Glaciation of Dartmoor -- new book

This new book, edited by David Evans and Stephan Harrison, has recently been published by GWLG and QRA -- £18 to non-members.  It's very nicely produced and is easy to read -- and has been intended for use as a field guide.

The basic findings have been reported on this blog already, following the publication of a 2012 paper:
http://brian-mountainman.blogspot.co.uk/search?q=+dartmoor+glaciation

In the first part of the book there are chapters on landscape evolution, peroglacial features, glacial geomorphology and the numerical modelling of the Dartmoor ice cap.

The second part of the book is mostly about key sites which can be visited, book in hand.

It's very useful indeed, and emphasises that Dartmoor must have been glaciated more than once.  The authors also tie in the field observations with the numerical modelling done by Alun Hubbard and others.  The basis theme is that during the Devensian -- and by implication during the Anglian as well -- there must have been several small cold-based glaciers and ice caps beyond the edge of the Irish Sea Glacier as it waxed and waned.

Baggy Point erratic at +80m altitude

 Bad quality photo, but this is the 50-tonne erratic at Freshwater Gut.  This one is on the shore platform close to sea level.

I came across this record of erratics in the Croyde Bay area.  It's interesting that a Scottish origin is assumed for at least two of these well-known erratics.

"High on the crest of these same cliffs (+80m OD) sits a large isolated 500kg block of epidiorite (SS 435407), a basic igneous rock, possibly of Scottish origin. If natural agents are responsible for emplacing this erratic block, it points directly to ice having overridden these cliffs, perhaps the same ice that deposited the Fremington till. This is by no means the only far-travelled boulder in the district; a whole series has been identified on the coast between Freshwater Gut on the north side of Baggy Point (SS427400) and Saunton Sands (SS445377). These boulders vary in composition, but most imply a northern origin. There is general agreement that they are glacial in origin, although their mode of transport is disputed. Unlike the Baggy Point epidiorite, these erratics are littoral in location, sitting on the shore platforms close to present sea level, prompting the suggestion that they arrived on, or within, grounding icebergs calved from ice-fronts lying to the north or west." (Keene 1996 p 36)

"The largest [erratic], approaching 50 tonnes, is a granulite gneiss somewhat similar in composition to rocks found in western Scotland. This coarse-grained altered granite boulder is located at freshwater Gut (SS 427400)....At Saunton (SS437379) a platform at 5m OD is well-developed. The giant erratic at Freshwater Gut rests on a platform at 7.5m OD. At Pencil Rock (SS423402) a higher platform, at 13.7m OD, notches the sloping cliff." (Keene 1996 p 37) The picture of the Giant Erratic, shown above, is from Photo 11 on p 38.

From a Devon CC web page:

Quaternary
The Saunton-Croyde coast is one of the most important Pleistocene
sites in Southern England. Large erratic boulders sit on wave-cut platforms and are
overlain by raised beach deposits. These erratics are thought to be Scottish in origin,
the most famous being the Saunton Pink Granite (SS 44013787), a gneissose
granitic boulder, weighing some 12 tonnes that might have come from similar
outcrops in Gruinard Bay, Wester Ross. The overlying raised beach deposits consist
mainly of sands with pebble layers and some shingle. Molluscan fauna obtained from
the sands indicates warm or temperate depositional conditions. Overlying these
raised beach deposits are sands of variable thickness which are considered to have
been deposited either in an aqueous environment or to be wind blown aeolian sand.
Stephens (1966) suggests that fossil dune sands were present, whereas Edmonds
et. al. (1979), suggests that marine deposits have been overlain with beach sands
that pass upwards into blown sands containing terrestrial shells. The chronology of
these deposits is disputed, as absolute dating is difficult due to the lack of organic
remains. As the erratics are of presumed Anglian or Wolstonian age (i.e. pre-
Ipswichian glaciation) and the shore platform on which they rest is likely to be of
earlier Pleistocene age. The raised beach deposits are considered to be of
Ipswichian age due to their temperate molluscan fauna.

If ever there was a case for OSL dating, this is it......

This is a section of the big BRITICE map which shows the location of the key glacial features of the British Isles.  It's not very accurate, because inevitably there are many generalisations in the cartographic process and because it's bound to be out of date -- new information is always coming forward in the specialist literature.

In order to confirm the basic hypothesis of two major glacial episodes -- the Anglian and the Devensian -- in West Wales, we really need some OSL dating.  To explain:

One way of dating glacial landforms is optically stimulated luminescence dating (OSL). OSL is used on glacial landforms that contain sand, such as sandur or sediments in glacial streams. The OSL signal is reset by exposure to sunlight, so the signal is reset to zero while the sand is being transported (such as in a glacial meltwater stream). Once the sand grain has been buried and it is no longer exposed to sunlight, the OSL signal starts to accumulate.

