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Saturday, 3 September 2011

Carmarthen Bay in focus

This map (reproduced from Sid Howells's excellent leaflet on the geology of Caldey) shows Caldey on the shore of Carmarthen Bay and shows how the island -- according to the accepted scheme of Devensian glaciation -- had an Irish sea glacier ice edge away to the west and a Welsh Ice cap glacier to the east.  This latter, composed of ice flowing down the Loughor and Tywi Valleys, is supposed to have pushed out into the bay in its shallow eastern section -- but it is supposed that it did not cover the Gower Peninsula at the time.  (The evidence for that might well need to be revisited.) Another lobe of Welsh ice is thought to have crossed the South Wales coast in Swansea Bay, and a third on the Glamorgan coast between Cardiff and Newport.

The dashed line shows the supposed outer limit of the glaciation, and the dotted line shows the supposed contact zone where Irish Sea ice and Welsh ice came together.

If we now adjust the map to take into account the evidence from Caldey Island and South Pembrokeshire, what do we find in Carmarthen Bay?  Well, it gets rather interesting, since there was either a narrow strip of land between two ice fronts (one moving in from the west and another from the east) or else a "collision zone" in which these glaciers collided head-on.  Such a thing is not impossible.  Here is an example from West Greenland:
The contact zone between these two colliding glaciers is pretty messy, with lots of debris and lots of meltwater flowing around.  maybe something similar happened in carmarthen bay.

The problem is that most of the critical evidence needed to sort this out lies beneath the surface of the sea.  We can get some clues from these sea bed maps:

The upper map shows the basal sediments -- which might well contain a lot of glacial and fluvio-glacial material (including bluestones!!)  The lower one shows the sediments mapped at the sea-floor surface in a recent project.

Neither map gives us any strong clues as to where the ice margins might have been -- but this all requires further research.


10 comments:

Constantinos Ragazas said...

Brian,

Glacier till is characterized by pebbles, gravel and stones completely surrounded by the 'matrix support' of finer silt and sand. Am I correct in this? Or there are other characteristics for glacier till.

If so, I have a question. If this characterization of glacier till is correct, what distinguishes glacier till from deposits made by mud slides and streams?

I am specifically thinking of the Caldey till you discovered and the photo of this you have posted. How certain are you that this deposit is a glacier till and not a mud deposit? Call it intellectual curiosity!

Kostas

BRIAN JOHN said...

Kostas, I could write you a whole thesis on this. Till can be very variable, and it is indeed variable in West Wales. Please do a bit of googling on this -- the web is full of info...

Constantinos Ragazas said...

Brian,
\
Where do you suppose I found the characterization of glacier till in my last post? I googled it!

“...the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as matrix support, is diagnostic of till”. (http://en.wikipedia.org/wiki/Till)
But I couldn't find comparisons with mud stream deposits!

I know there are many huge differences between glaciers and mud masses. For one think, their mass size and power to transform large landscapes. And the size and manner of the erratics they carry and the distance these are carried. Another big difference is that glaciers melt and leave deposits mainly exposed on the surface, while mud dries and 'seals' deposits.

But one similarity, however, between glaciers and mud streams is that in both cases we have debris uniformly mixed in with pebbles and rocks surrounded by a ,'matrix of support'. In some limited circumstances this attribute may be the only observable factor. In these limited circumstances, it may not be possible to distinguish glacier till from mud deposits if all we have to go by is the characteristic of 'matrix support'. Perhaps there are other signs to look for and make this distinction. That was my question to you!

My question aims at just such limited instances where glacier till is indistinguishable on sight by mud deposits. And if there are such instances, could Caldey Island till be such? In the case of the till deposits you found at Caldey Island, what would distinguish glacier till from mud stream deposits along the coast? And what deposition process would 'seal' a fissure containing bone fragments? Or leave huge 'manure' deposits to be sold to farmers as fertilizer, as if along the banks of the Nile?

Quoting from your own post,

“... the majority of the abundant bone from the (Eel Point) site was sold by the quarrymen for manure, or simply dumped into the sea (Smith, 1860) .”

and again,

“... the discovery in 1986 of sealed ļ¬ssures containing more Pleistocene fauna (Davies, 1989; Davies and Smith, 1989).  Pleistocene faunal remains and stone tools are well known from most of the Caldey Island sites, although the association between the two is far from certain.”

Kostas

BRIAN JOHN said...

Fair enough, Kostas -- these are perfectly valid questions, and I'll seek to answer them to the best of my ability. Because there are similarities between mudflow and landslide (catastrophic) deposits and some types of till, geomorphologists have got into the habit of referring to "diamictons" which gets them away from having to make judgments about origins and processes. A diamicton is defined as: "unsorted and unstratified sedimentary deposit of any origin, for example: till, landslide debris, mudflows, turbidites, etc"

But there are differences -- for example, mudflows and other materials seldom contain striated pebbles, and neither do they generally contain pebbles of all shapes and sizes, from many different source areas. I've seen many mudflows and landslide deposits, and they generally have stones which are not well rounded or sub-angular, and which can generally be identified as having come from a defined source -- not far away, and of course upslope.

I'm pretty sure of my designation of the Caldey deposit as till -- and indeed others have done the same. Sid Howells has seen striated pebbles in some of the till deposits on Caldey, although I have to admit that I have not.

