Saturday, 10 April 2010
The Last Glaciation of the Bristol Channel
O Cofaigh and Evans, 2007 -- LGM or maximum advance of British-Irish ice into the Celtic Sea:
From the Abstract: "The Irish Sea Till was deposited by the Irish Sea Ice Stream during its last advance into the Celtic Sea. We present 26, stratigraphically well constrained, new AMS radiocarbon dates on glacially transported marine shells from the Irish Sea Till in
southern Ireland, which constrain the maximum age of this advance. The youngest of these dates indicate that the BIIS advanced to its overall maximum limit in the Celtic Sea after 26,000–20,000 14Cyr BP, thus during the last glaciation. The most extensive phase of BIIS growth therefore appears to have occurred during the LGM, at least along the Celtic Sea and Irish margins. These data further demonstrate that the uppermost inland glacial tills, from the area of supposed ‘‘older drift’’ in southern Ireland, a region previously regarded as having been unglaciated during the LGM also date from the last glaciation."
I disagree strongly with this ice margin as it is portrayed in the Bristol Channel and in West Wales. Ice always moves perpendicular to the ice margin except where there is an exceptionally strong physical constraint. So the ice cannot have flowed from NE to SW along the Pembs coast -- it must have come in from the NW, flowing towards SE. That is exactly what the ground evidence says -- when one looks at striations and erratic transport. All of the ice that laid down the latest Irish Sea till deposits around the Pembs coast came in from the NW. And I cannot imagine an ice-free Bristol Channel either -- an ice lobe like this MUST have reached the coasts of Cornwall, Devon and Somerset around 20,000 years ago, even if the ice stream was behaving like a surging glacier and even if it had an exceptionally low long profile (ie a profile very different from the glacier "equilibrium profile".) Colm and David do discuss this as a possibility -- but I think they should have adjusted the ice margin on the map while they were about it!!
What has this got to do with Stonehenge and the bluestone erratics? Not a lot really -- except to flag up the idea of repeated glaciations, with relatively minor differences in ice flow directions.