Gower, Pencoed and the "Older Drift"

In previous posts I have pondered on the attribution of certain glacial deposits on Gower to the Anglian glaciation -- or to the "Older Drift Glaciation" as it used to be called.  The attributions were dodgy, to put it mildly, because for a period in the 1980's there was an obsession with assigning every deposit to one or another of the "lithostratigraphic" units invented by Prof David Bowen and because of faulty amino acid dating.  I agree with Prof Danny McCarroll and Dr Bethan McCarroll that all of the glacigenic deposits described in the literature on Gower are probably Late Devensian in age -- except maybe for the concreted till in Watch-house Bay and in Foxhole Cove, near Southgate, which could be Anglian.  

I think that the concreted till in that area might be the same age as the concreted till at Black Mixen, Lydstep, and the other concreted deposits at Ceibwr and Witches Cauldron in North Pembrokeshire.

The other South Wales deposit assumed in the past to be of Anglian age is the mysterious glacigenic deposit at Pencoed, not far from Bridgend, which might be linked with the pottery clay worked in the past at Ewenny.  Some of the erratics collected from that area are found in the Storrie Collection, which I have discussed here:

https://brian-mountainman.blogspot.com/2011/09/erratics-and-deposits-in-vale-of.html

In the Geological Conservation Review volume of 1989, Campbell and Bowen were rather circumspect about the evidence from Pencoed and Ewenny, but the authors of the BGS Bridgend memoir of 1990 were more forthcoming, and it is worth recording some of their text here.

Wilson, D, Davies, J R, Fletcher, C J N and Smith,
M. 1990. Geology of the South Wales Coalfield, Part
VI, the country around Bridgend. Memoir of the British
Geological Survey, Sheet 261 and 262 (England and Wales)
Second edition

https://pubs.bgs.ac.uk/publications.html?pubID=B01835

P 47. INTRODUCTION


The major climatic changes that occurred during the
Quaternary resulted in the southward spread of ice over
much of northern Europe. Several glacial events are known
to have occurred, separated by interglacial or interstadial
periods when the climate ameliorated. Two glaciations, with
an intervening warm temperate interglacial perind, have
been recognised in South Wales (Bowen, 1973a, 1973b,
1974). The deposits of the earliest (pre-Ipswichian) glaci-
ation occur south of the limits of the latest (Devensian) glaci-
ation. The Ipswichian interglacial is recognised from raised
beach depesits, mainly on the Gower Peninsula and in
Pembrokeshire.

The glacial deposits of the Bridgend district comprise tills,
morainic and fluvioglecial sand and gravel and laminated
silts and clays. They were mostly deposited during the
Devensian (the ‘Newer Drift’ of Charlesworth, 1929);
however, the occurrence of supposed pre-Ipswichian till at
Ewenny [SS 903 777] and Pencoed [SS 959 817] (Mitchell,
1960; Bowen, 1973a) has been cited as evidence of an earlier
glaciation in the Vale of Glamorgan by ice of Irish Sea origin
(Bowen, 1970, 1973a, 1974). There are no dated Ipswichian
deposits within the district and evidence for the interglacial is
largely circumstantial, being based on the recognition of sup-
posed palaeosols (Crampton, 1964, 1966) of this age
(Bowen, 1970, 1974).

Postglacial deposits and landforms range in age from the
late Devensian, through the Flandrian, to recent times. They
mainly comprise alluvial and head deposits, but also include
the products of contemporaneous coastal processes and man-
made features.

==============

GLACIAL DEPOSITS
Page 47

These deposits cover most of the north-eastern part of the
district, are present within the main river valleys, and occur
on the coastal tract between Kenfig [SS 790 620) and Porth-
cawl [SS 808 782] (Figure 2). The expanse of glacial drift in
the north-east is generally regarded as part of an end-
moraine of “Newer Drift’ (Charlesworth, 1929) deposited by
a Glamorgan piedmont glacier, the glacial drift of Kenfig is
envisaged as being deposited on the flanks of a second pied-
mont glacier which debouched into Swansea Bay (Charles-
worth, 1929; Bowen, 1970). The glacial deposits of the lower
Ewenny valley, hitherto regarded as remnants of an earlier
(pre-Ipswichian} glaciation (Strahan and Cantrill, 1904;
Mitchell, 1960; Bowen, 1973a, 1973b), occur south of the
limits of the Devensian ice.

The glacial deposits generally consist of gravelly tills, con-
taining lenses of sand and gravel. Fluvioglacial outwash
gravels occur within, and beyond, the limits of the former
Devensian ice-front, forming terraces in places and underly-
ing the alluvium of the main valleys. Laminated silts and clays
occur as lenses within the tills and outwash gravels.


