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

Saturday, 29 December 2012

Rhosyfelin Pleistocene Stratigraphy

The 2012 excavation, seen from near the higher part of the rock face --- the top of the sedimentary sequence is seen in the steps cut into the slope on the left.

I was looking at my photos from this summer, and happened on these.  They reminded me what an interesting stratigraphy MPP and his team have discovered at Rhosyfelin.  There is a real story in them thar sediments........  and by the way, I still see nothing at all that would make me think any of this is down to human agency.

 Stained and semi-rounded erratic (?) at the base of the 2012 excavation.  I suspect that this is at the top of a layer of till.

Blocks and rubble with some rounding of edges, at the base of the scree / rubble layer.


Coarse head with pseudo-stratification, resting on scree and rubble.  At the junction there is a darker layer which might be rich in organic material.  Is this rich enough to provide a sample for radiocarbon dating?

 The lower part of the sedimentary sequence, exposed in the lower part of the 2012 excavation pit.  This is not far from the pit deemed to have held a standing stone.  This is a very interesting stratigraphy, with at least six layers representing changing environmental conditions.  At the bottom is a reddish stained clay-rich layer that might be till.  Above that is a fine-grained layer containing streaks of darker (organic?) material.  Is this a deposit dating from Lake Brynberian?  Then there is a thin layer of broken head.  Above that is another layer of finer sediments incorporating broken rock debris, capped by a thin head layer and then another darker-coloured layer with finer sediment.  Probably the two holes mark the positions from which samples have been taken for C14 dating.

12 comments:

Constantinos Ragazas said...

Brian,

Thanks for this post! Much here to maul over. In your view, is there anything in the stratigraphy shown not consistent with my hypothesis Rhosyfelin was in water?

“Blocks and rubble with some rounding of edges, at the base of the scree / rubble layer“ makes perfect sense. Since the rubble at the base of the scree would have been longer exposed to the water torrents cutting and polishing the NW face of Rhosyfelin. While the blocks that fell later would had less time exposed to lesser flow and later none.

The only question is how long ago? My sense is it was not that long ago. And probably not any earlier than 4000BP. Which would make MPP's “human quarry” impossible! The method I outlined in an earlier post can answer this. ( http://brian-mountainman.blogspot.com/2012/12/rhosyfelin-jan-2012.html )

The enigmatic excavated pit looks more and more “fishy”.

Kostas

BRIAN JOHN said...

Kostas -- there is almost nothing here to confirm your "inundation" theory. There are no torrential fluvial or fluvio-glacial deposits expsed thus far -- although there may be some lower down. The only bit of the sequence that makes one think of standing or flowing water is the lowest fine-grained layer -- there are slight traces of lamination, and so this MIGHT be a lake deposit. Everything above it seems to tell a story of an oscillating cold and cold-temperate environment with periglacial processes occasionally operating....

Constantinos Ragazas said...

Brian,

Don't confuse me for Robert! I have no “innundation” theory. And much of what RJL argues I also disagree. Arguing the present river at Rhosyfelin was at one time wider and bigger (or even possibly a glacier Lake, as you also believe) – placing MPP's “quarry” in water – is NOT arguing for “innundation” a la RJL.

You write, “there is almost nothing here to confirm your "inundation" theory”.

That wasn't my question! I asked if anything shown is not CONSISTENT with my “---” theory!

Let me ask you directly. Do you believe the present river did NOT extend more widely to have at one time engulfed Crag Rhosyfelin just meters away?

Kostas

BRIAN JOHN said...

Sorry Kostas -- I must not allow you and Robert to get tarred with the same brush......

In answer to your question, the nature of the sediments is NOT consistent with your idea of the river being wider and deeper and engulfing the crag, at least in the period since the sediments started to accumulate.

But as I have said before, the gorge of the Afon Brynberian does look like a meltwater channel to me -- like many of the other spectacular channels in N Pembrokeshire. When was that gorge formed? Well, possibly during the Anglian Glaciation, c 450,000 years ago. Or maybe in one of the other ancient glacial episodes about which we know very little.

Constantinos Ragazas said...

So Brian,

“… you believe the present river did not extend more widely to have at one time engulfed Crag Rhosyfelin just meters away”. I accept this is what you believe.

What is your explanation of the enigmatic excavated pit? To site just one of the 'facts on the ground' that begs for explanation. Along with the rounded edges at the bottom of the scree and not at the top, the dirt heaps clinging the NW rock face, the orientation of the Crag same with the river and wider landscape embankments, the polished NW face of the Crag, etc.

Kostas

Constantinos Ragazas said...

Brian you write,

“the nature of the sediments is NOT consistent with your idea of the river being wider and deeper and engulfing the crag”

Can you speak in more detail about this? What is it about the several layers of sediment shown in your photos that goes against the Crag being engulfed in water?

Curious Kostas

BRIAN JOHN said...

These are not water-laid sediments. Please read up on a little simple sedimentology.

Constantinos Ragazas said...

Brian,

Took your advice and looked up the Wiki-article on Sedimentology (the only one I could find googling). Besides the obvious basic principles listed, nothing in the article provides an obvious explanation why you think it is so obvious these “are not water-laid sediments.”

But the article does bring up some recent research you may find interesting. Quoting from the article,

The research shows that some mudstones may have formed in fast-moving waters: "Mudstones can be deposited under more energetic conditions than widely assumed, requiring a reappraisal of many geologic records."

Geology is not fixed in stone afterall!

Kostas

BRIAN JOHN said...

Kostas -- we are not talking about the formation of mudstones here. We are talking about the formation of slope deposits under cold and temperate climatic regimes.

Constantinos Ragazas said...

Brian,

I know! Didn't mean to suggest the Rhosyfelin sediments have anything to do with mudstones.

Thought you may find this recent research interesting. And lead you to reassess your reliance on accepted geological knowledge.

I am still perprexed what you see in these photos that makes you so convinced the river at Rhosyfelin was not bigger and wider to have engulfed the Crag. Please tell!

Kostas

BRIAN JOHN said...

Kostas -- this is getting wearing! The nature of the sediments tells us what processes have operated in this locality in the past. This goes back to the earliest days of geology. Water-laid deposits look very different from periglacial slope deposits. They have different particle size distributions and different particle shapes. As simple as that. Time to move on.

Constantinos Ragazas said...

OK Brian. No problem. I accept you have concluded these deposits are 'periglacial slope deposits'.

But could you please describe the direction of flow of the river at Rhosyfelin in that aerial photo you posted and I have been refering to in my posts? Also, is the layer of broken rock debris as shown in the fourth photo in your post at the same level as the scree? How deeper if not?

HAPPY NEW YEAR EVERYONE!

Kostas