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Friday, 16 January 2015

Glacial confusion in North Somerset


There is great confusion in the specialist literature about precisely when the last glaciation of Somerset occurred. (Just for the avoidance of doubt, there is no doubt that there was one -- the evidence is there to prove it....)  I have been looking at this interesting PhD thesis by Anne Bridle, and she summarises very nicely where the confusion occurs.  Much of the problem has to do with the use of dating techniques, with several now available.  In 2006 it was widely assumed on the basis of "aminostratigraphy" (ie stratigraphy based on the use of the amino acid dating technique of calcium carbonate materials like marine molluscs) that the last glaciation of this area was during Marine Isotope Stage 16 or even before that.  however, there seem to have been major problems over calibration, and it now looks as if the deposits date from MIS 12 -- ie the Anglian Glaciation.  This is much more consistent with the evidence appearing from all over the place...... and of course it ties in much more neatly with recent dating of glacial deposits in Eastern England.

Bridle, A. (2012) The mid-to-late pleistocene palaeoenvironments of the Gordano Valley, North Somerset. 

PhD, University of the West of England, Bristol.

The publisher’s URL is
http://eprints.uwe.ac.uk/18104/

Extract from Chapter 2

Devensian (MIS 2) ice is assumed to have been confined to the north of the Severn Estuary (Jones & Keen 1993, Clark et al. 2004), all deposits in Somerset interpreted as glacigenic having been attributed to earlier (pre-MIS 2) glaciations (Bowen 1973a, Hunt 1998b, Evans et al. 2005). In keeping with the generally accepted age of the most southerly extent of ice when the deposits were first described, the age of the Somerset glaciation was originally assumed to be Wolstonian (MIS 6) (Hawkins 1972, Gilbertson 1974, Gilbertson & Hawkins 1978a) or earlier (Gilbertson & Hawkins 1978a, Andrews et al. 1984). The deposits were later assigned to either a MIS 10 or Anglian (MIS 12) glaciation (Hawkins 1977, Bowen et al. 1986, Jones & Keen 1993, Kellaway & Welch 1993, Keen 2001, Harrison & Keen 2005) and some have since been considered to be MIS 14 or 16 (Bowen 1991, Hunt 1998a). Deposits interpreted as till, possibly pre-MIS 15, are recorded at Kenn 4 km south west of the Gordano Valley, and may extend as far south as Greylake Quarry on the Somerset Moors (Hawkins & Kellaway 1971, Gilbertson & Hawkins 1978a and b, Hunt 2006a and e). Gravels interpreted as till and glacial outwash on the margins of the Gordano Valley (described in Chapter 3) have been correlated with this glaciation (Campbell et al. 1998, Hunt 1998a, Bowen 1999b, Campbell et al. 1999). Its age is uncertain, but it antedates MIS 5e and covered most of south Wales and undefined areas farther south (Bowen 1973a, 2005). At Kenn, gravel interpreted as till is overlain by younger interglacial deposits. Amino acid analysis of Corbicula fluminalis shells from these deposits indicates MIS 15 deposition (Andrews et al. 1984, Bowen et al. 1989), suggesting that the glacial deposits are MIS 16 or older. However, Hunt (2006h) advised that ratios obtained from Corbicula fluminalis are problematical, a view supported by Penkman et al. (2007). In their recent reappraisal of aminostratigraphy of the southern part of the North Sea Basin, Meijer and Cleveringa (2009) considered the AAR results for Kenn Pier and Yew Tree Farm to be aberrantly high; similarly high ratios have been reported for Corbicula fluminalis from Purfleet, Essex, which is assigned to MIS 9 on the basis of mammal biostratigraphy (Schreve 2001a). Meijer & Cleveringa (2009) suggest sampling was from the shell umbo, in which case the ratios are consistent with a MIS 9 age. Furthermore, the presence of Corbicula fluminalis is inconsistent with MIS 15 age as it is only known from pre-MIS 19, MIS 11, 9 and 7 deposits (Meijer & Preece 2000, Keen 2001, Meijer & Cleveringa 2009). This indicates that the Kenn deposits are more likely to be MIS 11 or 9, which would place the glacial deposits in MIS 10 or 12 (Keen 2001, Harrison & Keen 2005, Westaway 2010b).


