The North Devon Dilemma

On many occasions in the past I have discussed the impact of glacier ice on the coasts of Devon and Cornwall, and the ages of the glacial episodes themselves.   It's now widely acknowledged that the ice of the Irish Sea Ice Stream must (at some stage) have impinged on all of the coasts on the southern flank of the Bristol Channel.  Rather than arguing right now about whether this ice assault happened in the Anglian or the Wolstonian glacial episode, let's just follow my established convention of referring to the Greatest British Glaciation or GBG.

Basic question:  could the ice surface elevation of any ice sheet that carried erratics from West Wales towards Stonehenge have been around 200m -- ie sufficient for glacier ice to have been present on Salisbury Plain?  Answer -- yes, without a doubt.  The basic principle is that in order for forward motion to be maintained, and in order for erratics to be transported and emplaced, there must be a continuous surface gradient between source and dumping ground.  I thought originally that the ice surface profile had to conform with the standard equilibrium profile beloved of all glaciologists; but as work in the western approaches and the Celtic Sea arena has proceeded in the last decade or so (thanks to James Scourse and many others in the BRITICE team) it become clear that the ice surface gradient of the Irish Sea Ice Stream was very shallow indeed -- possibly less than 1m per kilometre. That's because of thick basal sediments and a highly deforming bed.  Maybe the bed characteristics were different as the ice flowed over the areas that are now dry land (Pembrokeshire, Vale of Glamorgan etc), but most of the route  that concerns us is now under water, where there are thick and mobile seafloor sediments.

So -- if Mynydd Preseli was overridden, as I think it was, to get glacier ice onto Salisbury Plain, at a surface elevation of over 200m, we need a drop of c 300m over c 200 km of distance.  That's an average gradient of 1.5m per km -- shallow, but perfectly feasible, and in line with recent research findings..........

I haven't drawn a suggested ice edge on the above map, because the situation would have been very messy, with ice caps on Mendip, Exmoor and Dartmoor and also extensive snowfields and firn accumulations on Salisbury Plain and the Wessex Downs.  These would have been aggregated in the landscape as the climate deteriorated towards the phase of maximum ice sheet extent -- it would have been difficult to see an ice edge if one had flown over in a time machine.....

Also, on the above map substitute very jagged lines for the smooth ones.  It was, I think, reasonable for Kellaway, many years ago, to have tried to differentiate "ice with Scottish erratics" from "ice with Pembrokeshire erratics" -- and indeed parallel ice streams can flow along side by side with not much mixing, but we now know that the Irish Sea Ice Stream was "out of sync" with the glaciers coming from the Welsh Ice cap, so erratics probably followed very erratic routes, with considerable mixing.

In the years that have elapsed since James Scourse (1997) used the word "impossible" with regard to the bluestone glacial transport hypothesis, many different computer modelling exercises have incorporated runs that have demonstrated that if the right combination of circumstances actually occurred, the ice edge could well have been located to the east of Stonehenge.

Two of the runs from Boulton & Hagdorn, 2006 -- crude, maybe, and demonstrably incorrect in some details, but nonetheless a valuable exercise. The "cool" run, on the left, incorporates the detached small ice caps of Dartmoor and Exmoor.

To return to the north coast of Devon. Ground truthing is difficult here, because the conventional wisdom is that there are hardly any glacial deposits between Saunton and Bridgwater, over a distance of around 80 km.  There are spectacular cliffs and coastal inlets where glacial and glaciofluvial deposits might be expected to survive -- but we have little go on, apart from records of erratics up to an altitude of c 175 m inland of Ilfracombe, spectacular meltwater channels in the Valley of the Rocks and elsewhere, and occasional records of erratic materials in gravels and soliflucted accumulations that seem to be of Devensian age. As it stands, the evidence suggests that pre-Devensian ice might well have affected this coast, but that in the LGM the outermost ice edge was either offshore or somewhere to the west.

Clearly the evidence here ties in with the evidence from the south coast of Glamorgan, between Porthcawl and Penarth, where the cliffs and coves appear to be free of recent coherent glacial sediments.  For that reason I have suggested a Late Devensian ice limit approximating to the one on this map: