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Sunday, 18 January 2015

The Erratics at Baggy Point, Croyde and Saunton (2)


 This is a map from Olwen Williams-Thorpe which represents the ideas of Geoffrey Kellaway.  You can see the location of Lundy Island just above the "100 km" symbol, and the the Baggy Point area to the right of that, where the longer arrow crosses the coast.  Note that Kellaway speculated about a "Scottish ice stream" with very far-travelled erratics crossing the Devon coast precisely in this area......  It begins to look as if he was very perceptive.

 The Giant Erratic at Freshwater Gut.  This photo was taken over 50 years ago, possibly by Nick Stephens.  The erratic appears not to have moved at all........

 These are the Saunton cliffs -- with a very jagged wave-cut platform and lots of cliff-fall debris close to HWM.  Excellent territory for erratic hunting........

In the "Cliffs of Saunton" booklet there is a double-page spread showing the distribution and lithology of 21 of the largest erratics on the Croyde - Saunton coast, based on the work of Paul and Rosemary Madgett.  All of those erratics are in excess of 50 cms on their longest axis -- ie they are very heavy to move, and highly unlikely to have originated as ship's ballast.

List (with locations) of the 21 largest erratics in the area.  Identified by Paul Madgett and published in "The Cliffs of Saunton" by Peter Keene and Chris Cornford.  (Keene, P. and Cornford, C.  1995.  The Cliffs of Saunton. 9780948444241.  44pp.  £2.95.)  For further detail, please buy the book here:
If you have difficulties in reading this, click to enlarge.

Paul and Rosemary (in their 1987 paper) have a listing of 37 boulders in excess of 25cm on the longest axis.  Please note that the numbering is different in each list. Since 1987 several more have been found, which aren't in the literature.

From Paul's notes:


The 1987 paper lists the 8 which had been noted up until that date (including the Ramson Cliff epidiorite), plus a metre-sized Contorted Gneiss that we had discovered hidden in a deep gully, almost spherical in shape - but Peter independently spotted this one and published in 1986 (I think he was alerted by Peter Robinson, who used to live in Braunton, who spotted it while doing an OU project). However, of those 8, Taylor's Dolerite (1956, one of two designated 5b by him) has disappeared long ago. It was a small slab, 29x13.5cm, in a small cave, and was moved at most spring high tides when big waves were running. I suspect it may have been broken up, possibly not helped by that cave being used by local lads & lasses as a "rave cave" at times! Taylor's other 5b, an Agglomerate, is still there, though moved intermittently within the cave.

I didn't list the extra ones referred to by Stephens in various papers, as I discovered these were either small pebbles or small cobbles, or were mistaken identifications confused with Taylor's 1956 Grey Spilite and Hughes' 1887 Quartz Porphyry (Taylor himself in his 1958 paper seems to have caused this confusion - see paper for discussion).

ASnother point is that the low cliffs of "head" immediately south of the stream on Croyde Beach (themselves simply the distal edge of the solifluction terrace around Saunton Down) have yielded to me a few small erratics - e.g. another tuff, and a porphyritic basalt. Thus while such erratics are far outnumbered by the purely local clasts in the Head, they are present. This may well have caused Stephens to assert that there are exposures of in situ till in the low cliffs of Croyde Bay; whereas I would tend to the conclusion that the soliflucted debris includes some re-worked glacially derived material.

Those thin sections are available for further study if Rob and Richard would like to see them. They include several "rhyolite" and "tuff" specimens. However, the numbering on them refers to yet another listing of mine, going back 30-odd years, which included some quite small cobbles. Thus I will need to do a bit of "archaeological excavation" in my study in order to hunt out the relevant listing and cross-reference the numbering schemes!

One rock-type that is quite well-represented in the Down End erratic suite is a Quartz Conglomerate, which I am pretty sure is from the ORS - and of course this conglomerate outcrops in South Pembrokeshire........?  There may well be other sedimentary rock-types which should be included in the erratic suite, but it is the igneous and metamorphic ones which are much more recognisably "foreign" to the area.

We also have to take into account that in the past small ships in ballast have foundered in the area - the list of wrecks in the Barnstaple/Bideford Bay area is very long. One rock-type represented on the south side of Croyde Bay by a spread of irregularly shaped cobbles, with only minor rounding of the corners, is a micro-granite showing granophyric texture - I suspect it might have been from Penmaenmawr, either simply ballast or possibly to be sold as road-stone once unloaded.

Smaller pebbles include a lot of porphyries and tourmalinised rocks, which quite possibly came down the Taw-Torridge (ice-rafted in melt-water?) from the Dartmoor area. However, I don't recall any substantially-sized cobbles and boulders of these lithologies, though there are some larger granites which might just be Dartmoor. The presence of two broken granite rollers of agricultural provenance (one on the Baggy foreshore, one along the base of the Saunton cliffs) makes me suspect that some at least of the granite cobbles originated from this kind of source!

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BSJ comment:

What's very interesting in all of this is the mention of dolerites, rhyolites and lavas in this erratic suite.  Where do such things occur in abundance?  Well, in the Fishguard Volcanic Series, for a start......
It looks as if some thin-section work on all of these samples might be rather rewarding -- whether or not there is any link with Pembrokeshire.




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