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Sunday, 25 October 2015

Occam's Razor on the Gower

Some of the members of the QRA / GLWG Field Trip to Gower, 22 - 25 October, near Horton in Port Eynon Bay

The Gower Peninsula, near Swansea, has a rather interesting location, since it has probably been affected at various times by both Welsh ice and Irish sea ice, and since it lies close to the margin of Devensian ice derived from the southern part of the Welsh ice cap.  Somewhere, around 20,000 years ago, the glaciers flowing down the valleys of the western coalfield skidded to a halt.  They reached Carmarthen Bay and Swansea Bay, that's for sure, but where was the ice edge on the Gower?

The glacial and fluvioglacial deposits in this area consist predominantly of sedimentary rocks -- sandstones, grits, conglomerates, shales, and mudstones from the ORS, NRS, Millstone Grit, Coal Measures and Pennant Sandstone.  And a lot of Carboniferous Limestone too, since that is the locally outcropping rock across much of the peninsula.  There are some igneous and metamorphic erratics from the west -- particularly in western Gower, but in the bulk of the thick glacially-derived materials looked at, they are extremely rare.  So the assumption is that these erratics have come from largely destroyed earlier glacial deposits related to the action of the Anglian (?) Irish Sea Glacier.

For decades, the story of the Quaternary on the Gower has been complex and confusing -- with a multitude of suggested ice edge positions and with a multitude of chronologies to go with them.  Now, at long last, it appears that common sense is breaking out.  Researchers based mostly in Swansea University have studied most of the key coastal and inland sites and when I joined the field trip yesterday, leaders John Hiemstra, Danny McCarroll and Rick Shakesby gave a coherent and convincing new interpretation of the events of the Late Quaternary.  Essentially, what they are saying is this:

1.  There are not multiple interglacial raised beaches on Gower, as suggested by DQ Bowen, but one raised beach which is sometimes cemented and sometimes not, and which occurs at a range of different altitudes depending on precise locations on peninsulas, in bayheads etc.  The beach always contains well rounded pebbles and in some places it is very rich in shell fragments and even complete shells.

2.  There is plenty of evidence of a prolonged periglacial episode following the formation of the interglacial raised beach, since here and there we see many metres of pseudo-stratified slope deposits or "head" above the raised beach and beneath the glacial and fluvioglacial materials.  (This is exactly the sequence we see in Pembrokeshire.)  Sometimes the head is cemented, and sometimes not.

3.  The glacial sediments exposed in the cliffs at Rotherslade, Horton and elsewhere are not ancient deposits moved into place by periglacial and slope deposits, but fresh materials emplaced at or very near a Late Devensian ice edge.  (There was a lot of discussion amongst field trip experts about whether some of these deposits were indicative of actual ice-contact situations, with chaotic and even catastrophic flowing and emplacement of till and mixed fluvioglacial materials at a time of rapid ice wastage, or whether they were carried from an ice margin some way away and deposited as "plugs" of debris with till-like characteristics in episodes of turbulent and high-velocity meltwater flow.  The jury is still out on this....)

4.  The Paviland Moraine near Horton, which has been interpreted by DQ Bowen as well beyond the Devensian ice margin (and hence a very ancient glacial feature) contains material which is no more ancient than any of the other glacial and fluvioglacial materials exposed in the cliffs.  It should therefore be re-interpreted as a Devensian feature -- deposited maybe during a long ice edge stillstand at this position, on a ridge not far from the southernmost tip of Gower.

More on some of this anon, when I have had a bit more time to examine my photos and to look at the excellent "Quaternary of Gower" Field Guide prepared by the field trip leaders.

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