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Wednesday, 12 August 2015

Rhosyfelin: Once upon a time in the west......

When I was teaching glacial geomorphology, I always found it useful to show students what certain types of glacial environments looked like in the past, by using parallels from today.  Analogies are always good, although of course none of them can exactly replicate the conditions you are trying to work out when you look at a glaciated landscape or a set of sediments.

Anyway, I am pretty convinced that at the time when the till and fluvio-glacial materials were being laid down at Rhosyfelin, about 20,000 years ago, the valley at Rhosyfelin would have looked rather like this:










In these ice-contact or glacier snout environments, we see vast amounts of till, water-carried sediments including boulders, cobbles, sands, gravels, silts and clays -- all mixed up together in very complex relationships.  Small lakes form quickly and drain quickly. These are very dynamic and dangerous environments because buried ice is melting, causing slopes to be very unstable.  "Catastrophic" events are commonplace.

I have seen the sorts of deposits we see at Rhosyfelin in many environments like this, in Greenland, Axel Heiberg, Norway, Iceland and Antarctica.

At Rhosyfelin, these glacial and fluvioglacial deposits seem to be intercalated with rockfall materials and buried beneath up to 2m of stony and pseudo-stratified slope deposits, accumulated over the past 20,000 years or so.  There are signs of water-lain deposits not far from the tip of the spur, suggestive of temporary lakes in the main channel.  The big "proto-orthostat" at Rhosyfelin seems to rest on these "glacial environment" deposits, and following its emplacement in a rockfall it was then buried beneath slope deposits.

If the archaeologists try to tell you that all the sediments around and on top of the big stone were laid down within the past 5,000 years or so, don't believe them.

7 comments:

chris johnson said...

Thanks for a super series - it does indeed make the picture clearer.

I wonder whether the debate about the extent of glaciation in the "twenty thousand years ago" frame is now resolved for Pembrokeshire. I recall reading Seymour a while back who postulated that areas of SW Wales might have been mild enough to serve as a refuge for plant life, and which I interpreted as meaning a more rapid flourishing of nature after the younger dryas. The general opinion seemed to be that Pembrokeshire might well have been tundra - I think the correct term is "prolonged periglacial environment" in the Devensian.

In other words, might it be that the pictures are more appropriate to the Anglian glaciation? This would give more time for the gentle countryside we now see towards the North to have developed.

I am very interested in how the first migrants to Pembrokeshire might have perceived the environment so I wonder if there is a clear opinion as to what happened in Prescelli during the Younger Dryas.

Lots of questions, I'm afraid

BRIAN JOHN said...

Chris -- suggest you do a search on this blog. Just type in "Devensian ice limits" and lots of posts will come up. In particular, take a look at this one:

http://brian-mountainman.blogspot.co.uk/2011/10/devensian-ice-limit-in-north.html

I'm increasingly convinced that a large part of S Pembs was ice-free tundra during the Devensian maximum -- so, yes, the idea of plant refuges etc is perfectly feasible.

And yes, there were people in Pembs around 20,000 years ago when the ice edge was close by. Put "Caldey Island" into the search box!

BRIAN JOHN said...

Sorry -- your point about "Which glaciation?" -- ice edges look like this at the end of every glaciation. When all the dead ice has gone the topography calms down rather a lot! And from that point in, denudation and rearrangement kicks in. The older the deposits, the more they are rearranged and even removed. Hence my speculations about Llangolman.....

chris johnson said...

Fascinating picture. So someone standing on Carn Menyn just after the ice had melted would be looking North at a ravaged landscape bare of all vegetation and highly perilous to visit - perhaps as recently as 8000 BC? Always presuming there were people, of course.

BRIAN JOHN said...

Chris -- take a look at this post:

http://brian-mountainman.blogspot.co.uk/2013/01/ten-thousand-missing-years.html

Hugh Thomas said...

All makes perfect, level headed sense and gives me an insight into what we are really looking at today in Rhos Y Felyn. Thank you for posting...

chris johnson said...

Thanks Brian,

It is likely there were some people around based on the Caldey finds, not to mention red ladies. What traditions they had and tales to tell we will never know, but the contrast between south and north preselli was likely stark. The sea levels were much lower so this would have been the mountains.

The Younger Dryas was much colder than I was imagining. Do you think the ice returned to cover rhosyfelin?