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Thursday 15 February 2024

Seabed features -- North Pembrokeshire

Thanks to Dan Soper for drawing attention to this resource. Courtesy the Admiralty, they have made available an incredibly detailed seabed "map" which shows up all sorts of fascinating things.  It's a bit clunky, being a Beta version, with intermittent and patchy coverage, and things flip in and out of focus as you zoom in and out -- but it can only get better. 

Here are some initial observations:

1. There are no clear recessional moraines in the southern part of Cardigan Bay, related either to the retreating Irish Sea ice margin or the Welsh ice margin. But the rough sea floor north of Dinas Head and Newport Bay may be made up partly of morainic or fluviuoglacial accumulations.

2. The "mega ripples" or ridges aligned NW-SE to the north of Dinas Head are aligned with the assumed ice direction movement of the Devensian Irish Sea Ice stream. Might they be meltwater features --ie giant eskers? It's a possibility, but care is needed because they may also be ripples related to sediment movements on the bed of the bay, associated with tides and currents.

3.  One's eyes might be playing tricks, but are there two very big channels offshore, around the northern edge of the Pen Caer peninsula?  Might they be meltwater channels, associated with the Gwaun - Jordanston meltwater channel system on land?  It's a distinct possibility.....

4.  There is an enormous feature off the coast of St Devids Head, top left in the upper image.  It seems to have lots of associated small ripples on its edges -- so my guess is that it is a very large sedimentary feature or sand bank linked to tidal and other currents.  Expert opinion needed........

5.  The very rough sea floor off the North Pembrokeshire coast to the west of Pen Caer must coincide with igneous outcrops of dolerite and volcanics including rhyolite.  This offshore belt of outcrops must be seriously considered as a "candidate area" for igneous erratics scattered across Pembrokeshire -- and also, of course, for the Stonehenge bluestones.  

6.  The deep channel between Ramsey Island and the mainland is seen here quite clearly, confirming earlier observations and conclusions.  My money is on this channel being another deep meltwater channel, maybe cut in an earlier glaciation but also used during the ice wastage episode at the end of the LGM.

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