As we can see from the map, there are three main groups  of fluvioglacial deposits in Pembrokshire -- shown brown on the map.  In the area hypothetically affected by Anglian ice only (shown yellow) the westernmost group is in the valley of the Western Cleddau river, mostly between Trefgarn Gorge and Haverfordwest.  The eastern group is in the valley of the Eastern Cleddau, around Gelli and Clynderwen.  And the third group is in the far north, in the Moylgrove-Monington area, in the area affected by the Devensian ice advance of the Irish Sea Glacier.

One of these groups -- the westernmost one -- should not be treated as a high priority for OSL dating because a great deal of meltwater related to the Devensian ice wastage phase was channelled through the Trefgarn Gorge and flowed southwards towards Haverfordwest and Milford Haven.  So the gravels in the river terraces and in patches on interfluves might well be made up of a mixture of very ancient gravels and newer ones -- that would give a confused OSL dating scenario, unless great care could be taken on a sampling programme.

But if we could get dates from sands in the other two groups of fluvioglacial materials, that would be fantastic.  I suggest initially -- as a test run -- that samples could be analysed from two locations:

(1) Llangolman gravel pit (described in a recent post here: http://brian-mountainman.blogspot.co.uk/2014/10/pre-devensian-glacial-deposits-south-of.html and characterised by bedded sands and gravels that LOOK very old indeed:

(2) Trefigin gravel pit near Monington -- that's the northern one of the two pits shown on this satellite image.  Here the sands and gravels look much fresher and cleaner, as befits deposits claimed to have been laid down at the peak of the Devensian glaciation, around 20,000 years BP.

The two lower photos are from Charlie Bendall's Report on the Trefigin Quarry (2013)

My guess would be that the sand from the Llangolman Quarry will give an OSL date of around 450,000 years ago, and the sand from Trefigin Quarry about 20,000 years BP.

A Trefigin date would be quite interesting, because it might be accurate enough to tell us whether the sands and gravels on the higher ground on either side of the Teifi Valley (occupied to Glacial Lake Teifi) were laid down during the ice advance phase, or the ice retreat phase.

Has anybody out there got the expertise, the money, and the inclination to undertake this vital piece of research?

KEY REFERENCES

Trefigin Quarry Sedimentology Report 2013
Dr Charlie Bendall, DGES, Aberystwyth University, 50pp

Etienne JL, Jansson KN, Glasser NF, Hambrey MJ, Davies JR, Waters RA, Maltman AJ, Wilby PR. 2006. Palaeoenvironmental interpretation of an ice-contact glacial lake succession: an example from the late Devensian of southwest Wales, UK. Quaternary Science Reviews, 25: 739-762.

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).

Bristol Channel Glaciation -- are things stirring?

At last, it looks as if things might be stirring.  Geologists and geomorphologists are apparently beginning to realise that it would not be a bad idea to know more about the glaciation of the Bristol Channel area -- and all credit to Phil Gibbard and Tom Spencer for creating a studentship at Cambridge for somebody to get stuck in on answering some of the problems highlighted on this blog.  Let's hope they get a really good student (presumably starting in Oct 2015?) capable of getting us closer to a definitive chronology.
 
Graduate Research Opportunities, University of Cambridge

C112: Pleistocene glaciation of the Bristol Channel area: sedimentology, stratigraphy and geochronology (Lead Supervisor: Philip Gibbard, Geography)

http://essdtp.esc.cam.ac.uk/programme/climate-theme/c112-pleistocene-glaciation-of-the-bristol-channel-area
Supervisors: Philip Gibbard (Geography), Tom Spencer (Geography)

Importance of the area of research:
Investigation of the Quaternary history of the Bristol Channel region is an important, yet neglected focus of research. Its critical position between S. Wales and the SW Peninsula means that it has been overriden by ice during at least two major glaciations, the ice advancing up the Bristol Channel from the at some time (so far undetermined) during the Pleistocene (Crampton 1966). This is confirmed by finds of Pembrokeshire and Irish Sea erratics in SE Wales in Quaternary sediments (Bevins and Donnelly, 1992). In addition, there has long been controversy about the presence or absence of glacial deposits in the Bristol/Somerset area (e.g. Kidson and Haynes 1972) and on the floor of the Channel itself. New data collected by the supervisors demonstrates that Lundy, at the Channel's western end, was glaciated from the NW during the last glacial cycle, yet the extent of this advance further east remains unestablished. The age of glaciations further east in the Bristol Channel remains unknown and controversial. The evolution of the Bristol Channel itself is only vaguely known. Its floor includes a deep channel of the River Severn and tributaries. The implications for the evolution of the River Severn drainage system are profound, particularly including the river's extension offshore. The results will cast new light on the evolution of this critically important region.

