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.......