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).