THE BOOK
Some of the ideas discussed in this blog are published in my book called "The Bluestone Enigma" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
To order, click
HERE

Thursday, 30 October 2014

EH makes monographs available for free


 Good for EH -- they have just made a long list of research monographs available for free download.  Some very interesting materials is included in the list........


http://archaeologydataservice.ac.uk/archives/view/eh_monographs_2014/index.cfm?type=Date

Abermawr -- one of the top 50 Quaternary sites in the UK?



 The deformed contact between underlying periglacial slope deposits of Early / Middle Devensian age, and the overlying Irish Sea till, laid down by Late Devensian ice moving onshore from the north-west

The QRA (Quaternary Research Association) is asking all its members for nominations for the "top 50" Quaternary sites in the UK.  I have nominated Abermawr -- it has to be up there with the best!


Here is my citation:
This is the most comprehensive exposure of Late Pleistocene deposits in West Wales.  There are exposures at both ends of the bay.  Storms have revealed an Ipswichian raised beach on a rock platform remnant, and above that there is a sequence of periglacial deposits made up of angular bedrock fragments, but incorporating far-travelled erratics.  Above that is a clay-rich Irish Sea till of Late Devensian age and containing striated clasts, fragments of carbonized wood and sea shells.  The main components of the till are sea-floor deposits, dredged up by glacier ice moving across the old coastline and later laid down by lodgement and shearing.  There are also flow-tills, and the glacial deposits are capped by fluvio-glacial materials, an upper head (referred to in the past as “rubble-drift”), sandy loam and modern soil.  The deposits represent a complete advance/retreat cycle close to a glacier margin. In the upper head there are fossil ice-wedges and involutions of Late Glacial age.  Beneath the storm beach there are peat beds and remnants of the “submerged forest”, and these organic-rich sediments can be examined in the marsh on the landward side of the storm ridge. There is a continuous stratigraphic record here, probably stretching back c 100,000 years.

Rijsdijk, K and McCarroll, D. 2001. Abermawr, in The Quaternary of West Wales Field Guide, QRA, pp 32 - 38.

John, BS 1970. Pembrokeshire, in  Lewis, CA (ed) The Glaciations of Wales and Adjoining Regions, pp 229-265

Wednesday, 29 October 2014

Was Cardiff Bay affected by Devensian ice?


With reference to my earlier post about the position of the Devensian ice front on the southern flank of the Welsh Ice Cap, there is still disagreement among geomorphologists.

This section of the BRITICE Devensian map shows abundant glacial / fluvioglacial deposits in the Cardiff - Newport area, but the compilers of the map decided that the Vale of Glamorgan was ice-free at this time, as was the coastal stretch to the east of Lavernock point.  The dotted line on the map shows the assumed maximum ice front position.

In contrast, Devensian ice lobes are assumed to have reached the Bristol Channel lowlands (ie beyond the present coastline) both in Swansea Bay and Carmarthen Bay.

My earlier post is here:
http://brian-mountainman.blogspot.co.uk/2014/10/flat-holm-erratic-hunt-6-lets-imagine.html

The extensive spreads of ice-related deposits shown on the map were assumed by Charlesworth in 1929 to represent the position of the South wales End Moraine, but it's now known that things were much more complicated than that.  Some of the deposits on the map are Devensian, and some may well be much older.......  In some of the reconstructions by Prof DQ Bowen over the years, he has an ice lobe pushing offshore between Cardiff and Newport, and in others the lobe fails to reach the coast.  Work in progress?

Saturday, 25 October 2014

Mystery

Just been alerted to the fact that 1470 people have looked at this blog post about braided rivers in Iceland......
http://brian-mountainman.blogspot.co.uk/2014/05/braided-rivers-in-iceland.html

Well over 700 have looked at it today -- I wonder why?  Does anybody else have any idea what's going on?  Admittedly, they are very beautiful images......

Tim's Long Barrow opens for business


Tim's long barrow is open for business -- and if you want to take a look, get over there right now!!  More info here on the BBC web site:


 and here, on Tim's site:




Wednesday, 22 October 2014

BRITICE map of Pembrokeshire's old glacial deposits


This is the relevant piece of the BRITICE map of glacial deposits and other features relating to the Last Glaciation.  It's a few years old now, and new information has made parts of it look dated, but it was the state of the art summary when it was published, and it's still a valuable aid to future research.  The whole map is in two sections available for download -- northern and southern sheets.  This extract is from the southern sheet.

The map shows the area assumed to have been ice-free in the Devensian (faint yellow colouring), main spreads of fluvio-glacial gravels (brown), proglacial lakes (blue), meltwater channels (dark blue), glacial limits (dotted lines) and assumed ice dam positions (yellow lines).

