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Sunday, 2 October 2011

The Glaciation of Somerset

Nobody has ever managed to work out where the easternmost limit of glaciation is in the county of Somerset.  A number of people have tried to find the glacial limit, and there have even been PhD theses written on precisely this subject, but the evidence is subtle, and much of it is buried beneath the thick peat of the Somerset Levels.  The map above shows where these low-lying areas are -- marked in blue.

Glacial deposits are known, of course, in the Kenn - Court Hill - Nightingale Valley area to the west of the Mendips, and at Bathampton, just off the top right edge of the map.  That area is over 25 km inland from the Bristol Channel coast.  There is another deposit that has been interpreted as a till at Greylake, right in the middle of the Somerset Levels, in the valley of the River Parrett.  As indicated elsewhere on this blog, there are also features that look suspiciously like glacial meltwater channels in the Mendip Hills. 

In trying to work out where the GBG ice limit might have been, we also need to think about glacier dynamics.  The photo above, from the edge of the Greenland Ice Sheet, shows how ice behaves in this sort of situation, fingering out into the lowlands whilst leaving intervening ridges and hill masses ice-free.  Where drainage is blocked by the advancing ice, ice-dammed lakes will be created.

If the ice reached Bathampton Down it must also have affected the Mendips -- and in that case it is inconceivable that ice did not fill the Somerset Lowlands probably to the east of Glastonbury.   My own guess is that it reached at least as far east as the chalk escarpment, about 50 km inland.  Why isn't there abundant evidence of that ice advance in the form of moraines, or buried glacial deposits, or in a scatter of erratics?  Well, if we want to, we can say that the erratic assemblage at Stonehenge, and in the district round about, is the only evidence we need.  Part of the reason for a relative paucity of glacial traces might be that the ice incursion was very short-lived -- maybe lasting no more than a few centuries.  It was also a very long time ago, and followed by almost half a million years of denudation and landscape change.  Another part of the answer might be to do with glacier bed conditions.  Was the Anglian ice sheet that affected this area wet-based or cold-based? 

If the Devensian glacier models are anything to go by, the ice should have been wet-based, but again -- with respect to the GBG -- this is a case of work still to be done.

3 comments:

The Stonehenge Enigma said...

Brian

Nice picture of the Islands of Somerset in the Mesolithic - expand it to the rest of the country that was covered by the same ice sheet and you can endorse my hypothesis.

RJL

BRIAN JOHN said...

No chance of that, Robert.

Constantinos Ragazas said...

Brian you write,

“Part of the reason for a relative paucity of glacial traces might be that the ice incursion was very short-lived -- maybe lasting no more than a few centuries.”

That puzzles me! Its when glaciers advance that glacier debris is carried into an area. And when glaciers retreat that debris is deposited in the area. So why would the duration of time a glacier sits in an area determine if that debris is little or lot?

More likely explanation is that the land was covered and protected by solid ice and most of the debris washed off into the sea!

Concerning the submarine ridges in the English Channel. I have been thinking a lot about this. Anything new come out on this since your post reporting it? And how close are the ridges to the Brittany coast. More specifically, how close are these to the small islands off Brittany that you visited and posted on earlier this summer? These are the ones with the very peculiar prehistoric vaulted chamber mounds. We discussed this before. But has been on my mind.

What puzzles me about these is not so much the vaulted chambers (I feel comfortable that these could have been made by natural processes!) Rather, its the huge mounds of stone piles. The stones look more like broken up slate. Where did these come from? Are there identifiable quarries in these small islands where prehistoric men would have dug them and carried them? And why so many? It seems so laborious and so unnecessary!

And that brings me to my query. If these slate stones did not come locally from quarries in the small island, maybe they were brought there by glaciers. And if they were brought there by glaciers, and the submarine ridges are glacier deposits, maybe these ridges are also made up of similar piles of slate. All aligned according to the glacier ice edge that brought them and dumped them over the ice surface edge and into the sea.

So, for the record. My view is that these submarine ridges in the English Channel off Brittany are either stone alignments (like in Carnac) that have collected silt and may be even buried now in underwater debris; or these ridges are huge distinct piles of slate (like the chambered vaulted mounds in the small islands) arranged in parallel rows by retreating sea glaciers. The alignment of the ridges suggest that retreat was in a W-NW direction. In agreement with what we would expect.

I know! You strongly disagree! But the more you reject my ideas the more difficult will be for you to embrace them later! In the meantime, your rejections are on public display!

Kostas