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Sunday 21 October 2018

Strange boulders on the coast of Brittany

Coast of Brittany, to the west of Roscoff.  Granite territory, with low cliffs and coastal tors everywhere -- in many ways similar to the coasts of the Isles of Scilly.

Thanks to Daniel Praeg for drawing this to my attention during our discussions on the extent -- and thickness -- of ice in the Celtic Sea area during the glaciations of the Quaternary.  Here is one key article:


Hallégouët, B. & Van Vliet-Lanoë, B. (1989). Héritages glaciels sur les côtes du massif Armoricain, France. Géographie physique et Quaternaire, 43(2), 223–232. doi:10.7202/032771ar

Rough translation:  Glacial traces on the coasts of the Armorican Massif, France

Relict ice rafting along the shores of the Massif Armoricain, France.  Occurrence of erratic boulders on the floor of the English Channel and along the shorelines of the Massif Armoricain is a common feature.  The distribution and the lithology of these erratics suggest ice rafting as the main transport mechanism and dispersion from local, regional and far away (basalt) sources areas.  They were transported either by river ice, sea ice or icebergs.  Drifting direction was controlled by prevailing winds assisted by tidal currents.  One episode of shore rafting has been identified in shore formations and associated to a short transgression close to the end of the last interglacial.  The observed relict sedimentary faciès is comparable to modern shore environments influenced by seasonal ice processes. Sea ice action on shore dynamics is believed to have taken place repeatedly during the Pleistocene along the coast of the English Channel.  It could partly be responsible for the shaping of the shore platforms in the area.

This is not easy territory for spotting erratic boulders -- the shoreline for mile after mile is littered with thousands of boulders, many of them heavily stained and covered with lichens and algae..... 

The area on the map below is one I know quite well, since we in Newport are twinned with the village of Plouguin, not far to the north of Brest.  Been there many times.......

I can pick up the gist of what is going on in French literature, but I'm rather concerned here that the assumption seems to be right from the outset of this article that the erratic (non-granite) boulders -- and there are many -- are all ice-rafted, and that the rafting episodes occurred during short-lived transgressions during the last interglacial.  Now we know that that was a time of relatively high sea-level -- somewhat higher that that of today. The implication is that it was warmer then then it is now -- and how much ice-rafted debris to we find coming onto the coasts of Western Britain and Brittany today?  How many erratics are recorded as coming onto these coasts as a result of floating ice transport since sea-level arrived at approx its present level, around 5,000 years ago?  As far as I know, none at all.

The authors of the paper ascribe the occurrences of boulders to the sort of coastal processes which David Sugden and I described in the "Coastal Geomorphology of High Latitudes" monograph in 1975.  They even invoke evidence of coastal currents and prevailing wind directions in order to explain how boulder-bearing ice floes reached the shoreline of western France during a warm interglacial.  It all seems rather forced, for two reasons:

1. The sorts of processes that are well known from the Arctic and the Antarctic today require very severe sea ice conditions with permafrost on land and sea ice anchored to the shore for at least six months of every year.  Such conditions cannot have prevailed during the last interglacial in western France.

2.   No account has been taken by the authors of isostatic and eustatic effects, which are complex to say the least. This is an area of complex isostatic and tectonic effects which have distorted palaeo-shorelines and raised beaches, but overall this is a sinking coast -- which means that during the last interglacial the shoreline might have been higher, with respect to current sea-level, than it is today. The paper below gives some further information.  

I think we have to assume that if these boulders were carried onto the coast either in ice floes or in icebergs and bergy bits blown ashore from the west, this happened much closer to the  peak of a glacial episode, and not during an interglacial.  Might this have been the Late Devensian glaciation?  That seems unlikely in view of the fact that the boulders and boulder beds are deeply buried under slope deposits and seem to be associated with raised beaches -- so it is much more likely that they date from either the Anglian or Wolstonian glacial episode -- or maybe both of them. 

