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Wednesday, 17 September 2014

Rhosyfelin -- melting ice and wholesale chaos


Last year I predicted that if the Rhosyfelin dig was extended out onto the valley floor, there was a fair chance that  fluvio-glacial materials would be revealed somewhere near the base of the sediment sequence.  And so it has come to pass........  the chaotic mess exposed by the archaeologists is something I have seen scores of times in the Arctic and Antarctic where rocky outcrops occur in the vicinity of melting ice and where torrential meltwater streams are also a part of the scene.  The photo above shows what has been revealed beyond the end of the rhyolite spur.  There is a mixture of gravelly and sandy till and fluvio-glacial materials consisting of sub-rounded and rounded cobbles and boulders in a gravelly matrix.  This is typical of violent and high-velocity meltwater flow close to a wasting ice edge.  My guess is that the deposit dates from around 20,000 years ago.  Here is a close-up:

  

You can see from the exposed surface of the dig that the till occurs here and there, and is mixed in with the jumble of rockfall slabs and boulders.  There is no "Neolithic floor" -- the broken blocks of rhyolite are scattered about at all levels -- on top of the till and fluvio-glacial materials, incorporated within them, and also beneath them.  Blocks have been falling off this crag intermittently before, during and after the last glacial episode to have affected this area.  This is exactly as I described the situation in "A Long History of Rhosyfelin."  

The glacial and fluvio-glacial deposits coincide with the iron-rich hardpan or iron-pan about which I have written in previous posts on this blog.  Above the iron-pan there are gleyed soils in some places and otherwise organic-rich soils and colluvium.  It will be interesting to see what radiocarbon dates come from these deposits.  My guess is that they span the whole of the Holocene -- so dates as early as 18,000 yrs BP are quite possible.



In other places (for example out on the valley floor) there are other gleyed materials rich in silt and clay BENEATH the hardpan -- and things get even more interesting.  The two photos above show very fine-grained and relatively stoneless silts, clays and sands on top of the glacial and fluvio-glacial materials at Rhosyfelin.  In one case (see the upper photo) there is a "hummock" of gravelly material with these finer sediments draped over the top of it.  Are we looking at a dead-ice or melt-out feature here, or is this a frost-heave structure?  Somebody needs to examine it carefully.....

So have these fine sediments been deposited in a lake?  It's quite possible.  Small and short-lived lakes are very common around the edges of melting glaciers, and are created when ice masses block the natural outlets which meltwater would like to use.

So could this lake have been Glacial Lake Brynberian, about which we have speculated many times?  Possible, but the evidence relating to Glacial Lake Teifi shows that this lake -- and many others linked to it -- formed and drained while the Devensian ice was still advancing into North Pembrokeshire and the Teifi Valley.  So if the stratigraphy here is behaving as it should, any sediments relating to Glacial Lake Brynberian should occur BENEATH the fluvio-glacial materials that are currently exposed on the floor of the dig.  Keep digging, lads and lasses -- it will be very interesting indeed to know what's even deeper down........


2 comments:

Constatinos Ragazas said...

Brian,

Very interesting indeed! You write,

”There [at the Rhosyfelin excavations beyond the outcrop towards the flood-plain] is a mixture of gravelly and sandy till and fluvio-glacial material consisting of sub-rounded and rounder cobbles and boulders in a gravelly matrix. This is typical of violent and high-velocity meltwater flow close to a wasting ice edge.” (The bold highlight is mine!)

On 26 November 2012 17:58 (http://brian-mountainman.blogspot.com/2012/11/rhosyfelin-is-it-quarry-site.html ) I wrote, ”I am intrigued by the flat smooth polish of the N-W face at Rhosyfelin ....

Could [the N-W face at Rhosyfelin] have been cut and polished by meltwater torrents streaming down that side shaping the smooth N-W side but not the S-E side? And wont that explain all the fallen stones? As it would also explain the soil cover (just a meter? deep) that had them buried – as the stream volume and flow making such deposits eased over time.”
(added highlight my own).

The new evidence validates my observations re: Rhosyfelin from two years ago! Yet your comments continue to ingnor mention of these. That wont be as objectionable to me if it wasn't for the heated debate and rejection at the time my observations provoked in your blog. Good to know I was right two years ago after all!

Next step? Demonstrate Crag Rhosyfelin was engulfed in water at the time. And prove that time was more recent (post Stonehenge) by C14 dating fresh water shells taken from deep cracks at the very top of the Crag.

We are coming near to resolving MPP's “quarry” fantasy.

Kostas

Davey said...

I am not sure that you are looking at the same stones that MPP described as the 'fulcrum' standing stones. The only one i am certain of is the one at the base of the large slab that was once described as lying on rock tracks (although oddly, no mention of tracks last night, the tracks having seemingly been replaced by a man made terrace).

Beyond this stone was the semi circular 'revetment'.

There was another at the end of the outcrop where the rock is indented and where Ixer had taken the samples from and i think the third was in the bowl area.

I forgot to mention yesterday that he also said that right at the end of the outcrop was signs of habitation, the area has darker soils and there is an entrance way. He mentions it as having a long episode of having hearths.