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Sunday, 18 April 2010

Sir Archie was no fool

An old map showing the proposed ice limits of the "Older Drift" glaciation of Great Britain, according to Sir Archibald Geikie (1894) and Charlesworth (1957)

I found this old map, which I made in 1962 to show the proposed ice limits of Geikie and Charlesworth. Note that Geikie in 1894 had an intuition that the ice coming in from the sea extended all the way up the Bristol Channel and pressed well across the present coastlines of Devon and Somerset. The ice margin out in the Celtic Sea does not make a great deal of sense, given what we know today, but nevertheless this is a pretty impressive piece of deduction on his part, made on the basis of very scanty evidence. He knew that St George's Channel had been occupied by a massive Irish Sea Glacier -- that had already been demonstrated in 1885 by a geologist named Hicks, who recorded striations and erratic movement in the St David's area that could only be explained by ice coming onshore from the NW. In 1891 Hicks also demonstrated that the lowland parts of Pembrokeshire had also been inundated to a great depth by ice coming in from St George's Channel.

Where Geikie went seriously wrong was in his assumptions about the sources of the ice in W and S Wales -- he thought that the ice affecting Devon and Somerset was WELSH ice coming from the north. He also thought that the ice overriding Pembrokeshire was Welsh ice flowing into Cardigan Bay from the uplands, and flowing across the county from NE towards SW. In saying that, he appears to have forgotten the evidence -- already in print -- from Hicks.

The term "Irish Sea Glacier" was coined by Carvill Lewis in 1903. The most important work in the early part of the twentieth century was that of TJ Jehu in 1904 -- he worked out the Pleistocene stratigraphy of North Pembrokeshire, and confirmed that the traces of ice action (striations and erratic transport) all showed a dominant ice flow from NW towards SE.

Given this convincing history of earlier research, it is all the more amazing that HH Thomas, when he proposed his "human transport" theory as an explanation for the presence of bluestones at Stonehenge, dismissed out of hand the idea that ice overrode Pembrokeshire and extended far to the east......... carrying erratics with it.


Anonymous said...

From where did Hicks demonstrate was the source of the ice in South and West Wales and is that now the current view accepted by the scientific community?

Brian said...

As I recall (it's 50 years since I read the Hicks papers!) he just did some simple observations in Pembrokeshire which showed that erratics had been carried from NW towards SE, and that that accorded with the evidence of striations. He deduced that the ice could not have been WELSH ice, coming from that direction. All other workers have accepted that view subsequently -- maybe except for HH Thomas, who wanted a small Preseli ice cap which could conveniently drop off assorted bluestones in the Cilymaenllwyd area, ready for building into a proto-Stonehenge, which could then be dismantled and shipped off across the sea to Salisbury Plain! I have personally seen NO evidence to support that strange HH Thomas idea.....