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Sunday, 1 May 2011

The Bristol Channel Lowlands

 With reference to my recent post about Cantre'r Gwaelod (The Lowland Hundred) and the tradition of Mesolithic flooding of the coastal landscape, I came across this excellent map in an article about the Eel Point finds.  If you look at the submarine contours (goodness knows why the authors chose that peculiar contour spacing) you can see just how extensive the tundra lowlands of the Bristol Channel were at the peak of the Devensian Glaciation; in fact, the coastline at the time was probably so far off towards the west that it would have been off the edge of the map.

Even given that these contours would have been a little different (because of glacial sedimentation and the post-glacial redistribution of submarine sediments by tidal streams etc) we cannot escape the fact that this was a vast undulating plain crossed by a number of meandering rivers -- of which the largest was the Severn -- and supporting tundra vegetation and herds of "Ice Age" animals including woolly mammoth, woolly rhinoceros, red deer, wolf, wild boar, giant ox etc.  As the sea-level rose gradually after the glacial episode, many of these animals became extinct as the climate and vegetation changed and the coastline approached its present position.  This coastline is also, of course, approximately the coastline of previous interglacials.

Source of the map:
A Mid-Upper Palaeolithic human humerus from Eel Point, South Wales, UK
Rick J Schultinga, Erik Trinkaus, Tom Higham, Robert Hedges,
Michael Richards, Bernice Cardy
Journal of Human Evolution, 2005

4 comments:

Constantinos Ragazas said...

Brian,

Interesting map! Any way you can pinpoint Stonehenge in it? Perhaps in reference to the inlaid map of the UK?

Question: How do we distinguish between “rising of sea levels”, “lowering of coastal land” and “melting of frozen sea channels” without the use of GPS technology? Wont all three have the same evidential characteristics?

Kostas

BRIAN JOHN said...

Stonehenge is more or less where the coast of Yorkshire is on the inlaid map -- have a look at some of the other maps on this blog.

Eustasy and isostasy are covered already on this blog -- type the terms into the search box, and all will be revealed. Transgressive shorelines are difficult to interpret when you have nothing much to go on -- as in some of my early research in Greenland and Antarctica. But once approx curves are established, then you can begin to get a reasonably reliable picture. Isostatic curves have to be established in areas where isostatic depression has taken place; eustatic curves are established by reference to coastal sediments and shorelines in the tropics and subtropics, where large-scale isostatic effects due to glaciation can be discounted. The melting of frozen sea channels has zero effect on shoreline positions.

Constantinos Ragazas said...

Brian,

Thanks for your explanation.

You write, “The melting of frozen sea channels has zero effect on shoreline positions.”

I was thinking of “shoreline” in the sense of that line which marks the edge between land and water. In this perhaps restricted definition certainly the “shoreline” will change when a frozen sea channel melts.

Kostas

BRIAN JOHN said...

No, Kostas, you are wrong. I have seen plenty of frozen sea channels in my time, and you can see them from space in Arctic Canada, Antarctica and elsewhere if you use Google Earth. In those channels, when the sea ice melts every spring, the shoreline is exactly where you would expect it to be.