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Saturday, 4 March 2017

The West Angle Enigma (3): two tills, or one?

 As indicated in the first post on West Angle, there is a considerable dispute about whether there is one till, or two, at West Angle.  In my own work, in spite of extensive excavations using a JCB and several drill-holes in the vicinity of the old brick-pit, no trace was found of a "basal till" underneath the raised beach cobbles and gravels.  Even in summer, waterlogging in the sandy layer beneath the beach frustrated attempts to expose any deeper sediments.  David Unwin also failed to see anything beneath these sands.

D.Q.Bowen


On the other hand, DQ Bowen has asserted, on many occasions, that he has encountered this basal till.  In the Quaternary of Wales GCR volume, edited by Campbell and Bowen, there are no less than eight DQB references cited in support of this assertion, including these which appear to be the "primary" references:

BOWEN, D. Q. (1973a) The Pleistocene history of Wales and the border-land. Geol. J. 8, 207­224.
BOWEN, D. Q. (1973b) The Pleistocene succession of the Irish Sea. Proc. Geol. Ass. 84, 249­272.
BOWEN, D. Q. (1974) The Quaternary of Wales. Chapter 17 in T. R. Owen (Editor), The Upper Palaeozoic and post-Palaeozoic rocks of Wales. Cardiff. 373-426.

But as I recall not one of them actually contains the detailed stratigraphic and other field evidence that we need to see.  Each one of the articles cited makes the same assertion, and cites lots of other DQB articles,  but fails to show us the evidence.  In 1977 Bowen promised an article by "Ribbon and Bowen et al" arising from work in progress, but as far as I can see this was never published.

So my default position has to be that Bowen and his colleagues saw a stony grey Irish Sea till at West Angle ABOVE the raised beach, and mistakenly assumed it to have been stratigraphically lower. The reason for this can be seen in this part of my surveyed section:

The critical cutting here is numbered B.  At this point a "classic" Irish Sea till is exposed, reddish in colour, with many ORS clasts and other striated cobbles included -- and even lignite and shell fragments.  There are thrust structures in the underlying sediments.  As we can see, no raised beach can be seen in the vicinity, and we can see from the adjacent cutting C that the raised beach there is at a very low altitude -- only at about 2m OD.  It may therefore dip northwards and disappear somewhere between cuttings B and C.  Because the exposed Irish Sea till in the gully is at a relatively low altitude (just over 5 m OD) Bowen might have assumed that the raised beach was above the till, since the raised beach is indeed at about this level in cutting H.

Another possibility is that Bowen identified raised beach cobbles in the sediments above the till and mistakenly interpreted this finding.  Where ice moves inshore across an old coastline it incorporates older sediments -- including raised beach cobbles -- which are then redeposited in lodgement till or in flowtill or fluvioglacial gravels.  We see evidence of this in the Isles of Scilly, and also in other coastal districts of West Wales. 

If I am wrong, and if Bowen has indeed observed raised beach deposits above his supposed lower till, I will be very happy to publish the evidence and revise this opinion.  If any reader is in possession of such evidence, please get in touch.

E.E.L. Dixon


So we are left with the evidence from Leach and Dixon, collected in the early 1900's.  Now it gets even more interesting.  This is the original source:

The geology of the south Wales coalfield. Part XIII, the country around Pembroke and Tenby, being an account of the region comprised in sheets 244 and 245
Author: E E L Dixon; Geological Survey of Great Britain
London : H.M.S.O., 1921.
Series: Memoirs of the Geological Survey of Great Britain. England and Wales.
Chapter 14, pp 189-203

 This original source has been cited, over and again, by Bowen and other researchers in support of the assertion that there is Irish Sea till BENEATH the raised beach at West Angle, and that this was seen by both Leach and Dixon prior to the publication of the Memoir in 1921.  But when you go back to source, as I did last night, you see that this is nothing better than a myth.  Geologists and geomorphologists are just as susceptible to mythology as archaeologists are!

A careful reading of the Memoir chapter reveals the following:

1.  In his introductory remarks Dixon says: "Stiff "till" has been found only at West Angle Bay.  It is overlain by gravels and loams with drifted plant-remains, which may include a representative of the raised beach....." (p 195)  It is true that the till is at a low level in the northern part of the section and that silts with plant remains are at a higher level in the southern part of the section -- but that does not mean that the latter are stratigraphically higher than the former.  The till lies in a gully or channel cut into older sediments.

2.  On page 197 Dixon says that he looked at the drift cliff section in the company of Arthur Leach and that they could not decide which gravels were of glacial origin and which were related to the raised beach.  Quote:  "No agreement was reached, however, as to which, if any, of the beds might be taken as a representative of the raised beach." (p 197).  It is clear that the two did not undertake any excavations, and that they simply tried to sort out the stratigraphy from glimpses of it here and there -- since the bulk of the section then, as now, was obscured by slumping.  I think that like David Bowen, they were misled by the presence of glacially recycled raised beach cobbles in the glacial suite of sediments.