Let's forget about the north Pembrokeshire features for the moment and concentrate on those of central Pembrokeshire.  The patches of glacial deposits (gravels and till) shown are generalised from the BGS Geology of Britain map at the 1:50,000 scale.  Those of the Western Cleddau valley are well shown, as are the sand and gravel patches to the east of Haverfordwest, in the Clarbeston Road - Clynderwen area.  But the patches to the north, in the foothills of Preseli, are not shown, probably because they are not extensive enough.  But they are rather intriguing, as I suggested yesterday.  What do they tell us about the glacial history of West Wales?  I'm on the case......

http://mapapps.bgs.ac.uk/geologyofbritain/home.html

http://www.britice-chrono.group.shef.ac.uk/current-sampling-map/

http://www.sheffield.ac.uk/polopoly_fs/1.22760!/file/britice_2004_s.pdf

Tuesday, 21 October 2014

Pre-Devensian glacial deposits south of Preseli


Fluvio-glacial sands and gravels (shown in pink) in the area around Llangolman and Maenclochog, to the south of the Preseli upland.

 Some more original field research here, for those who may be interested.......

Thanks to Huw Absalom of Bush Farm, Llangolman for the opportunity to look at his splendid gravels today.  Very interesting indeed.  I have been frustrated for some time because although there are plenty of patches of glacial and fluvio-glacial deposits in the central and southern parts of Pembrokeshire, as shown on the geological maps, they are very seldom exposed for proper examination.  They should be pre-Devensian if we have got our ice limits in the right places -- but hard evidence of great age was required.  

So when I got a message from Sid Howells telling me that Huw wanted somebody to take a look at the gravels in his small pit close to the Church of St Colman, I got onto the phone and organized a visit.  So I have been over there today, and things are starting to look good.

The pit (grid ref SN 115268) is not a commercial one -- it's just used for providing hardcore and concrete ballast for use on the farm.  It's about 30m across and 10m deep, exposing fluvio-glacial gravels and sands in the full height of the face.  It's cut into the top of an undulating terrace which has clearly undergone a long history of erosion, especially on its northern flank, where the slope drops down into a small tributary valley which is part of the Eastern Cleddau river system.  The valley is about 25m deep.  Huw tells me that clay is exposed on the floor of the valley, and that there are also exposures of sand not far from the stream.  It's a reasonable assumption that clay-rich till underlies the sands and gravels.

The exposed gravels are relatively  fine-grained, with no cobble-sized beds or boulder beds -- indicating turbulent but not violent torrential flow.  There are a few interbedded sand horizons, suggesting periods of quieter flow.  The bedding dips quite steeply from NW towards SE -- so this may indicate the presence of an advancing delta front close to an ice edge.  This is supported by the presence of some larger stones in the gravels, up to football size, and quite angular.  There is one large chunk of bluish rhyolite which looks as if it has come from a larger boulder.  I didn't see any striated pebbles, but I would not be surprised if some were to turn up......  But the gravels are packed with erratics, including assorted Fishguard Volcanics, rhyolite, dolerite and one quite distinctive pebble of Carboniferous Limestone.  (Where on earth did THAT come from?)

Typical gravels exposed in the Llangolman quarry face.  Note the overall alignment of long axes from top left to bottom right.  Note also just how many pebbles are heavily weathered if not rotten.

One of the black layers cemented with manganese oxide, above a sand bed about 20 cms thick.  


Fragments of the lowest black "manganese concrete" layers in the quarry, which was so hard that it had to be broken up with a tractor grab.  Note also traces of foxy-red "iron oxide" concrete.

I have never seen so many layers of manganese concrete in a single small quarry face.  There are at least six of them, including one just a metre or so beneath the ground surface.  This fact, together with the extremely rotten character of many of the pebbles in the gravels, is suggestive of great age.  Maybe we are looking at deposits from the Anglian glacial episode of about 450,000 years ago.

Great age is also indicated by the presence of "churned gravels" near the ground surface which have lost all trace of their original bedding, and several distinct fossil ice wedges and frost fissures within which many of the pebbes are "standing" with their long axes vertical.  That is indicative of pebbles falling or sliding down into an open frost track or wedge from which the ground ice has temporarily melted.  The assumption?  Thick and long-lived permafrost -- maybe for many thousands of years.



The two photos above have been annotated to show the outlines of the ice wedge casts.  Each one is between 2m and 3m deep.  Note how the gravels and sands are "churned" within the casts and normally bedded on the flanks.  I have also annotated the surface layer of gravels, where signs of frost heave are abundant.  This layer is sometimes just 50 cms deep, and in other places well over 1m deep.

Next time I visit the pit, Huw has kindly offered to be in attendance with his digger -- so we can go down deeper, and maybe find some till........

And speaking of till, it is exposed on the flank of a small stone quarry at Plascwrt, at SN 118274.  It's clay rich, with smallish pebbles and cobbles some of which are well rounded far-travelled erratics.  It is friable to the touch, and foxy red in colour.  So again the impression is that it is severely weathered as a result of many thousands of years of exposure to weather of many types.....


The reddish till layer above fractured bedrock in the small stone quarry at Plascwrt.  It is clay-rich and contains abundant erratic pebbles.


Tentative conclusion: these are very old glacial deposits which display a quite different combination of features from those exposed on the northern side of Preseli, which are deemed to be of Devensian age.  My guess is that these are of Anglian age -- or maybe Wolstonian.  (That's a glacial episode that we know very little about, at least in Western Britain.)