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Tuesday, 19 August 2014

The Avon Ice Lobe

This useful map from the GCR volume on SW England shows most of the key Quaternary sites in this area.  It also shows the terrain -- and that's rather useful too. I am increasingly convinced that when the last glacier ice to affect this area came in from the west, it split into several lobes, as suggested a long time ago by Kellaway (1971).  This would be entirely natural, from what we know about the behaviour of ice sheet edges:

So although the hill masses like Exmoor, Mendip and Dartmoor might well have supported their own cold-based ice caps at the time of the Greatest British Glaciation (GBG) the hill masses would in ant case have acted in a diversionary role, forcing incoming ice streams along pre-existing lowland corridors.

We can speculate, therefore, that two main ice streams affected Somerset:  in the north the Avon Ice Stream and in the south the Parrett Ice Stream.  I have speculated before that the latter might well have reaches at least as far east as Glastonbury, and maybe all the way to Salisbury plain, while the northern branch reached at least as far east as Bathampton Down and Kinsdown, to the east of the city of Bath.

This is a map of ancient glacial (?) deposits to the east of the city, from a 1995 publication by Donovan.

Here are some notes:

Donovan, DT. 1995 High level drift deposits east of Bath.  Proc Univ Bristol Spelaeol. Soc. 1995, 20 (2), pp 109-126

Varney (1921) recorded on Bathamption Down deposits including angular and sub-angular flints, flint pebbles (often broken by frost), quartz pebbles (white, pink, brown), fine and coarse sandstone, in flat pebbles, occasional pieces of conglomerate containing quartz pebbles, fragments of Palaeozoic shales, as seen in Coal Measures, rare pieces of coal, rolled and subangular pieces of Oolite (local).

Other geologists have found ORS and Pennant material as found in S Wales.  Upper Palaeolithic sandstones  -- from S Wales?  Palaeozoic shales as in Coal Measures?

Donovan speculates that these deposits are from very ancient tills from which most limestone fragments have been weathered away.  Some sandstones may also have been destroyed, leaving only the most resistant (highly silicious) rocks in any quantity. Flint and chert pebbles and larger clasts have come from old Cretaceous deposits and residues in the Bristol-Bath area?  Coal fragments could survive in glacial deposits, possibly all the way from the South wales Coalfield.  The clay matrix is also reminiscent of till -- these are not river deposits.  The residual deposits are a mixture of Tertiary residual clays and glacial deposits?  If there was a glaciation here, it was probably Early or Middle Pleistocene.
These deposits are classified to the Kenn Formation.  At Kenn, the glacial materials are overlain by Cromerian (MIS 15) deposits and by Holocene silts and clays.  Geographical limits:   Restricted to Avon and north Somerset; occurrences at Kenn, Court Hill, Nightingale Valley, Failland Ridge, Bathampton Down and Bleadon Hill.

Another "glacial" site is in the railway cutting beneath Hampton Rocks.

This starts to get interesting.  If, as appears to be the case, there are no known Pembrokeshire erratics in the area affected by the Avon Ice Lobe, this means that the ice might have come partly from the Welsh Ice cap, carrying Coalfield erratics with it.  If you look at the map at the top of this post, that would make sense.  There must have been some Irish Sea Ice involved, because there is a large erratic of "white limestone" from Northern Ireland at Kenn, which has to be accounted for.

But it looks increasingly apparent that Kellaway was right, and that the main thrust of the Irish Sea Glacier into Somerset was to the south of Mendip and across the deep embayment now occupied by the Somerset Levels.   Watch this space.....


TonyH said...

Your map " of ancient glacial (?) deposits to the east of the city [of Bath], from a 1995 publication by Donovan" shows a possible glacial flint feature at Hayes near Freshford, to the west of the river Avon, south of Monkton Coombe and some way NW of Bradford on Avon.

I would just add that Hayes Wood, at 500 feet, has produced evidence of a Neolithic and Iron Age settlement. In 1935 JFS, or Jack Stone - the same archaeologist with links to Stonehenge and to the Greater Cursus bluestone finds of the late 1940's that Rob Ixer has examined in recent years -undertook trial excavations at Hayes Wood and wrote up the proceedings in the Somerset Archaeological and Natural History Society Proceedings Vol 82. Amongst other evidence, they found flint flakes. I have obtained most of the above from Margaret Wilson's "Limpley Stoke Valley", 1994.Pages 27 to 31 are worth a look on prehistory and settlement. Bradford Museum Research Group is continuing to investigate the hinterland of Bradford as far as Winsley and Inwood following its LIDAR survey of in particular the plateau to the north.

TonyH said...

The same map from Donovan's 1995 publication that I remarked upon yesterday does not reference the Hampton Rocks geological site that was discussed in Brian's Post. I gather this is at NGR 779666. If so, it is where the Bath - Swindon -London rail line separates from the Bath - Bradford-on-Avon -Southampton line.