Project summary:
This project will rely on detailed reconstruction of the three-dimensional field mapping and reconstruction of the Quaternary and associated sediment sequences. Field investigation for ground truthing will be carried out from available exposures on land throughout the area. This will be supported by using remote sensing (involving satellite imagery and aerial photographs), borehole logs from both on shore and offshore (from the BGS and other archives), sediment analysis and the application of appropriate geochronological techniques (U-series to date cemented deposits in limestone areas, OSL to date sands, radiocarbon to date organic deposits).

References:
Crampton, C.B., 1966. Certain effects of glaciation in the Vale of Glamorgan, South Wales. Journal of Glaciology 6, 261-266.

Hughes, P.D., Gibbard, P.L., Ehlers, J. 2013. Timing of glaciation during the last glacial cycle: Evaluating the concept of a global 'Last Glacial Maximum' (LGM). Earth-Science Reviews vol.125, pp.171–198.

Bevins, R.E., Donnelly, R., 1992. The Storrie Erratic Collection : a reappraisal of the status of the Pencoed ‘Older Drift’ and its significance for the Pleistocene of South Wales. Proceedings of the Geologists’ Association 103, 129-142.

Gibbard, P.L. & Clark, C.D. 2011 Chapter 7 - Pleistocene Glaciation Limits in Great Britain. p. 75-93. In: Ehlers, J., Gibbard, P.L. & Hughes, P.D. (eds) 2011 Quaternary Glaciations - Extent and Chronology - A Closer Look. Developments in Quaternary Science 15.pp. 75-93.

Kidson, C., Haynes, J.R. 1972. Glaciation in the Somerset Levels: the evidence of the Burtle Beds. Nature 239, 390-392.

Rolfe, C.J., Hughes, P.D., Fenton, C.R., Schnabel, C., Xu, S., Brown, A.G. 2012. Paired 10Be and 26Al exposure ages from Lundy: new evidence for the extent and timing of Devensian glaciation in the southern British Isles. Quaternary Science Reviews 43, 61-73.

 

The Ixer Compendium

As a service to mankind, here is a link to all the published papers by Rob and his colleagues.  Most of the recent papers relate to the bluestones at Stonehenge and in Pembrokeshire.  No excuses for anybody to plead ignorance of the main findings -- even if the detailed petrography is often rather challenging for mere mortals......

Thanks to Rob and the Academia people for making these papers accessible.  A lot of my old papers are on a similar website called Researchgate.

You can either read the papers online or download them.  Happy reading!

http://independent.academia.edu/RobertIxer

I realised that not all the key papers are there, available for download.  Here is a fuller list, thanks to Rosie's web site!

Richard E. Bevins, Rob A Ixer and Nick J.G.Pearce 2014. Carn Goedog is the likely major source of Stonehenge doleritic bluestones: evidence based on compatible element geochemistry and Principal Component Analysis. Journal of Archaeological Science, 42, 179-193.

Rob Ixer and Richard Bevins 2013. Chips off the old block:the Stonehenge debitage dilemma. Archaeology in Wales, 52, 11-22

Richard E. Bevins and Rob A. Ixer 2013 Carn Alw as a source of the rhyolitic component of the Stonehenge bluestones: a critical reappraisal of the petrographical account of H.H.Thomas. Journal of Archaeological Science, 40, 3293-3301.

Rob A. Ixer and Richard E. Bevins, with a contribution from Mike Pitts. 2013. A re-examination of rhyolitic bluestone ‘debitage’ from the Heelstone and other areas within the Stonehenge Landscape. Wiltshire Archaeological and Natural History Magazine, 106, 1-15.
 
Richard E. Bevins, Rob A. Ixer, Peter C. Webb and John S. Watson. 2012. Provenancing the rhyolitic and dacitic components of the Stonehenge landscape bluestone lithology: new petrographical and geochemical evidence. Journal of Archaeological Science,39, 1005-1019.

Rob Ixer and Richard Bevins. 2011. Craig Rhos-y-felin, Pont Saeson is the dominant source of the Stonehenge rhyolitic ‘debitage’. Archaeology in Wales, 50, 21-31.

Richard E. Bevins, Nick J.P.Pearce and Rob A.Ixer. 2011. Stonehenge rhyolitic bluestone sources and the application of zircon chemistry as a new tool for provenancing rhyolitic lithics. Journal of Archaeological Science, 38, 605-622.

Rob.A.Ixer and Richard E. Bevins. 2011. The detailed petrography of six orthostats from the bluestone circle, Stonehenge. Wiltshire Archaeological and Natural History Magazine, 104, 1-14.

