Trebetherick revisited

When I was a student, back in the Stone Age, I went on a field trip that included a visit to look at the Trebetherick boulder bed (or "boulder gravel") and thought that it was self-evidently a till.  But at that time tills were not supposed to occur so far south, and the deposit was explained away as something else.......

Anyway, the site has been discussed in a multitude of research papers and book chapters, and still confuses people.

Here are some attempts at explanations:

The Arkell paper:

 https://pdfslide.net/reader/f/the-pleistocene-rocks-at-trebetherick-point-north-cornwall-their-interpretation

THE PROBLEM OF THE NATURE, ORIGIN AND STRATIGRAPHICAL POSITION OF THE TREBETHERICK BOULDER GRAVEL

by B. B. Clarke

Abstract. The small patch of gravel within the head of the Camel estuary near Trebetherick Point (sx 927781 is described, and various suggestions about its origin reviewed. It is believed to have originated as a till left by the south Irish Sea Riss ice sheet, and suffered frost heaving and solifluction to its present site during three phases of the Würm.

http://ussher.org.uk/wp-content/uploads/2021/01/PUS_1969.pdf

See also p 288 of the 1970 volume on"The Glaciations of Wales" and the section devoted to Trebetherick Point by Campbell on p 184 of the GCR volume on SW England (1998).  James Scourse has considerd the si5te in a number of publications.

To quote Clarke:

It is 2 m thick, thinning rapidly both ways, and only 50 m in lateral extent. It consists of pebbles and boulders up to 300 mm across. Most are roughly rounded, a few angular, and a few well rounded and water worn. They rarely touch and are set in a matrix of bright orange clay. The gravel differs from the local head in the colour of the matrix, and the rarity of slate except at the margins, and from the present day beach gravel in the clay matrix, the rarity of flint and the abundance of granite. The most striking feature is the junction with the three heads. There is a zone of interpenetration in each case although the heads are believed to be of different ages. Many of the stones in the boulder gravel are from local quartz veins and elvans but there are also many granites, one of which may be from Lundy, quartzites, indurated shales, flints, one foliated granite which shows a much higher grade of metamorphism than belongs to this area, and a number of unidentified igneous rocks.

Most of the literature on Trebetherick is taken up with rather sterile arguments attempting to sort out the lithostratigraphy and with attempts to fit the various layers of sediment into pre-existing schemes of glacial and interglacial episodes.  There is no great merit in trying to summarise all of that.  

But two points are critical in any modern assessment:

1.  Following the work by James Scourse and other members of the BRITICE Chrono team, it is now well established that the Late Devensian Irish Sea Ice Stream extended far to the south of the Cornish coast, meaning that nobody should be surprised if actual glacial deposits from the LGM should be found on the coast in favourable locations.

2.  It is inescapable that the Trebetherick "boulder bed" was emplaced above an erosional contact with the slope breccia (otherwise referred to as the "lower head" or "main head") and that it is younger than both the raised beach and the sandrock / beachrock which is everywhere associated with it.

So the boulder bed is in exactly the same stratigraphic position as the Late Devensian till in the Isles of Scilly, all around the coasts of West Wales and on the Gower Peninsula.  If the deposit really is a "remobilised" or "recycled" deposit from some ancient glaciation, if would be an extraordinary coincidence for it to have been slid into precisely this position in a complex suite of deposits, and nowhere else.

The conclusion has to be that the boulder bed is a genuine Late Devensian glacial deposit emplaced here by the same ice stream that affected the Isles of Scilly.  It may have been subjected to some remobilisation and redistribution shortly after original emplacement, but that would be par for the course,  and should surprise nobody.  That is what happens to almost all glacial deposits in ice-marginal situations.