With ref to this interesting site report from John Mason:
https://geologywales.co.uk/storms/winter14c.htm
I was intrigued by the images of far-travelled metamorphic erratics found on the beach at Tonfanau -- on the northern coast of Cardigan Bay. Thanks to John for allowing the use of the pics.
John says that these rocks are not local, and that they remind him of some of the rocks in the Lewisian / Torridonian / Moine sequence in northern Scotland. In the text of his blog, John says:
"All sorts of other rock-types are to be found here (ie on the beach): perhaps the most exotic are the rare boulders and pebbles of high-grade metamorphic rock, reminiscent of the ancient, 2-3 billion year old rocks of NW Scotland. They consist of quartz, pink and white feldspars, glittering spangles of white and black mica and, in some cases, garnets - small examples of which are visible (small, intense red areas) in the photo with the 50p piece."
"There is one area on the beach, usually covered over by sand, where a number of large blocks of these metamorphic rocks lie embedded in the moraine, like the one in the image below (found in 1998), which is about half a metre long. Its more angular-looking underside is where it was embedded in the clay matrix of the moraine. The general scarcity of high-grade metamorphic rocks, and the occurrence of so many of them in the one spot, has led me to suspect that it all arrived together in one mass of ice - perhaps an iceberg, calved off some far-distant glacier and incorporated into the ice-sheet - that subsequently grounded here and, melting away, released its payload of rocks that it had brought from far away."
Interesting stuff. It's known that at Tonfanau, in the Devensian Glaciation, Welsh ice from the uplands of Snowdonia and Cader Idris flowed out into the bay across the coast -- but later this ice was displaced by the ice of the Irish Sea Ice Stream, which must by then have been immensely powerful. I don't accept that these boulders can have been carried by floating ice -- sea level was far too low at the time, near the time of the glacial maximum..
Of course, we know of other assumed Scottish erratics in Pembrokeshire and on the Bristol Channel coasts, but if any of these have come from these ancient rock outcrops in NW Scotland, that means they must have come from north of the Highland Boundary Fault -- but this area was (according to all the text books and learned papers) affected by ice flowing west or north-west, ie on the northern flank of the ice shed. On the southern flank of that same ice shed the ice fed the Irish Sea Ice Stream and flowed southwards.
Hi Brian - I meant that the blocks of this rock could have been in an iceberg captured by and incorporated into the Irish Sea ice sheet many hundreds of kilometres away and fortuitously stranded at Tonfanau with part of that ice-sheet. Would explain the very localised distribution of the larger and angular-rooted blocks.
ReplyDeleteAh ...... I see! Yes, that would be feasible. Or dropstones in deep sea sediments or from an old shore zone, picked up by overriding ice and incorporated into glacigenic deposits later on. Not necessarily in the same glacial episode....
ReplyDeleteInterestingly, the larger erratics were all found below high water mark. A few hundred metres north of their location, Welsh diamicton is present at roughly the same topographical level. The huge blocks of Middle Jurassic oolitic limestone in the latter area all come from a higher datum and are eroding from the low (3-4 metres) cliffs that back the foreshore. An interesting puzzle, the limestone being the easy bit!
ReplyDeleteThanks John -- I don't know this site, but it sounds interesting! I'm convinced that the "concentration" of far-travelled erratics in the intertidal zone is more apparent than real -- and this is true of all the western coasts of Britain.
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