Doctoral thesis 1965

I had forgotten that my old doctoral thesis, dating from 1965, is available on line from the Oxford University Archive.  I'm not sure when it was digitised.  Looking back at it, it's amazing how crude it is, with hand-drawn maps and diagrams, and a a very rough typescript.  I think we only ever did three copies, which was the limit for things typed with carbon paper in those far-off days.  How different it all is today, with digital technology.......

https://ora.ox.ac.uk/objects/uuid:de091cbb-5d45-4ed7-aee4-cefc15bb28be

There are two volumes, one with the text and the other with the photos, maps, diagrams and laboratory results in graphic form.  There were some large maps too, but they seem to have disappeared.  What's quite reassuring is that in the 53 years that have passed since I submitted this thesis for cross-examination by Drs Farrington and Beckinsale, virtually nothing has happened to shift its key conclusions.  Rather, I should say that there has been a vast amount of new research -- but none of it has falsified my central hypothesis or invalidated my observations or field measurements or lab results.  We now have a rather more accurate idea of where the Devensian ice edge may have run, but that's about all.

The plug of Irish Sea till and related Devensian deposits which fills an old valley at Druidston

My diagram of the Abermawr drift cliff as it appeared in 1964.  The site is now featured as one of the "top 50" Quaternary sites in the UK.

Anyway, in the hope that some of this might be of historical interest, here are the Abstract and Summary of Main Findings.


Aspects of the glaciation and superficial deposits of Pembrokeshire

Brian S John, 1965

Abstract:

"Since the publication of the work of Jehu and Charlesworth the distribution and significance of the glacial deposits of the Irish Sea have remained in a somewhat confused condition." So said Professor W.D. Evans in 1964. This thesis represents an attempt to remove the confusion from one small corner of the Irish Sea basin; it is concentrated largely in North Pembrokeshire, and aims to resolve some of the stratigraphic and chronological problems concerned with the Pleistocene deposits of the region.

There is a long history of research into the glaciation of North Pembrokeshire. Most of the early work was undertaken in Dewisland (the St. David's Peninsula), but in the major contributions of Jehu (1904) ond Charlesworth (1929) something was revealed of the Pleistocene history of North Pembrokeshire as a whole. Subsequent work has been greatly influenced by these two authors, and there has been no detailed field study of the area since Charlesworth. As a result, many controversies have arisen in recent years; these range from the purely stratigraphic problems of the classic tripartite drift succession and the nature of the Upper Boulder-clay to the absolute age of the Pembrokeshire drifts. Again, there is doubt concerning the number of glaciations represented in North Pembrokeshire; about the existence of the South Wales End-moraine, the pro-glacial lakes, and the so-called overflow channels of Charlesworth; and about the age and origins of the coastal features of the county. These are among the problems that this thesis attempts to answer. [Please see pdf. for full abstract]

Blocky head at Pen Daladeryn at the southern end of Ramsey Sound.

Sub-glacial meltwater channel cut across a spur at Dinas

Fluvioglacial gravels exposed in the Mathry Road gravel pit, in the western Cleddau Valley

Thesis Summary:
(there may be mistakes -- subjected to OCR of old faded typescript!)

"Since the publication of the work of Jehu and Charlesworth the distribution and significance of the glacial deposits of the Irish Sea have remained in a somewhat confused condition." So said Professor W.D. Evans in 1964. This thesis represents an attempt to remove the confusion from one small corner of the Irish Sea basin; it is concentrated largely in North Pembroke- shire, and aims to resolve some of the stratigraphio and chronological problems concerned with the Pleistocene deposits of the region.

There is a Iong history of research into the glaciation of North Pembrokeshire. Most of the early work was undertaken in Dewisland (the St. David's Peninsula), but in the major contributions of Jehu (1904) and Charlesworth (1929) something was revealed of the Pleistocene history of North Pembrokeshire as a whole. Subsequent work has been greatly influenced by these two authors, and there has been no detailed field study of the area since Charlesworth. As a result, many controversies have arisen in recent years; these range from the purely stratigraphic problems of the classic tripartite drift succession and the nature of the Upper Boulder-clay to the absolute age of the Pembrokeshire drifts. Again, there is doubt concerning the number of glaciations represented in North Pembrokeshire; about the existence of the South Wales End-moraine, the pro-glacial lakes, and the so-called overflow channels of Charlesworth; and about the age and origins of the coastal features of the county. These are among the problems that this thesis attempts to answer.

