tag:blogger.com,1999:blog-1228690739485734684.post7666815297233753101..comments2024-03-28T14:00:12.372+00:00Comments on Stonehenge and the Ice Age: More thoughts on ice movementBRIAN JOHNhttp://www.blogger.com/profile/00413447032454568083noreply@blogger.comBlogger2125tag:blogger.com,1999:blog-1228690739485734684.post-25267703746059930002009-11-06T08:16:58.748+00:002009-11-06T08:16:58.748+00:00Yes, this is an interesting problem. Short-lived ...Yes, this is an interesting problem. Short-lived entrainment is the norm rather than the exception -- because you need quite exceptional glaciological conditions for entrainment to happen on a glacier bed (or by rockfalls ONTO a glacier, as in the case of the Foothills Erratic Train in the USA). Glacier beds beneath ice sheets are often "protected" with nothing much going on; in some cases there can be streaming and areal scouring, but if the bed is frozen there will be no erosion. So a cluster or short stream of erratics would not be a problem for a glaciologist.....<br /><br />On the matter of the Altar Stone and the transport of erratics from the Brecon Beacons, see my late October posts. The Altar Stone erratic must have been transported southwards initially by Welsh ice, and must then have been transported eastwards along the contact zone. My current theory is that the ice carrying this mixed material (Welsh and Irish Sea erratics) came in on the north side of the Mendips -- and we know that there are some interesting erratics at Stanton Drew.<br /><br />Another p[ossibility is that the Altar Stone was carried southwards during one ancient glaciation, dumped somewhere near the South Wales coast, and then picked up and carried eastwards by the Irish Sea glacier in a later glaciation. "re-entrainment" of this type occurs wherever glaciers and ice sheets operate, since the sequence of events in a glaciation is never precisely duplicated.BRIAN JOHNhttps://www.blogger.com/profile/00413447032454568083noreply@blogger.comtag:blogger.com,1999:blog-1228690739485734684.post-28299257893291215392009-11-06T00:08:15.081+00:002009-11-06T00:08:15.081+00:00A short lived entrainment zone as you suggest woul...A short lived entrainment zone as you suggest would presumably mean fewer erratics being transported. Could that therefore explain why we don't see much evidence of large boulder erratics now? <br /><br />What I don't understand is if the stonehenge bluestones are a representative sample of the transported erratics why are there apparently more from sources in the Preselis than from say the Brecon Beacons. Would a different entrainment process operate in the 2 different regions which favoured extraction from the Preselis?Chris Rnoreply@blogger.com