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Monday, 10 January 2011

The Devensian climate in the South-West

About 50 years ago (OK -- that really dates me!) I concluded that the stratigraphy of the deposits around the coasts of SW Wales and SW England could best be interpreted as showing a big early glaciation -- with the Irish Sea Glacier flowing across Pembs and up the Bristol Channel and onto the coasts of Somerset, Devon and Cornwall; an interglacial period in which sea-level rose a little above that of today; and finally a rather complex period of cold climate culminating in a short-lived glaciation.

Since 1965 (when I finished my doctorate) nothing has appeared in the literature to make me change my mind.  In the stratigraphic sections above, look at the items numbered (4) -- these are quite complex and thick layers of solifluxion deposits, indicative of prolonged cold-climate conditions.  There are at least 4 different facies, and the beds contain at least one weathering horizon which seems to suggest a warmer interval.  We find similar deposits on the coasts of SW England too.

Click to enlarge the diagrams.

If one tries to fit these deposits (4) into a known framework of climate change during the Weichselian / Devensian, we can refer to this period as Early and Middle Devensian.  It lasted for about 45,000 years, and there were some periods of boreal climate and others of tundra / permafrost climate, when the ground would have been deeply frozen and when solifluxion processes would have been very important during the summer thaw season.  Since solifluxion layers do not simply accumulate layer on layer ad infinitum, but sometimes "flush out" older deposits and rework material already present on a solifluxion slope, matching up these thick pseudo-bedded layers to specific climate phases is difficult......

But at last, in the Late Devensian, the ice of the Irish Sea Glacier arrived, dumping glacial deposits (made from recycled sea floor deposits, for the most part) on top of the accumulated solifluxion layers -- but generally not flowing with such strength that these old layers were everywhere removed.  In many embayments (such as Abermawr, West Dale, Druidston, Poppit and many others) what seems to be almost a full sequence is preserved -- and we can also see what happened when the glacier ice melted out, leaving a cap of flowtills and thick deposits of sands and gravels behind.

Then there was another short-lived and very cold periglacial phase which led to the creation of frost-heave features, fossil ice wedges etc.  It's difficult to decide whether this was the Older Dryas phase, or the Younger Dryas, or a combination of both.

So that's the story -- and I think that the stratigraphy of coastal and inland deposits backs it up.

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