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Wednesday, 4 July 2018

Glaciation of the Bristol-Gloucester region

This is from Wikipedia — I have quoted from this Regional Geology before, but I still get people saying “But there is no evidence of glacier ice ever having extended to the east of the Bristol Channel.” Well there is evidence, and it not disputed. There is a huge amount of research, and there are abundant papers in the peer-reviewed literature.

Glacial deposits, Quaternary, Bristol and Gloucester region
Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)

The glacial deposits of the region are mostly scattered remnants and provide difficult problems of interpretation. The earliest drift deposits are represented by remaniĆ© patches of erratic pebbles of quartz, ‘Bunter’ quartzite and, less abundant, strongly patinated flint lying on the surface of or within fissures in the Cotswold plateau up to a height of 300 m above OD. On the eastern boundary of the present region and in adjacent areas to the east, there are scattered patches of sandy and clayey drift with similar erratics, which are now known collectively as the ‘Northern Drift’. The general opinion is that the deposits are heavily decalcified and probably include both tills and the fluviatile deposits derived from them. They predate organic Cromerian deposits in the Oxford area and thus provide evidence for pre-Cromerian glaciation (see summary in Bowen et al., 1986)[1].

High-level plateau deposits in the Bath-Bristol area comprise poorly sorted, loamy gravels with abundant Cretaceous flints and cherts and have been correlated with the ‘Northern Drift’.

The Anglian glaciation is better represented in the district. In the Vale of Moreton there is a three-fold sequence. At the base lies the Stretton Sand, a fluviatile, cross-bedded quartz sand, which has yielded a temperate fauna including straight-tusked elephant and red deer. This was formerly dated as Hoxnian in age but now must be considered to be older. The Stretton Sand is similar to the supposedly younger Campden Tunnel Drift (see below), and it has been suggested that the temperate fauna in it is derived from an earlier interglacial deposit. The overlying Paxford Gravel, which comprises local Jurassic limestone material, has yielded mammoth remains and has an irregular erosive contact with the Stretton Sand. At the top, up to several metres of ‘Chalky Boulder Clay’ with derived ‘Bunter’ pebbles may be present. Thin red clay is locally present immediately beneath the till, possibly representing a feather-edge remnant of the glacial lake deposits of Lake Harrison.

At the northern end of the Cotswolds, in the gap between Ebrington Hill and Dovers Hill, the Campden Tunnel Drift consists of well-bedded sand and gravel with ‘Bunter’ pebbles and Welsh igneous rocks, and two beds of red clay with boulders, probably a till. The deposits occupy a glacial overflow channel, up to 23 m deep, caused by the ponding of the Avon and Severn valleys by the Welsh glacier farther downstream.

Evidence in Somerset and Avon, combined with that from South Wales, for an Anglian glacier moving up the Bristol Channel has been accumulating in the last decade or so. The construction of the M5 motorway through the Court Hill Col on the Clevedon–Failand ridge led to the discovery in the bottom of the col of a buried channel, 25 m deep and filled with glacial outwash deposits and till. Drilling has since proved similar drift-filled channels in the Swiss and Tickenham valleys crossing the same ridge. South of the ridge, and rising from beneath the Flandrian alluvium of Kenn Moor, marine, brackish and freshwater interglacial sand and silt overlying red stony and gravelly till and
poorly sorted cobbly outwash material were disclosed in drainage trenches and other works. AAR results indicate that whilst the bulk of the interglacial deposits are Ipswichian in age, samples of Corbicula fluminalis from fluvial deposits directly overlying the glacial deposits give a much earlier date and suggest that the latter are Anglian in age (Andrews et al., 1984[2]). Similar local occurrences of possible till have been reported beneath the Burtle Beds of the Somerset levels. In the light of these and other discoveries, the glacial overflow hypothesis of Harmer (1907)[3] for the cutting of the Bristol Avon and Trym gorges has been revived to explain why these rivers cut through hard rock barriers in apparent preference to easier ways through adjacent soft rocks.

