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Tuesday, 8 June 2010

Sea ice and giant erratics



Giant's Rock at Porthleven -- weighing 50 tonnes. Origin unknown.....



Freshwater Gut, Croyde Bay, N Devon. The big boulder (I shall say this only once) is of garnetiferous hypersthene-bearing granulite of gneissose type. Origin unknown.......

Having taken a look at Neolithic sea-levels, what about Interglacial sea-levels in Southern England? This is an interesting question -- one which has been answered (to some degree) by Ian West:

West, Ian M. 2010. Sarsen Stones and Erratics of the Wessex Coast
http://www.soton.ac.uk/~imw/Sarsens-Erratics.htm

Extract (CONCLUSIONS)

"The English Channel glacier theory is interesting and stimulating but it is not strongly supported by firm evidence. It does not accord with the evidence of the flint gravels, there is no evidence for a "Glacial Lake Solent". The erratics were probably mostly transported by floating sea ice. There is clearly is a problem concerning dates of transport of the erratics and relative sea-levels. A phase of ice-floes, significantly earlier than the Eemian (Ipswichian) Interglacial accounts most satisfactorily for most features. The source of the erratics of the Hamphire-Sussex coastal plain seems to have been largely the south or southwest, the coasts of Brittany, the Cotentin Peninsula and the Channel Islands, but some more rocks of more distant origin seem to be present.

Some sarsens may have been transported by ice or flood action in the braided rivers of the periglacial environment. This may explain examples at Milford and other places away from the raised beach, unless these (low) area were still within reach of sea-ice. The higher level sarsens of Portland require further consideration; they are somewhat mysterious but there is a possible source area on the chalk downs northwest of Weymouth from which they could have been transported in some way. However, the deposit in which they occur is not sufficiently understood and the possibility of a high level raised beach on Portland has not been eliminated.

The erratics of the low-level Hampshire-Sussex coastal plain were transported to the area earlier than 128 thousand years BP at an unknown date. They were present near the surface during the Devensian. In the early Flandrian those on the present coastal plain and others under the sea to the south or in the channel of the Solent were also on dry land. The deeper ones were submerged round about about 8000 BP and large areas south of Hayling Island, for example, in post-Neolithic times. The erratics include sarsens (greywethers) particularly just east and southeast of the Solent, but the known examples are smaller than the stones of Stonehenge, and often weathered and crumbly to some extent. Dolerite, like the Stonehenge Bluestones, does not seem to have been recorded. There is no evidence that the sarsen stones were transported into the area by man and ice was the agent. Evidence has not yet been presented that any sarsens or other rock types from here were used in any way in connection with Stonehenge, but equally there is no evidence against. They were used in wall-construction in historic times.

Studies of the examples of erratics offshore could shed new light on the theories of ice-transport and date of origin. Unusual rock types might more clearly indicate the source areas. Any possible evidence of human (transport?)is clearly worth seeking."

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This is interesting too:

Boulders, Salcombe Fishing Grounds, English Channel

Hunt (1880, 1881, 1883, 1885) found a considerable number of foreign blocks in the Salcombe fishing grounds, some 30 to 50 km south of the Devon coast. Of 40 blocks described, there is granite, microgranulite, serpentine, syenite, gabbro, diorite, basalt, "diabase" (dolerite), trachyte, gneiss, quartz grit, conglomerate, sandstone and chalk flints and other rock types. They are discussed further by Prestwich (1892). The serpentine is precisely like the Cornish varieties. Surprisingly the other igneous rocks could not with certainty be ascribed to the English or French coasts. The gneiss resembled Hebridean gneiss from Scotland.

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The famous pink granite erratic at Saunton, lying BENEATH the sandrock and other deposits dated to the last (Ipswichian) interglacial. This gives us a clue to the age of these erratics. They have been here for a very long time......

OK -- the erratics may date from a number of different glacial episodes, but the consensus seems to be that they were transported and dumped either in the Wolstonian glacial episode (around 250,000 years ago) or in the Anglian glacial episode (around 400,000 years ago).

Most (but not all) of them are beneath 10m asl -- but as many have pointed out, nobody knows how many similar erratics there may be in SW England buried in soils and slope deposits. Absence of evidence is not evidence of absence. The 'concentration" around present sea-level may be more apparent than real, since on the coastline overlying deposits have been eroded away, leaving the erratics behind on ancient wave-cut platforms. This has led geomorphologists to expend a great deal of hot air on ice-floe transport and iceberg transport. I'm not very happy with that, since all the evidence points to LOW sea-levels at all times when glacier ice might have been calving icebergs carrying big stones into deep water. Whichever glacial episode we are talking about, and however much glacial ice there might have been in the Bristol Channel and in the Celtic Sea, I cannot envisage sufficient isostatic depression and rebound to have matched an early interglacial eustatic rise of sea-level similar to that of the Holocene.

No -- I still prefer to think of ancient glacial deposits in the South-West. and this brings us all the way back to the glacial transport of the Stonehenge bluestones.......

2 comments:

Kostas said...

Brian,

… with your latest posts you got me studying whole areas of the geology of the UK that I never before imagined I would be interested in doing! Hard to digest all that, especially if one does not have the background and prior training on geology. So you understand. My approach is simply 'common sense pragmatism' taking and using the information that I am given. But always keeping a fresh and uncommitted mind without prejudice, moved only by the search for truth.

Some preliminary comments.

Quoting I believe from Clement Reid in the article by Ian West you referenced:

"About a hundred of these pits were examined, and the conclusion seemed irresistible that they afforded clear evidence of the agency of floating ice. Drift-ice grounding on the ancient foreshore dropped its burden of erratics between the tide marks."

And also,

"It is thus that I account for the occurrence of emptly pits, for they seem to mark the former sites of blocks which may have shifted their position several times before finally coming to rest."

If these huge sarsens were carried to shore by floating sea-ice, how thick the floating ice need to be in order to support the weight of a 50 ton sarsen? Certainly the ice had to be considerably thick to bare the weight. If so, wont the floating ice dump its cargo much deeper into the water than carry the sarsen far up on the beach?

I don't understand the thinking that allows drifting sea-ice carry the huge sarsens ashore, but does not consider land-ice bringing the sarsens to the lowest point from higher elevations.

Interestingly also, we once again are witnessing 'empty pits' (just like the empty pits of the Stonehenge Layer). And once again the explanation is that the same sarsens were once placed in one pit, than latter in another and then another. With the Stonehenge pits, this was done through the agency of men (so goes the explanation) while with these shore pits, the agency is nature and the sea.

I don't doubt that the nature and the sea have the power to carry these huge stones from place to place. But if they have the power to transport them and transplant them, wont you think that the same power would also eliminate any vestiges of the previous pits? It doesn't make sense.

But here is an explanation that can explain the 'empty pits', whether at Stonhenge or at these shores. The same agency of land-ice (according to my theory) that would have brought sarsen stones to these locations would also bring equally huge broken blocks of ice. And just as sarsens going over the edge would hammer and embed themselves into the ground, ice-blocks would do the same. But ice-blocks would over time melt and disappear, leaving behind their 'empty pits', while sarsen will remain firmly embedded into the bedrock for us to wonder and theorize.

Constantinos

Brian said...

Kostas, I agree that we need to look vary carefully at ice foot processes. Clement Reid was a careful observer, and his suggested mechanism for pressing or grinding erratics (on a shore platform made of relatively soft rock) into pits or depressions could make sense, given tidal oscillations and other changes in the thickness, appearance and composition of the ice foot.

I have explored these matters in a few other posts.