THE BOOK
Some of the ideas discussed in this blog are published in my new book called "The Stonehenge Bluestones" -- due for publication on June 1st 2018. After that, it will be available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click
HERE

Sunday, 20 May 2018

Seymour thesis on vegetation development in Holocene Preseli


I had not realised that this thesis from Philip Seymour had been digitised.  Anyway, it is a useful resource in that it describes environmental change since the last "cold snap", as recorded in the pollen record.  It's very tightly focussed, and I would have like to see a bit more awareness of the wider context; my sense is that there was a concentration at the time on working out what anthropogenic changes there were -- still a reaction, maybe, to the old ideas of environmental determinism......

Some of the sites examined were in the eastern Preseli area -- referred to by the author as the "Bluestone Area".  One interesting thing is the author's unswerving allegiance to the human transport thesis; he says there is so much evidence of human occupation and activity in the area around Foel Drygarn and Caen Meini that Kellaway's glacial transport thesis becomes "unnecessary" !!  Hmmm.... It was a long time ago, and we'll let that pass.

But a useful document nonetheless......


The environmental history of the preseli region of South-West Wales over the past 12,000 years

Seymour, W. Philip
Date: 1985

Citation:
Seymour, W. P. (1985) 'The environmental history of the preseli region of South-West Wales over the past 12,000 years', Institute of Geography and Earth Sciences, Aberystwyth University

Abstract:
The project involves a detailed palynological investigation into the environmental changes that took place during the Late-Devensian Lateglacial and Flandrian periods in the Preseli district of northern Pembrokeshire, south-west Wales. The approach adopted specifically takes into account the considerable diversity in terms of subenvironments and ecological habitats within the field area, with representative sites on the northern coastal plain, the exposed ridges of the Preseli Hills, sheltered valleys which dissect the uplands, and the flanking plateaux. In this manner and through definition of local pollen assemblages, unrepresentative extrapolations are minimised and an unbiased regional chronology has been produced. Results indicate that the distinctive climatic character of Pembrokeshire was probably manifest throughout the entire period under discussion. Thus, Corylus was locally present during the Late-Devensian Lateglacial Interstadial as it expanded from refugia to the south and west, and its extension very early during the Flandrian is also recognised. Conversely, Betula was relatively subdued during the Lateglacial and Early Flandrian, therefore suggesting that migration across the Cambrian uplands to the east was inhibited, particularly with the prolonged influences of the Loch Lomond (Younger Dryas) stadial on the high ground. The early establishment of mixed oak forest on the coastal plain is also recognised, although with some variation in its distribution within the field area. Apart from iiilocalised occurrences of carr woodland, however, the main Alnus rise did not occur until c. 6800 BP, when it is suggested that the rising sea-level may have been largely instrumental in creating suitable habitats on the littoral lowlands. During the later part of the period in particular, the variable activities of prehistoric populations are evident. Especially notable is the centre of activity during the Late Neolithic - Early Bronze Age near the site associated with the origins of the Stonehenge Bluestones. During the post-Roman period several cycles of increased exploitation and abandonment are recognised and these correlate well with historical evidence.

Saturday, 19 May 2018

Blanket peat on Waun Mawn


Exposure in the side of one of the drainage channels on Waun Mawn, on a gentle south-facing slope.  Here we see about 30 cm of iron-stained / gleyed regolith made of broken meta-mudstone debris with signs of cryoturbation.  Above that there is a thin layer with streaks of organic material, and above that about 10 cm of "blanket peat" containing a network of roots from the present-day turf layer at the surface, which is only 5 cm thick.  This is quite typical of the moorland hereabouts.  There is no sign of Devensian till at this location.


I have been taking another look at the "progress report" written by Prof MPP for the Rust Family Foundation:

http://www.rfamfound1.org/proj23find.html

In the section on Waun Mawn some stress is placed on "the date of peat formation" as a guide to the age of the sockets and the speculations surrounding stone removal.

As far as the biggest recumbent stone in the putative "proto-Stonehenge" circle is concerned, MPP says this: Its former stone socket is lined with many packing stones, and the peat fills of this socket indicate that the stone fell after the onset of peat growth."

On the other hand, "The smaller recumbent stone excavated in 2017 is on the east end of the arc and is just under 1m long (fig.4). The fill of the stone socket contains only brown loam and no peat, indicating that it filled before the growth of peat. Thus this stone came down before peat growth."