It has been suggested that at the height of this glaciation ice advanced eastwards up the Bristol Channel, affecting both sides of the Bristol Channel and impinging on the Somerset coast (Figure 2.6A) (Campbell & Bowen 1989, Ballantyne & Harris 1994), whilst ice derived from Wales may have blocked the Severn Estuary (Gilbertson 1974, Green 1992). Stephens (1970) suggested that this combination of ice, pressing southwards against the coast, may have formed a pro-glacial lake in lowland Somerset, the limits of which were controlled by the Bristol Channel ice front and the surrounding high ground. However, there is no unequivocal evidence for glaciation of the Mendips or south Somerset; consequently this scenario has been dismissed (Hunt et al. 1984, Farrant & Smart 1997, Hunt 1998b).


Despite there being little evidence for the limits of a pre-MIS 15 glaciation (Harrison & Keen 2005), Gilbertson & Hawkins (1978b) were able to infer its extent and direction of ice flow, and this is illustrated in Figure 2.6B. Their direction of ice flow agrees roughly with evidence for a glaciation of uncertain age, usually correlated with either Anglian (MIS 12) or Wolstonian (MIS 6) stages, found on the northern plateau of Lundy Island where there are extensive scatters of pebbles of erratic lithologies at 107 m above Ordnance Datum Newlyn (OD; the standard mean sea-level datum for Britain) and where west-north-west to east-south-east ice movement across the island has been inferred from ice moulded granite (Bowen 1973b, Harrison & Keen 2005).

15 comments:

Constantinos Ragazas said...

Brian,

Quoting from your post,

It has been suggested that at the height of this glaciation ice advanced eastwards up the Bristol Channel, affecting both sides of the Bristol Channel and impinging on the Somerset coast (Figure 2.6A) (Campbell & Bowen 1989, Ballantyne & Harris 1994), whilst ice derived from Wales may have blocked the Severn Estuary (Gilbertson 1974, Green 1992). Stephens (1970) suggested that this combination of ice, pressing southwards against the coast, may have formed a pro-glacial lake in lowland Somerset, the limits of which were controlled by the Bristol Channel ice front and the surrounding high ground. However, there is no unequivocal evidence for glaciation of the Mendips or south Somerset; consequently this scenario has been dismissed (Hunt et al. 1984, Farrant & Smart 1997, Hunt 1998b).

This supports my claim of a glacial lake formed by glaciers advancing up the Bristol Channel encroaching along the Somerset coast. As for the claim there is no unequivocal evidence for glaciation of the Mendips, Alex Gee may have something to say on this.

Kostas

kostadinos@aol.com

BRIAN JOHN said...

The idea of a glacial lake in the Severn Estuary, impounded by ice pressing across the Bristol Channel from Wales towards the Somerset and Devon coasts, has been around for a long time. People have searched for laminated lake clays and have not yet found them -- but I have always thought the idea reasonable. Maybe there are lake sediments on the floor of the Severn Estuary, beneath the later sea-bed sediments.

Constantinos Ragazas said...

Brian,

Your idea of lake evidence beneath the later sea-bed sediments is very reasonable. Since clearly the sea level rise and inundation of the area would have come later and after the glacial lake demise.

Do you know of any studies made of soil-core samples taken from the Severn Estuary? Perhaps the announced grant on the glaciation of the Bristol Channel may include the search for evidence of glacial lakes.

Kostas

kostadinos@aol.com

BRIAN JOHN said...

Quite a lot is known about the Severn Estuary and Bristol Channel sediments. Use the search box and all (well, some) will be revealed.....

Constantinos Ragazas said...

Brian,

I recall from previous discussions Bristol Channel is littered with megaliths. Do we know the source of these? Should these be included among the "missing" glacial erratics?

You argue the stones at Stonehenge to be an assemblage of glacial erratics. Can the same argument be made for all the other stone circles and alignments?h

Kostas

BRIAN JOHN said...