Jump up ↑Bowen, D Q, Rose, J, McCabe, A M, and Sutherland, D G. 1986. Correlation of Quaternary Glaciations in England, Ireland, Scotland and Wales. Quaternary Science Reviews, Vol. 5, 299–340.
Jump up ↑ Andrews, J T, Gilbertson, D D, and Hawkins, A B. 1984. The Pleistocene succession of the Severn Estuary: a revised model based upon amino acid racemization studies. Journal of the Geological Society of London, Vol. 141, 967–974.
Jump up ↑ Harmer, F W. 1907. On the origin of certain canon-like valleys associated with lake-like areas of depression. Quarterly Journal of the Geological Society of London, Vol. 63, 470–514.


TonyH said...

You allude to what you draw to our attention in this Post, on page 196 of your new book, in Chapter 8, "The Work of The Ice".

There you also say, quote:-


"We should also be prepared to consider the possibility that some,at least, of the clay - rich
deposits found in pockets on Salisbury Plain are ancient glacial deposits.They have never been properly studied. They may or may not carry far - travelled erratics."


"Furher to the WEST,it would be reasonable to assume that there are glacial deposits (and maybe the remnnants of a terminal morraine) somewhere beneath the thick peat layers in the Bridgewater - Glastonbury district AND ALSO SOMEWHERE NEAR BRADFORD - ON - AVON NEAR THE CONFLUENCE OF THE AVON AND FROME RIVERS[my italics].

Could you enlarge on why it is you think there are glacial deposits in the district I have capitalised, somewhere near Bradford - on Avon?

Alex Gee said...

Tony: I presume that Brian is referring to the fluvio-glacial gravel deposits marked on the BGS map but hitherto unexamined? there are similar deposits just to the north of and near the centre of Warminster? I've seen similar erratic material in the ramparts of Tedbury Camp.

BRIAN JOHN said...

There are intriguing mentions of deposits here and there in Somerset and western Wiltshire which might have glacial or fluvioglacial origins — rather than the periglacial and fluvial origins assumed for most of the mapped superficial deposits. I’m also using a hunch — no more than that — that if the Mendips were glaciated during the Anglian then it is inevitable that ice also covered the whole of the Somerset Levels depression (as suggested by Gilbertson and Hawkins many years ago) and probably pushed all the way up to the Chalk scarp. Glacier ice always fills depressions rather than overriding hills, if it can........ and as I have often suggested, the ice margin must have been crennelated here rather than straight. Geomorphologists always draw straight lines for ivce edges in territory that they do not know very well — and then over time, as research expands, the lines become ever more wobbly.....

TonyH said...

Thank you, Alex & Brian, for that: I don't yet look at the BGS maps for my personal "catchment area" [Bradford - on - Avon, Trowbridge, Bath, Westbury, Warminster etc], so I shall have to soon!

Just stumbled upon a website for ICE AGE EUROPE: NETWORK OF HERITAGE SITES. Worth a look, it promotes protecting these places.

Alex Gee said...

I would suggest that the Anglian Ice overrode the Mendips as far as a North/South line somewhere to the west of the summit of Blackdown; there after the ice stream split and passed to the North and South. I would also suggest that there was an independent plateau ice cap covering Mendip at the time;similar to the current situation in modern day Iceland!;for which there is also some evidence!

BRIAN JOHN said...

Interesting comment, Alex. I must look into all of that with respect to some of the earlier posts on this blog. Without knowing tha area in detail, the info on the record looks rather persuasive......

TonyH said...

Why, Brian, are the References at the end of this Wikipedia article marked "Jump To"??

BRIAN JOHN said...

On my version it says “jump up’! Nothing to de with exhortations and excitement — they are symbols relating to page layout in the original format of the article. I couldn’t work out how to get rid of them on my new iPad......