Then again:  "Emptied stone sockets with stone packing (but no surviving monolith) were identified beyond both ends of the arc of monoliths. The socket on the west side was a circular pit (0.85 m- diameter and 0.3 m-deep) containing large packing stones set vertically. The emptied socket had filled with brown soil before any peat formation. Deformation of the edge of the pit showed that its former standing stone had been removed towards the north."
"We discovered two empty stone sockets on each end of the arc, suggesting that these stones may well be the remains of a dismantled stone circle (figs.5,6). Megaliths were removed from these sockets before the onset of peat growth on this site, indicating that the stone
circle was dismantled in the distant past."

Tying things up and seeking to demonstrate (even at this very early stage) that there WAS indeed a stone circle here, on the basis of very scanty evidence, MPP concludes:  "It can be assumed that the lack of peat in three of the stone sockets indicates that their standing stones were removed before the growth of blanket bog. This is likely to have started growing around 3,000 years ago, which would indicate that the stones came down in the Neolithic or earlier Bronze Age."

This is all seriously confusing.  MPP suggests that blanket peat formation here did not start until 3,000 years ago, which would place it in the Sub-Atlantic climate phase (pollen zone VIII), well after the elm decline and 3,000 years later that the date normally assumed for blanket peat development in the uplands of Wales.  In most of the texts and in Gallego-Sala et al (2015) it is suggested that in upland Wales blanket peat development probably started early -- maybe as early as 7,000 years ago --  during the Atlantic "climatic optimum" when it was warm and wet.  It's also suggested that around 3,000 years ago, at a time of lower rainfall totals, and an increase in ash and birch cover, blanket peat development might actually have slowed.  After 3,000 years ago, blanket peat initiation occurred only in a smallish number of "less favoured" locations. 

In the Preseli uplands, we can reasonably assume that blanket peat development will have started at the same time as in the other uplands of Wales where there were acid soils and high precipitation rates.

Nothing seems to fit.  So we have a problem........

Could it be that the things being called "stone sockets" are not stone sockets at all, but are simply surface depressions or irregularities that have nothing whatsoever to do with standing stones?

================

Gallego­-Sala, A. V., Charman, D. J., Harrison, S. P., Li, G. and Prentice, I. C. (2015) Climate­-driven expansion of blanket bogs in Britain during the Holocene. Climate of the Past Discussions, 11 (5). pp. 4811­-4832.
ISSN 1814­9359 
Available at http://centaur.reading.ac.uk/48736/

PS.  The only detailed work on the development of vegetation in the Preseli - North Pembrokeshire area is a thesis by Philip Seymour, completed in 1985.  It can be seen here:
http://cadair.aber.ac.uk/dspace/handle/2160/3307

It's essentially a pollen analysis study based on a variety of upland and lowland suites, recording changes in pollen frequencies in sediment sequences.  It makes the point that the development of blanket peat bogs was never very great in this area, partly because of the lack of extensive plateau surfaces where waterlogging could occur.  So drainage -- mostly on gentle slopes -- was generally sufficient to prevent blanket peat development.  This is borne out by the generally thin peat layers which we find across most of the landscape -- 10 cm is a rather typical thickness.  Did all of the peat start to develop at about the same time?  And was that time associated with the Neolithic / Bronze Age increase in land clearance associated with forest burning and increased grazing activity?  Seymour suggests that this was the case, and that peat development before the Neolithic was not very marked, especially on fairly well-drained slopes.  He takes a rather anthropogenic approach, suggesting that peat and soil development was very much influenced by settlement and land use practices.  But there is a danger of circular reasoning -- was the environment causing man to make certain land-use decisions, or were cultural decisions shaping the environment?  Walker and McCarroll (in the QRA Field Guide for West Wales, 2001) take a more nuanced approach, agreeing that periods of peat development are associated with periods of increased rainfall, leaching, iron pan creation and waterlogging  -- while admitting that there is such a wide range of dates for the "onset of peat development" in West Wales that land use practices and settlement pressure must have some role to play.

It will be interesting to see what turns up when Waun Mawn is examined in greater detail.....





The raised beach platform at Lydstep Point

Looking east

Looking west

This is probably the most spectacular raised beach platform in Pembrokeshire -- it's about 100m long,  and up to 25m wide, and is tucked into the little bay between Lydstep Point and Whitesheet Rock.   It cuts across near-vertical strata, and appears to have nothing to do with any faults or fractures in the Carboniferous Limestone.  It's difficult to photograph because it is so extensive -- but everywhere it has quite a gentle gradient down from a distinct notch cut into the cliff slope, and at its outer edge there is a sharp drop down into the sea.

