coastal platform.  Because virtually all the land hereabouts is within the Castlemartin tank firing range, examination of the ground surface is not encouraged;  but rumour has it that deposits of quartz pebbles are quite widespread.  So the pebbles might have come from the existing land surface to the west of the find site, or from a clifftop area which has subsequently been destroyed by marine erosion.

I have no idea how thick or extensive this till layer might be, but it is certainly more than a metre thick in places.  The matrix is sandy and silty, and the deposit is not cemented, suggesting that it is probably Late Devensian in age.  If you look in detail at the photos, you can see pebbles of all shapes and sizes, and the main rock types represented are limestone (as one would expect), shales and mudstones, sandstones and grits (some of them clearly from Old Red Sandstone outcrops either offshore or near the mouth of Milford Haven), and flints.  I did not have time, on a very quick visit, to hunt for igneous pebbles -- but I would not be surprised if they are present in small numbers.

So the conclusion has to be that the ice topped the cliffs in the Stackpole Warren area.  We should not be surprised by this, given that till (apparently of Late Devensian age)  is also to be seen at Swanlake and on Caldey Island.

I'm intrigued by the similarities between the tills at Swanlake and Stackpole and on Caldey Island with the tills exposed in the low cliffs of the Isles of Scilly.  They look very similar, and they must surely be the same age.........

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