You still have the wrong picture of the bones in these caves. They were not found in piles -- they were scattered through varied cave deposits and (in the case of Eel Point) were probably collected by quarrymen in the limestone quarry and sold because they could be crushed for bonemeal.

There are many processes that can seal a fissure and thus protect any bones and artifacts that may be contained.

Constantinos Ragazas said...

Brian,

Thanks for your explanation.

Let's say there are various scenarios here that would account for the till at Caldey Island and various explanations for what is there and what is not there. I wont test your patience on this so I wont pick on specifics in your reply that could be further argued!

But just to clear a misunderstanding you have about my ideas, you write:

“You still have the wrong picture of the bones in these caves. They were not found in piles -- they were scattered through varied cave deposits and (in the case of Eel Point) were probably collected by quarrymen in the limestone quarry and sold because they could be crushed for bonemeal.”

I do not think of the bones in the caves as being “found in piles”. Far from it! I see these bits of bone fragments as mixed in the sediments found at Caldey -- whether in caves, or fissures, or I would even argue on the limestone quarry itself! And I see in all these deposits evidence these bone bits were carried to Caldey Island by meltwater and mudflows from higher elevations in Wales over a frozen Carmathen Bay. That would answer the question about these deposits being found along the northern and eastern coast.

If the 'bone deposits' in the quote from Smith (1860) were only found in caves, why would quarrymen bother and take time and energy to clear these caves of these deposits only to dump these in the sea (Smith, 1860)? Or if they did collect the bone deposits to sell to farmers as 'manure' why dump any of this into the sea when it could have been sold to farmers?

Makes sense that these deposits were on the limestone quarry itself and were interfering with the quarrying activities. I believe the 'bone manure' Smith references in 1860 were sediments rich in bone bits and soil rich in fertilizing nutrients (like with flood deposits along the Nile) that have build up over time along the northern limestone coast of Caldey Island.

All this, of course, is consistent with my hypothesis that Bristol Channel was a solidly froze waterway during the Devensian over which meltwater streams and mudflows from the mountains of Wales could have flowed carrying such debris of bone bits and pebbles and stones and sand and soil.

The 'beach terraces' at Caldey Island and till deposits at higher elevations also make perfect sense according to this hypothesis.

Kostas

BRIAN JOHN said...

Kostas -- my patience is sorely tested!! The caves are on the north side of the island because that is where the limestone is. The bones are scattered through the sediments in the caves -- sometimes fragments, sometimes whole bones. Some caves were not even known about until the quarrymen discovered them -- and of course they didn't want either the sediments or the bones, and got rid of them any way they could. The raised beach platforms and sediments do nothing at all for your mudflow hypothesis -- they are stratigraphically in quite the wrong positions.

Constantinos Ragazas said...

Brian,

From previous posts, you believe that the bluestones at Stonehenge were carried there by glaciers some 450,000 years ago. The Devensian ended some 10,000 years ago and according to the Devensian ice limits in your map and others, Devensian glaciers never reached Salisbury Plain.

My question: Why are you interested in the Devensian? Is this interest in any way connected with your glacier transport theory for the bluestones?

Kostas

BRIAN JOHN said...

We still don't know for sure where the GBG and LG limits were -- and we were all surprised when they got those surprisingly recent radiocarbon dates from the Scilly Isles. So the discussion (and the science) is ongoing. My best guess currently is that the bluestone erratics were carried in the Anglian glaciation, but I might be wrong.....

Constantinos Ragazas said...

Brian,

If as you write “We still don't know for sure where the GBG and LG limits were”, why do you summarily dismiss the possibility that Bristol Channel may have been frozen during the Devensian?

Let me address your past argument: since these 'global average temperatures' in the chart you posted are 'averages', they do not account for 'seasonal temperature variations' which would result in local ice melting during the summers and freezing during the winters.

I quote from a NASA article on average global temperatures:

“A one-degree change is significant because it takes a vast amount of heat to warm all the oceans, atmosphere, and land by that much. In the past, a one- to two-degree drop was all it took to plunge the Earth into the Little Ice Age. A five-degree drop was enough to bury a large part of North America under a towering mass of ice 20,000 years ago.” http://earthobservatory.nasa.gov/Features/WorldOfChange/decadaltemp.php

What evidence would a solidly frozen lake (for example) leave behind when it forms and melts in place and there is no ice flow?

I argue that 'local ice' formed by frozen bodies of water is significantly different from glacier ice. And looking only for evidence of glacier ice may not be conclusive of local ice once being present in an area.

Kostas

BRIAN JOHN said...

Kostas -- once and for all, these large frozen water bodies that you talk of just do not happen. This is simply a matter of physics. Ice floats, and then insulates the water beneath. Even in the most severe of periglacial climates (cf Siberia and Antarctica) there is a seasonal summer melt. That will have been the case at the peak of the Devensian glaciation as well. And in the Anglian or GBG. The only sorts of "frozen water" you can have are in seasonally frozen lakes, floating sea ice, and ice shelves. Each of those has water at depth, even in the depths of the Antarctic winter. Shallow ponds may entirely freeze over, but deeper water bodies do not, and fishes and other aquatic creatures survive in what seem impossible conditions. Look at the Antarctic today and you have your parallels -- and you have to face up to the fact that they are considerably difficult for you to cope with, given the other-worldly scenario which you keep on pushing.