The most fascinating thing about this platform is that it is incredibly chopped up -- criss-crossed with fissures and chasms and undermined by caves.  It is actually quite difficult to walk across it because of these surface irregularities.  This, to me, indicates very great age --  the chasms, pits and collapsed caves are all signs of marine processes currently destroying something formed a long tome ago, at a time when sea level was rather stable, around 15-20 m above its present level.  I think that this raised beach platform is at a higher level than that of Broad Haven -- which is also cut into a limestone coast.

In spite of a thorough search, I found no traces of a raised beach here (cemented or loose) and no trace of any till.  But there is an area of about 10m x 10m where cemented limestone breccia rests on the platform and has survived subsequent erosion -- storm waves certainly get onto this platform when there is a southerly gale combined with a high tide.


Here the breccia is about 1m thick, and about 2m  thick in a few places -- and it has to be related to the limestone breccia on the neck of the small peninsula just 450m to the west.  The other interesting feature of the platform is the presence of a number of widened fissures and "slit caves" cut into the face of the old cliffline at the bach edge of the platform.  These are perfect locations in which animal remains and maybe other organic materials might be found.  These would be invaluable in working out the chronology of this site.


My instinct is that there might be raised beach cobbles -- and maybe ancient till -- beneath the cemented limestone breccia, waiting to be discovered.  The rock platform itself may even predate the Anglian glacial episode -- but it could of course be a composite feature, freshened up during several interglacial high stillstands of the sea.



Hut circle on Waun Mawn


The hut circle on Waun Mawn -- some stones visible, and others buried in the turf.  Too small to be a stone circle, and too big to be the remains of a cromlech, I suspect......

I went over to Waun Mawn to see if I could find the little hut circle shown on some of the old maps.  I found it all right --- it's very small indeed, less than 5m across, so if it was a hut it must have been very cosy.......

It's located about 200m from the single standing stone to the north of the Gernos Fach track.  Go directly upslope from the standing stone, and you'll see the small grassy mounds and the stones towards the eastern edge of a grassy area surrounded by low gorse bushes.


The single standing stone on Waun Mawr, on the north side of the farm track

Friday, 18 May 2018

Lydstep ancient till site confirmed


National Trust map of Lydstep headland, showing key Pleistocene sites


Further to my earlier note about the suggested ancient till at Lydstep, I have been back there today to check it out -- and it is very impressive indeed.

https://brian-mountainman.blogspot.co.uk/search?q=+Lydstep

https://brian-mountainman.blogspot.co.uk/2014/03/classic-pembrokeshire-pleistocene.html

The first record of an ancient till at this site is from Arthur Leach, in his papers and manuscripts held in Tenby Museum:


I also visited the site and published a brief record of it in 1974.  The key location is a little peninsula that extends southwards from the "heel" of the big peninsula.  Here, by a freak of nature, a vast expanse of cemented limestone breccia has been protected from extensive marine erosion.  An area of at least 30m x 20m supports a most unusual -- and slightly surreal- -- landscape of concreted ridges and hummocks of limestone debris (mostly sharp-edged fragments which are partly frost-shattered and partly natural scree slope accumulations) on an undulating rock platform about 20m above sea-level.  I agree with Leach that this is not a wave-cut platform -- it is too high, and it does not have a wave-cut notch against an old cliffline.  I have struggled a bit to work out where on earth all this shattered scree has come from, and  have come to the view that it cannot all have come from the cliff slope to the north.  I think this is the floor of an old karst dry valley like the one that runs northwards from the western edge of Lydstep Headland today.  The southern side of this valley has, I think, been completely lost to marine erosion.  This is an indication of the great age of the breccia and everything that lies beneath it.


Cemented limestone breccia about 2m thick on the little peninsula at Lydstep, with the karst coast beyond (looking west)


Cemented limestone breccia about 3 m thick, at the foot of the northern valley slope.  There is no till exposure at this location.


Cemented limestone breccia sheet about 2m thick, resting on broken limestone bedrock.  To the left of centre is a chasm that falls straight down to the sea below.  


Looking straight down through another gaping hole in the sheet of limestone breccia.  The breccia now forms the roof of a gigantic cave, and eventually it will all collapse into  the sea........

At the eastern edge of the breccia exposures, there is a spectacular overhang or projection of the breccia sheet, sticking out eastwards by about 2m -- and beneath is is the classic exposure of ancient till.  The grid ref is approx SN 088975.  


The spectacular overhang beneath which the ancient till deposit has been protected.

