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Saturday, 8 January 2011

A Glacial Map of Southern England

Here's my latest attempt to summarise all the ground data (stratigraphy, geomorphology, geology, sediment studies, pollen analyses, radiocarbon dates etc) and modelling which has appeared in the literature in recent years.  There will inevitably be tweaks to the ice edge positions shown.  Click on the map to enlarge it.

I have taken the Anglian Glaciation (450,000 yrs BP?) to be the Greatest British Glaciation (GBG) because that is where most of the dating information seems to point.  At that time there was a powerful component of ice coming down from the North Sea and also a very powerful Irish Sea Glacier which pushed up the Bristol Channel and flowed around the edges of the Mendips as far east as Bath, Glastonbury, Street and maybe Nunney -- so the Somerset Levels and much of the lowland of Somerset was inundated.  That would have been the ice stream that carried the Stonehenge bluestones.  Whether the erratics were carried all the way to Stonehenge, or were left some distance to the west, is still an open question......  Beyond the ice edge -- and blending into it -- there would have been several small, thin local ice caps (cold based or frozen to their beds) that played a more or less protective role.  On the map I have just shown thin ice on the western uplands -- maybe the North and South Downs and other areas further east were also affected.)

The isostatic loading of Southern England at this time would have been sufficient to depress the land surface -- and the Channel coasts -- to the level at which ice rafting of giant erratics onto a shoreline at more or less its present position would have been possible.  (This is an issue that has been largely avoided by most geomorphologists -- but I have devoted a few posts to it.)  All of Southern England beyond the ice edge will have been affected at this time by intense permafrost conditions -- and in many areas there will have been a more or less permanent snow-cover, melting out for maybe just a month or so each summer.  River systems will have operated intermittently and seasonally, but fluvial erosion might have been considerable during warmer episodes, with much river terrace formation.

During the Devensian, the British Ice Sheet was more lop-sided, with much less ice flowing in from the Scandinavian Ice Sheet and the North Sea.  The Irish Sea Glacier affected the northern and western coasts of Pembrokeshire and pushed some way up the Bristol Channel, and it may just have reached the Bristol Channel coasts of Devon and Cornwall.  My guess is that the Somerset coast was NOT affected by the Irish Sea Glacier at this time.  The glacier reached the Scilly Isles and also extended further out into the south-western approaches -- maybe forced by a very powerful ice stream from Southern Ireland.  Again there would have been small, thin cold-based ice caps over the uplands of  Somerset, Devon and Cornwall, and extensive permafrost across the whole of Southern England.

My instinct is that during the Devensian there was not sufficient isostatic loading in Southern England for the land surface to be depressed to -135m; the result was that any ice rafting would have dumped Devensian giant erratics at a level well below present RSL -- so they are offshore and out of sight.  (We know they are there on sandbanks and shoals, since there are records of them in the literature.....)

So that's my latest hypothesis -- all comments gratefully received.

PS.  Re the erratic boulders in the Channel, see my post of 8 June 2010.  It included this extract:

Boulders, Salcombe Fishing Grounds, English Channel
Hunt (1880, 1881, 1883, 1885) found a considerable number of foreign blocks in the Salcombe fishing grounds, some 30 to 50 km south of the Devon coast. Of 40 blocks described, there is granite, microgranulite, serpentine, syenite, gabbro, diorite, basalt, "diabase" (dolerite), trachyte, gneiss, quartz grit, conglomerate, sandstone and chalk flints and other rock types. They are discussed further by Prestwich (1892). The serpentine is precisely like the Cornish varieties. Surprisingly the other igneous rocks could not with certainty be ascribed to the English or French coasts. The gneiss resembled Hebridean gneiss from Scotland.

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