Littered with megaliths? That's a bit of a leading question! We know that there are abundant stones of all sizes on the floor of the Bristol Channel in certain areas -- and in some areas finer sediments. I might wish to call them "erratics" instead...... but because of where they are they are very difficult to collect and study. Sometimes they are exposed at low tide (very big tidal range) as on that strange ridge extending out from Steep Holm.

Myris of Alexandria said...

But no megalithics in the archy sense.
Like Brian, they should be called erratics. Big erratics.
Sweaty Neolithics punting megalithic bluestones along the Welsh coast remains firmly unproven.
M

BRIAN JOHN said...

I'll take that as a compliment, since eccentricity is counted as a great virtue in the UK.....

PS. I used to have a duffle coat too...... and a multi-coloured sweater that was my pride and joy.

Constantinos Ragazas said...

Brian,

I used the term “megaliths” in its original meaning in Greek. “Big boulder erratics” will do just fine. Where are these found in abundance? And what is the current explanation how they got there?

Are the assemblage of stones in other Neolithic sites of circles/alignments also glacial erratics, as you argue Stonehenge is? Even if these did not come all the way from Wales? Shouldn't all these various stones found in Salisbury Plain be counted in the argument for glaciation?

Kostas

ps I too had a green duffle coat! But no colorful sweater that I recall!

BRIAN JOHN said...

You will naturally find boulders in abundance in the offshore zone near any cliffed coastline around the Bristol Channel. You don't need any complex explanations for most of them -- in spite of tsunami theories etc. See this: Bryant, Edward and Haslett, Simon (2007). "Catastrophic Wave Erosion, Bristol Channel, United Kingdom: Impact of Tsunami?". Journal of Geology 115 (3): 253–270. I prefer on balance just to assume that where there are sea cliffs, they occasionally break up and large chunks of stone end up on the foreshore -- and some of them get carried into deeper water. I don't know of any systematic survey of how many far-travelled erratics there may be, but I suppose there will be more in the areas of till on the sea bed and where there are thick sediments up to 20m thick which may have till at depth. The nice erratics on Flat Holm were not preferentially dumped there by overriding ice. Let's assume, quite logically, that every sq km of the Bristol Channel / Severn Estuary has several erratics, if Flat Holm is anything to go by. That adds up to an awful lot of erratics........






Myris of Alexandria said...

Dear Kostas
I too know the Greek meaning but qualified my usage by saying archy.
So drop stones could be erratics fallen onto the beach. Better than rafting ice. Methinks.
M

BRIAN JOHN said...

Drop stones are traditionally understood as stones dropped from floating and wasting ice onto sea-bed or lake sediments. There has to be movement down through a water column, or at the very least the dumping of erratics onto inter-tidal mudflats when an iceberg gets stranded -- a scenario described in detail in "Acts of God." !!

Don't quite understand your comment.... how would erratics fall onto a beach? From where? From the clifftops, as a result of cliff retreat? Are you suggesting that the giant erratics around the SW coasts are not dropstones at all, but erratics from the plateau that have rolled down onto the rock platform intermittently over a very long period of time? Reasonable hypothesis....

Constantinos Ragazas said...

Brian,

And what can explain the "empty pits" along the coast? I have argued years ago these are formed by ice blocks dropped over the ice edge of a glacier covering the area. The same consistent explanation as for all the "empty pits" in the Stonehenge bedrock!

Kostas

Alex Gee said...

Surely erratics falling onto the beach from clifftops, still have to be transported to the top of the cliff by glacial ice in the first place?

Wouldn't it also lend more credibility to the ice transport hypothesis because the ice would have had to have been thick enough to cover the top of the cliff, and hence extend laterally to a greater extent?

BRIAN JOHN said...

Very true, Alex. Instead of a scenario in which there is an ice front somewhere to the west, with floating ice then reaching the shore and dropping erratics on the shore platform, we have to accept ice pressing inland across the whole coastal zone, leaving erratics behind up to an altitude of 100m or more. I now have some fascinating new info from Paul Madgett -- watch this space.