Rob.A.Ixer and Richard E. Bevins. 2010. The petrography, affinity and provenance of lithics from the Cursus Field, Stonehenge. Wiltshire Archaeological and Natural History Magazine, 103, 1-15 

Timothy Darvill, Geoffrey Wainwright, Kayt Armstrong and Rob Ixer 2008. Strumble-Preselli ancient communities and environment study (SPACES): Sixth Report 2007-08. Archaeology in Wales. 48, 47-55.

T.Darvill, R.V. Davis, D.M.Evan, R.A. Ixer and G.Wainwright 2006. Strumble-Preselli ancient communities and environment study (SPACES): Fifth Report 2006. Archaeology in Wales. 46, 100-107.


R.A.Ixer and P.Turner. 2006. A detailed re-examination of the petrography of the Altar Stone and other non-sarsen sandstones from Stonehenge as a guide to their provenance. Wiltshire Archaeological and Natural History Magazine, 99, 1-15 

The Stonehenge bluestone lithology list

After all the recent geological work, what we have is a picture of multiple bluestone sources, many of which have been fixed with a degree of confidence, but with others still unknown.  The known lithologies, are itemised in the list below.  As Myris reminds us, there has been much confusion in the past about terminology, with old-fashioned terms being replaced by modern ones, and with some rock groups being amalgamated while others are split up and redefined.

Our attempts at understanding are further confused by our geological friends, who have more than once in the last few years placed rock samples into particular groups and have then changed their minds and re-classified them............


So let's try, not for the first time, to do a service to mankind by providing an up-to-date list. 

** There are 31 dolerite orthostats, of which 14 were sampled in 1991 and 2008.  Some are standing stones and some are stumps.  Some are spotted and some are unspotted.  Bevins, Ixer and Pearce (2014) analysed 22 Stonehenge dolerite samples, and suggested that they were clustered into three groups, with one sample petrologically distinct from all three.  Every one of the samples is unique, and the possibility must be entertained that every one has come from a different geographical location in eastern Preseli.

** There are five crystal vitric ash flow tuffs represented in the orthostat collection (stones 40, 48, 46, 38, 52c).  There seem to be four distinct types.  There is not much debris to match these in the Stonehenge debitage, but similar fragments are found in the great cursus field (Pitts, 1982).  Research is ongoing, but orthostats 38 and 48 and related fragments may come from north Pembrokeshire (Bevins et al, 2012).

** There are four other volcanic ashes -- stumps 32c, 33e, 33f, 41d. (These are not yet analysed.)

** There is one calcareous volcanic ash stump -- number 40c (This is not yet analysed.)

** There are two micaceous sandstone stumps -- numbered 40g and 42c.  There are also lumps of Lower Palaeozoic sandstone scattered about in the debitage -- the largest lump weighs c 8.5 kgs.  There are possible sources in the Lower Palaeozoic rocks of north-west Pembrokeshire.

** The Altar Stone (stone 80) is a greenish calcareous sandstone, probably from the Senni Beds of Carmarthenshire or Powys. (It is not from Milford Haven.)  There is some debitage related to this stone, but it cannot be established that the fragments came from the Altar Stone itself (Ixer and Bevins, 2013).

** In the debitage there are many fragments of rhyolites, some with planar fabrics -- and most matching the rock outcrops at Craig Rhosyfelin.  There appear to be three distinct groups of "rhyolites with fabric", all assumed to have come from the Pont Saeson area.  There may be a match with orthostat 32e or 32d (Ixer and Bevins, 2011).

** There are also some basic tuffs in the collection of fragments from the debitage -- two lithologically different types (Ixer and Bevins, 2013). Also from the Fishguard Volcanic Series?

** Other lithics in the stone collections from the debitage (eg. haematite (from the Reading Beds?), greensand, slate, limestones, Mesozoic sandstones and gabbros) appear genuine, and need further research.

Ixer and Bevins (2014) state that there are “about ten types of bluestone” represented in the orthostat / debitage samples, but they also show that these have nonetheless come from around 20 different locations.  It is estimated on the basis of the above points that there are at least 30 different rock types represented in the full "bluestone assemblage" -- especially when due respect is given in this count to cobbles as well as orthostats and flakes, for reasons frequently recited on this blog.  It should also be noted that the majority of the 43 bluestone monoliths/stumps at Stonehenge have still not been sampled and analysed.  The new work reinforces the idea that the Stonehenge bluestone assemblage is made up entirely of rock types from the west. 


Finally, a plea to the geologists -- if there is anything wrong with this list, please tell us about it, and give us your corrections and citations.  I'll publish them without fear or favour!