If, as we have argued on this blog, the Celtic Sea was deeply inundated with glacier ice  on at least two occasions, and if on both occasions the ice reached the edge of the shelf 250 km SW of the Isles of Scilly, it is not unreasonable to speculate that the western coasts of France were isostatically depressed just as the floor of the Celtic Sea was.  Under those circumstances, if the sea was washing the western coast of France at approximately its present relative level, floating ice from the ice edge to the west could well have reached the coast, and it could well have dumped erratic boulders which were later incorporated into slope deposits and raised beaches.

And then we have the intriguing possibility that if the ice edge was only 100 km offshore, to the west of the tip of Brittany, might it have come even closer, and might it even have reached these low granite cliffs?  Watch this space.........

Middle Pleistocene raised beach anomalies in the English Channel: regional and global stratigraphic implications. (2000) by B van Vliet-Lanoë et al
Journal of Geodynamics
Volume 29, Issues 1–2, January–February 2000, Pages 15-41


Palaeo-shore positions help to evidence long-term eustatic changes and assist in the understanding of tectonic movements at a regional scale. Raised beaches anomalies exist in the Channel region and may result from deformations induced by neotectonic or by glacio-isostasy. The aim of this paper is to re-analyse, within their geodynamic context in the Channel and Dover Strait regions, the stratigraphy and the datings of palaeo-shores of Middle and Upper Pleistocene ages. This sector of Europe is characterised by strong geological contrasts and is controlled by two main geological boundaries: in the north, the Variscan Overthrust (corresponding approximately to the position of the Dover Strait) and, in the south, the Northern Branch of the Southern Brittany shearing zone. These two boundaries border a domain which seems to behave rather homogeneously on a large scale under the control of plate tectonics. Today, shorelines are subsiding north and south of this ‘Channel’ region. Episodic uplift largely controlled the open or closed status of the Dover Strait, especially after the Messinian and Early Quaternary, by reactivating Variscan structures. After 400 ka, global cooling allowed supplementary deformations in the area to be induced by glacio-isostatic rebound and clustered seismic activity during the phase of ice sheet building. Evidence of eight different transgressions dated by ESR from Oxygen Isotopic Stage (OIS) 13 to the end of OIS 5 shows the complexity of the sea-level records in a region unstable for isostatic and neotectonic reasons. Due to glacio-isostatic depression, transgressions are possible in late glacial times as well as during full interglacials. Most platforms were initially cut, during the Late Miocene, and seem to have been re-trimmed several times, especially by shore ice rafting since OIS 9. Regionally, the sea apparently rose to about the same level in O.I. Stages 11, 9 and 7. Glacio-isostatic and glacio-eustatic relative displacements of the sea level together with background tectonic movements have modified coastal positions and have temporarily altered the intensity of tidal currents. The oldest shoreline deposits are preserved only in subsiding areas, controlled by the deep crustal pattern. Neotectonics related to Variscan structure reactivation still dominates glacio-isostatic deformation and basin subsidence. The OIS 7 positive anomaly seems related to a regional relaxation event.


More relevant info:

West, R.G . and Sparks, B.W. 1960. Coastal interglacial deposits of the English Channel. Philosophical Transactions of the Royal Society of London, Series B., Biological Sciences, No. 701, Vol. 243, pp. 95-133, 27th October, 1960 

Near Arromanches, at St Come de Fresne and Asnelles-Belle-Plage, two deposits showing a change from marine to freshwater sediments were investigated. The analysis of pollen and the Mollusca showed the prevalence of pine forest and its replacement by open steppe-like conditions as the marine regression occurred. After the regression, limon covered the freshwater deposits. The fossiliferous deposits are tentatively correlated with zone i of the Eemian Interglacial.  The relative land and sea-level changes indicated by the deposits are considered. It is concluded that in the English Channel, during the Ipswichian (Eemian) Interglacial, sea-level rose above its present height in zone f and fell below it during zone i. The Selsey-Brighton raised beach and the Normannien II raised beach are correlated with the same marine transgression. It is pointed out that if the Selsey-Brighton raised beach is to be correlated with the Monastirian II level of 7-8m, then this level should be correlated with the Ipswichian (Eemian) Interglacial.

This is an old piece of work, but it does seem to show that during the Ipswichian interglacial relative sea level on the coast of NW France was if anything higher than the sea-level of today.

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