Some space Is devoted to a discussion of the methods end techniques used durirg the drift studies in North Pembrokeshire. The techniques include the interpretation of head deposits, using some of the criteria of such workers as Dylik and Budel; the field mapping of drifts; stone- counts for the recognition of drift boundaries and the differentiation of drifts in composite exposures; the mechanical analysis of drift matrices in order to establish whether the different drifts have diagnostic grain-size distributions; the measurement of preferred stone orientation in order to recognise soliflucted deposits and perhaps to establish accurately the direction of ice-movement across North Pembrokeshire; and the analysis of pebble- roundness in order to recognise the mode of transport and deposition of certain drifts. A brief outline is given of the potential and problems of dating drifts on the basis of their derived aspemblages of marine mollusca, and of the uses and methods of radiocarbon dating of organic material in order to assess the absolute age of drifts.

Twelve of the most important coastal localities in West Wales are analysed in detail. The localities in North Pembrokeshire are grouped under the broad categories of North Pembrokeshire coast, West Dewisland coast, and South Dewisland coast, while related sections from the coasts of South-west Pembrokeshire and South Cardiganshire are also examined. Drift stratigraphy is virtually identical in all these areas; although the tripartite drift succession of Lower Boulder-clay, Intermediate Sands and Grovels, and Upper Boulder-clay is still accepted by many workers, it is not supported by evidence in the field. The term "Lower Boulder-clay" is unsatisfactory, for it includes drifts of several facies that are not always the strptigraphic equivalents of one another. There is no evidence from any coastal section that sands and gravels are overInin by a till of a later glaciation, and nowhere are they "intermediate" between two tills. The Upper Boulder-clay is not a till at all in most localities, although the term has been used in the past to describe some of the land facies of the Irish Sea till; in general, the Upper Boulder-clay is seen from detailed fabric analyses to be a rubble-drift of head, till, and sands and gravels soliflucted during or after the wastage of the last ice-sheet. It is always closely associated with the Irish Sea till.

There is no evidence from the coastal sections in support of CharLesworth's "South Wales End-moraine", and no trace of any large pro-glacial lakes. The sequence of deposits exposed in the coastal drift cliffs is the same well to the south, and to the north, of the so-called Newer Drift limit.

From the coastal studies it seems that there have been two glaciations of West Wales. These glaciations are not the equivalents of the Older Drift and Newer Drift Glaciations of earlier authors. An Early Glaciation, which may have been powerful and prolonged, apparently had an ice-gradient sloping down from the north-east; however, few traces of it remain apart fron the erratic boulders which rest upon the raised beach platforms of North Pembrokeshire. After the Poppit Interglacial, in which the raised beaches of the area were deposited, there occurred a Dewisland Glaciation from the Irish Sea. The ice of this glaciation extended at least as far south as Milford Hnven, and deposited Irish Sea tills where it assaulted the coast and local land facies where it moved off the land. Fluvio-glacial deposits were laid down in some coastal localities during the wastage of this ice. During the first phase of the Dewisland Glaciation the Irish Sea ice failed to reach North Pembrokeshire, and there is abundant evidence for a prolonged period of fluctuating periglaclal climate. From the association of coastal features with the drifts it is suggested that the drowned valleys of West Wales, the configuration of the coastline, and the raised beach platforms of the area were all features of the landscape prior to the Early Glaciation.

The characteristics of the drift covar are examined in Western Dewisland and in the Fishguard area in order to establish whether the coastal sections are truly representative of the complete drift succession. In Dewisland, after an examination of the drift characteristics, several quantitative techniques are employed to test the Lower Boulder-clay/Upper Boulder-clay subdivision, assuming, as a working hypothesis, that the moorland tills of the area are Lower Boulder-clays and the sandy tills elsewhere Upper Boulder-clays. The findings confirm those for the coastal sections, and suggest that the surface drifts on the moors are simply remanie drifts which are no different from those on the rest of the land surface. There is but one till sheet in Western Dewisland; it is thin and sandy, and has no "ideal" preferred stone orientation which indicates the directior of ice-movamant across the sea. Although the Irish Sea ice was at least 600 ft. thick over Dewisland, it appears to have been relatively weak at the time of till deposition.

In the Fishguard area there is hummocky topography of sands and grovels; this is dead-ice topography with interspersed and interbedded patches of ablation till. As indicated in the coastal sections, there is no true Upper Boulder-clay in the area, no trace of the South Wales End-moraine, and no trace of Glacial Lakes Manorowen, Gwaun, and Nevern. All the depositional features of North Pembrokeshire, including the hummocky topography and the valley sandur remnants, are related to the wastage of the extensive Dewisland Ice-sheet.

The meltwater channels of North Pembrokeshire are analysed in some detail in Part V of the thesis. A re-examination is made of Professor Charlesworth's hypothesis that the meltwater channels of the Gwaun-Jordanston system are Newer Drift features cut by overflows from pro-glacial lakes. It is suggested that there are many inconsistencies in the hypothesis, and that there are difficulties in accepting that lake overflows, even in a highly complex sequence, could have been responsible for the cutting of the channels. On the other hand the channel system has characteristics which suggest that it was cut predominantly by sub-slncial meltwaters. In association with the Gwaun-Jordanston channel system there are several features which can be attributed to meltwater flow marginally or sub-marginally alongside downwasting ice. These features provide support for the conclusions reached on the basis of depositional evidence concerning the mode of ice-wastage at the end of the Dewisland Glaciation.