The till exposure is about 10m long, and the till is about 1 m thick.  It is solidly cemented, like the breccia above it.  It rests directly on an undulating bedrock surface that has the appearance of ice moulding -- but no striations could be seen because of heavy manganese oxide staining. 


Cemented till (here containing mostly broken limestone fragments) resting directly on a stained bedrock surface


Approx 1 m of cemented till exposed beneath the overhang.


Large erratic block and smaller cobbles in a sandy till matrix, beneath the overhang.


Erratics in the cemented till, including ORS, buff sandstones, mudstone and shale fragments, as well as many local limestone fragments.


Here and there above the till is a cemented deposit of sandrock or sandy loam, foxy red in colour and full of interesting holes.  Animal burrows, or root holes, created when the sediment was fresh and soft?

I have a full record of the site, and the Pleistocene sequence seems to be:

4.  Up to 3m of cemented limestone breccia, in places blocky from catastrophic rockfalls, and in places full of smaller (frost-shattered?) fragments.  No clear stratification.

3.  Cemented sandrock containing some till and limestone fragments -- up to 20 cms thick.

2.  Cemented ancient till up to 1 m thick, containing abundant foreign erratics.

1.  Bedrock floor of old valley, apparently smoothed beneath glacial deposits.

There are no raised beach deposits here, and no fluvioglacial deposits either.  The best guess must be that the till dates from the Anglian glaciation, that the brickearth represents a climatic amelioration (interglacial?), and that the thick cap of limestone breccia has accumulated during the whole span of the Devensian glacial episode.  There is no fresh Devensian till here, but it is found a short distance away (about 200m) at the head of the creek leading into the dry valley.

As far as I am aware, this is the most extensive and most accessible ancient till deposit in Wales.






Devensian till at Lydstep



While out on o bookselling trip today I was inspired by the gorgeous summer weather to take a small diversion to Lydstep, where I wanted to check out an old record of mine regarding an ancient till deposit.  More of that in another post.

What I also discovered, on descending to the beach (which you can only do at low tide), was a splendid exposure of fresh till which has to be Devensian.  It's located in the stream gully where one has to scramble down from the grassy floor of the dry valley to the boulder-strewn beach.  The grid reference is SN 087976.

The exposure reveals at least 2m of fresh till, with a sandy and gravelly matrix and a wide assortment of striated, faceted and worn cobbles and smaller stones of many different lithologies -- including ORS, grey and buff sandstones and quartzites, blackish mudstones, shales, flints and quartz pebbles.   There's very little rounded material, suggesting that no raised beach or Pliocene pebble beds have been incorporated here.

The till is exposed almost up to the ground surface, and is capped only by the thin soil layer.  There is a lot of slumping on the exposure, but the till appears to be  underlain by a foxy red clay-rich deposit (at least 1 m thick) that appears to be relatively stone-free.  I could not determine whether this was a basal clay-rich till layer or a locally-derived gash breccia such as we see in many fissures along the limestone coast.

Anyway, this is one of the most coherent and easily accessible fresh till exposures on the South Pembrokeshire coast,  and there can be little doubt about its age, since it is completely uncemented (in an environment dominated by calcium carbonate) and has no scree or slope deposits on top of it.  It has to be Late Devensian in age.

As a matter of interest, this boulder rests on the beach just a few metres away, in a jumble of other large boulders.  It looks to me like gabbro -- similar to the gabbro that outcrops near St David's Head.  But that's just an educated guess......


Heavily abraded boulder of gabbro (?) not far from the till exposure at Lydstep.  It's one of the largest erratics seen thus far along the south Pembrokeshire coast.


As a matter of interest, the other posts describing Devensian till on the South Pembrokeshire coast are here:


https://brian-mountainman.blogspot.co.uk/2011/08/glaciation-of-south-pembrokeshire.html


https://brian-mountainman.blogspot.co.uk/2018/04/the-devensian-ice-limit-in.html

https://brian-mountainman.blogspot.co.uk/2011/08/glaciation-of-caldey.html

Thursday, 17 May 2018

Glaciation of the Bristol - Gloucester region



This is an interesting summary from the Earthwise (BGS) web site.  Found here:


It's bang up-to-date, having been revised in January 2018.  As the authors say:  "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."

Show this to anybody who suggests that ice never flows eastwards, or that the Irish Sea Glacier never crossed the Bristol Channel and penetrated far into Somerset.........

There is no assessment here of the likely easternmost limit of the Anglian ice.

----------------------------------------


Glacial deposits, Quaternary, Bristol and Gloucester region


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

Glacial deposits

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.


References


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.
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.
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.

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