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Thursday 14 September 2023

The conundrum of the coastal erratics

The Ramson Cliff erratic, deposited 80m above sea level by floating ice? I think not........ (Photo courtesy Paul Madgett)

My recent spat with Tim Daw about the possible ice rafting of the large erratics found on parts of the coasts of SW England was quite fun, arising from the fact that he tends to accept some of the things said about ice rafting by other geomorphologists, whereas I do not.  It's actually quite easy to find references to ice rafting in the specialist literature, including the GCR volume on South West England in 1998 and a more recent paper by Phil Gibbard and others in 2017.  Tim cites them both on his blog, and seems to think that I am blissfully unaware of their contents.  Let that pass with a reminder that I have cited both of these sources on innumerable occasions on this blog, sometimes agreeing with the authors on assorted matters, and sometimes not. 

So Tim can be forgiven for thinking that many geomorphologists who have studied the coasts of Devon and Cornwall think that the big boulders at Porthleven, Croyde, Saunton and elsewhere have been "rafted" by floating ice from the north and west.  That may have been true 25 years ago, but now that we understand isostatic and eustatic interactions much better than we did, any current author who proposes an ice rafting mechanism must be challenged.  I cannot see any possible combination of environmental circumstances that would permit the long-distance ice rafting transport of large erratics onto the coasts of the Bristol Channel and the English Channel near or above present sea level. At the beginning and end of each glacial episode, when ice rafting might have occurred, the coastlines of the day were scores if not hundreds of kilometres away from their current positions. We must remember that the epidiorite erratic on Ramson Cliff (near Baggy Point) is 80m above sea level, and we must assume that the "apparent concentration" of big erratics between the present-day high and low tide marks arises simply because that is where large stones are washed clean by wave action and are exposed to view.  It is entirely logical to assume that there are many more such erratics, some above (buried in sediments) and others in deep water below the current intertidal zone.

So this isn't really an argument between archaeologists and geomorphologists.  I think that some geomorphologists in the past have been rather careless in stating that the big erratic boulders of the South-West are possibly or probably ice-rafted, and have failed to take account of isostatic and eustatic oscillations and their timing with regard to climate change.

Clean sea ice off the coast of Antarctica.  Debris tends to be carried in floating remnants of glaciers -- in icebergs and bergy bits.  Other mechanisms for the incorporation of  erratic boulders and other debris are considered in "Coastal Geomorphology of High Latitudes", a monograph written by David Sugden and me, published by Edward Arnold.


New insights into the Quaternary evolution of the Bristol Channel, UK
ISSN 0267-8179.
DOI: 10.1002/jqs.2951

See these posts:


Tony Hinchliffe said...

So, how do you explain the arrival and positioning of the well - known erratic at Shebbear, many miles inland from the coast of North Devon?

BRIAN JOHN said...

Just type "Shebbear" into the search box -- I have discussed it on several occasions. This is an extremely erratic erratic -- and we can pull out various theories about how it got to the village and where it came from. If we dismiss human transport as vanishingly unlikely, the most reasonable theories are
(a) that the boulder was "dropped down" onto an evolving landscape from a previously higher land surface, rather as the big sarsens of Salisbury Plain are assumed to be residuals dropped down onto the chalk downs over many millions of years; or
(b) that the boulder is a residual from a destroyed patch of glacial deposits, dating from an old glaciation in which ice transgressed far inland from the Bristol Channel coast.
The latter theory is in my view far more reasonable, and is on line with much other evidence of at least one glacial transgression.