The orientation of the Gwaun-Jordanston channel-system is something of an anomaly. The sub-glacial meltwaters of the Dewisland Glaciation should have flowed approximately parallel with the direction of maximum ice-surface gradient, ie towards the south-east. However, the channels are orientated for the most part towards the south-west, possibly indicating that they were cut during the Early Glaciation when Welsh ice may have been dominant and the ice-sheet gradient sloping N.E.-S.W. There is other evidence in the Fishguard area for the Early Glaciation age of the channels, but the most convincing evidence comes from Dewisland; here the deep coastal meltwater channels (previously thought to be features of sub-aerial stream rejuvenation) support head beneath till of the Dewisland Glaciation. The channels cannot therefore date from the Dewisland Glaciation.

After a tentative reconstruction of the Pleistocene hiotory of West Wales on the basis of all the lines of evidence pursued, the proposals made are considered in the wider context of the Irish Sea Basin as a whole. The drift stratigraphies of South-East Ireland, Lleyn, Gower, and the north coast of Devon and Cornwall are compared with that of Pembrokeshire. In spite of minor differences of opinion between various authors, it seems that the drift stratigraphy is consistent as far south as the Scilly Isles. The Irish Sea till is a good datum for correlation, and it seems that the chronology proposed for North Pembrokeshire may hold good for all areas except South-East Ireland and Lleyn, where later glacial fluctuations may be represented. The Dewisland tills are probably the equivalent in age of the Eastern General and Fremington tills.

Pleistocene sea-level movements in Pembrokeshire are considered briefly in the light of evidence from further afield. Little attention has been paid in the past to low Pliocene and Pleistocene sea-levels, for the scheme of a high regressive sea appears to have been widely accepted. Several of the findings from Pembrokeshire contradict recent viewpoints: the prolonged period of low sea-level prior to the Early Glaciation, the great age of the coastline, and the separation of the periods of rock platform cutting and raised beach deposition by a full glacial stage appear to support the findings of Orme rather than Zeuner and Bowen.

In the final part of the thesis an attempt is made to establish an absolute ciironology for the Pleistocene of West Wales. Faunal analysis suggests that the shell content of Irish Sea till and outwash gravels is the same, indicating that they are the products of the same glaciation. The mollusc assemblages of the drifts may also be correlated with that of the Wexford Manurial Gravels and some other Pleistocene shelly deposits. Some of the implications of the mixing of mollusc species in drift deposits are discussed, and it is concluded that the shell assemblages in the Pembrokeshire drifts probably represent temperate to cold conditions, and are of Last Interglacial age or younger.

The Dewisland Glaciation, which dredged up the shells from the floor of the Irish Sea, may therefore be the Last Glaciation. This dating is confirmed by C14 dates for four organic samples from drift. Shell samples from outwash gravels yielded the date
         
37,960 -1400/+1700 years B.P.
                 
and 37,310 -1275/+ 1515 years B.P.

and samples from till yielded the dates > 36,300 years B.P. and c.40,000 years B.P.

The glacial drifts of Pembrokeshire were probably deposited within the last 38,000 years; it is probable that the Eastern General till of Ireland, and other tills as far south as the Scilly Isles, are of the same age. Since the Dewisland Glaciation was the Last Glaciation of the Irish Sea Basin, it follows that neither the South Wales End-moraine nor the Midland General - Clynnog Fawr moraine can be considered true terminal features; the latter feature may represent a local readvance during overall deglaciation. By correlation with Europe, it is suggested that the dated shells were alive during the Middle Warm period of fluctuating interstadial climates, while the Dewisland Glaciation was the equivalent in time of the European Main Wurm Glaciation. There is no evidence in West Wales to support the so-called Early Wurm Glaciation of other areas in western Britain; during the Early Wurm in Pembrokeshire there was intensive solifluxion, perhaps culminating in a local mountain glaciation at New Quay. By further correlation, the Poppit Interglacial is probably the equivalent of the North European Eemian, while the Early G1aciation may be equated with the Continental Riss. In the Midlands it has been proposed by some authors that the Main Irish Sea Glaciation was Early Wurm, while in Ireland several workers have mentioned that the Eastern General Glaciation was the equivalent of the Riss; it is suggested that neither of these datings is compatible with the evidence from Pembrokeshire, and that careful stratigraphic studies and further C14 dating of Irish Sea drifts may resolve this difficulty.

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Just as it did 53 years ago, the answer lies down there.........