3.  Dixon says (p 198) that the boulder clay was seen in just one location, "in the bottom of the cliff".

4.  It is also clear that he and Leach did not see the genuine raised beach at all, since it is at or below the level of the current pebble beach and is only very rarely exposed.  If they had seen it, they would have recognized it immediately, since it is very different in appearance from all of the other gravelly deposits in the West Angle sequence.

The main drift cliff exposure as described by Dixon is as follows:

7.  Shingle of limestone pebbles - less than 1 ft (modern)
6.  Gravel and sand - 6 ft
5.  Buff fine loam -- up to 2 ft
4.  Grey fine loam with plant remains - more than 5 ft thick but thins northwards
3.  Gravel and shingle, up to 4 ft, thins northwards
2.  Sand, up to 3 ft, thins northwards
1.  Boulder clay with scratched stones (including igneous), in stiff purplish clay, 6 ft max
x.  Black clay with debris of silicified Carb shells, at least 5 ft

Dixon says that the loams are "cut out for some distance by the overlying gravel, which appears to fill a channel in them", so it is clear that he did see the same slip face / erosional contact as others have seen half a century later:



To the north of this point, Dixon also describes a sequence in the cliff face, as follows:

Stony soil, some pebbles
Buff loam with weathered bedrock debris (= 5 above) - 5 ft
Grey loam, with stony sand below (= 4 above) - 2 ft
Current-bedded buff sand up to 1.5 ft thick
Coarse gravel, stones, some well-rounded, of all sizes, including igneous rocks, sandy matrix - up to 1 ft
Lower limestone shales, forming a platform c 13 ft above HWM

This is very confusing, since there is no high rock platform at the northern end of the drift cliff -- and I think that Dixon is  interpolating or trying to make sense of a complex situation.

Inside the brick-pit, Dixon found the following sequence (p 197):

8.  Angular stony loam - head?  3 ft
7.  Sand with abundant rounded bedrock flakes (?!!) plus erratics (including one from N Pembs) -- 4 ft
6.  Well bedded gravel and sand -- 2 ft
5.  Buff laminated loam including erratics -- 5 ft
4.  Dark blue loamy clay with wood and plant fragments -- 5 ft
3.  Irregular loam, sand and fine gravel  - 3 ft
2.  Grey clay or loam -- 1.5 ft
1.  Clean buff sand -- 1 ft

Conclusions


Dixon's recorded sequence has been mis-reported by Bowen and others, and it is now apparent that the gully or channel which was cut into the sequence of interglacial silts and clays at West Angle, and its filling by periglacial, glacial and fluvioglacial deposits, has caused great confusion about the stratigraphy both inside the clay-pit and on the cliff section facing the beach.  Old and new deposits are not exactly inverted, but to the south of the gully it appears that Ipswichian interglacial silts and clays (probably more than 70,000 years old) are found about 8 m above OD while Late Devensian glacial deposits (around 20,000 years old) are found  below 5 m OD to the north of point E in my surveyed section.  We do not know how this gully was formed;  but I suspect that a substantial quantity of pre-existing sediments (including raised beach cobbles and shingle) were removed by the ice of the Devensian Irish Sea Glacier as it moved onshore from the NW, and that these sediments were incorporated into the rather complex reddish "gully fill" deposits that we see today in the northern part of the cliff section.

So my conclusion is this.  Much as I would love to see a pre-Ipswichian Irish Sea till at West Angle, I don't think Dixon and Leach saw one, and neither did David Unwin or myself.  Neither, I suspect, did David Bowen and his colleagues -- which might explain why the promised paper by Ribbon and Bowen never did appear in print.

So -- there is just one set of glacial deposits at West Angle, associated (like all the others around the coasts of Pembrokeshire) with the Late Devensian glaciation.


The outer (northern) end of the drift cliff at West Angle.  Here, overriding ice has removed the bulk of the interglacial sediments, and the bulk of the sediments exposed in the cliff face are related to the Devensian glaciation.  The raised beach is at considerable depth, or is absent.  The best exposures of Irish Sea till occur around point B.


The middle part of the surveyed section.  At E we can see the slip face / slump face / erosional contact, to the left (north) of which the bulk of interglacial sediments have been cut out.  At D the bulk of materials exposed are glacially related.  Between F and H the bulk of the exposed sediments are fine-grained interglacial silts and clays overlying the raised beach.



The southern part of the surveyed section, extending northwards from the bedrock slope.  The bulk of the deposits exposed are interglacial, resting on the raised beach.  Around J and K glacially-related materials sit on these deposits at the top of the section.


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