================

Correlation of glacial sequences

The earliest glacial deposits in South Wales are undated, but
preeded the Ipswichian interglacial. They are preserved at
several localities (Bowen, 1973a, 1973b, 1974), and it is
generally accepted that they were derived from Irish Sea ice,
which entered the Bristol Channel and impinged on the present
coastline.

Glacial deposits at Pencoed and Ewenny have been
regarded as till of Irish Sea derivation (Mitchell, 1960;
Bowen, 1970, 1975, 1974) from the reputed occurrence of
shelly material and igneous erratics at the former locality
(Howard and Small, 1900; Strahan and Cantrill, 1904).
These deposits have also been cited as evidence that the Vale
of Glamorgan was glaciated during the pre-Ipswichian
period (termed the Pencoed Cold Stage by Bowen, 1970).
The Pencoed deposit, however, has none of the character-
istics of a till, nor is there any indication that it is older than
the Devensian, the laminated sands, silts and clays revealing
evidence of deposition in a temporarily impounded lake that
developed at the margin of the Devensian ice-sheet.

The Ewenny deposit is a complex of till, gravels, silts and
clays. The till contains abundant clasts of Silesian sandstones
and appreciable amounts of weathered chert, probably
derived from Dinantian or Jurassic limestones, but the
occurrence of erratics of undoubted western provenance
(Strahan and Cantrill, 1904) has not heen verified. The till
probably impounded ephemeral lakes, in which the silts and
clays were deposited. The species of foraminifera within the
Ewenny silts and clays are indicators of a temperate, marine
environment. but are probally derived, with the Bristol
Channel as the most obvious source. They may have been
transported and redeposited by westerly derived ice, or
introduced by aeolian reworking of coastal flats exposed dur-
ing a glacial low-stand in sea-level: in the latter case, the
foraminifera give no indication of the source of the ice that
deposited the Ewenny till. The origin of this till is, therefore,
equivocal; it may have been derived from the west during a
pre-Ipswichian glaciation (Mitchell, 1960; Bowen, 1970,
19730, 1974), but equally may have been deposited during
the Devensian, either by ice from the north-east which
penetrated the lower Ewenny valley ahead of the main ice
from, or from westerly devived Devensian ice (Woodland
and Evans, 1964),

There are no further deposits within the district that can
be ascribed to the pre-Ipswichian gliciation, although
circumstantial evidence has previously been cited to suggest
that the Vale of Glamorgan was covered by ice during this
period. It has been suggested that exotic heavy mineral suites
within soil profiles indicated contamination from a relic
cover of glacial material of Irish Sea derivation (Crampton,
1960, 195), but they are more likely to have been intro-
duced by aeolian action. Cobbles of Lower Lias limestone,
within soil profiles overlying Lower Lias bedrock, were
formerly thought to be of glacial origin (Crampton, 1966),
but this is unlikely because they are generally derived by in-
situ weathering of Lower Lias bedrock. The occurrence af
westerly derived erratics in the Ely valley and as far east as
Cardiff (Griffiths, 1939) has not been confirmed (Waters and
Lawrence, 1987), there being a general absence of erratics
south of the Devensian ice front.

There are no proven Ipswichian deposits in the Bridgend
district, but the terra rossa and terra fusca affinities of local soil
profiles have been interpreted as evidence of former warm
temperate climate (Crampton, 1964, 1966), and Bowen
(1970, 1974) has suggested that they are palaeosols of
Ipswichian age. There is, however, no evidence to suggest
that these are anything other than modern soil profiles. The
‘terra rossa’ soils on Newton Down [SS 838 795] are probably
due to the weathering of Triassic bedrock or palaeokarst
horizons within the Oxwich Head Limestone, both of which
commonly impart a characteristic reddening to soil profiles.
The shallow, free-draining “terre fusca" soils on parts of the
Lower Lias outcrop generally reflect the limestone to
mudstone ratios of the bedrock.

It has been suggested (Trenhaile, 1971) that ledges along
the coast, between low tide and 12m above OD, are rem-
nant interglacial shore-platforms. These occur at Sker Point
[SS 786 796], Porthcawl [SS 812 766] and Black Rocks [SS
867 742], but no deposits have been found on them to pro-
vide evidence of their age and, in general, the features
appear to be the result of differential weathering of the Car-
boniferous and Triassic rocks.

The Devensian ice front in the district is broadly similiar to
that previously described (Charlesworth, 1929; Bowen,
1981; Figure 2), In the north-east, ice trom the Glamorgan
piedmont glacier debouched onto the low ground south of
the coalfield, impinging on, and locally overriding, the
Dinantian limestone escarpment on the southern limb of the
Cardiff-Cowbridye Anticline. Devensian ice encroached
onto the low ground north-west of Coity [SS 918 820], but
north of Bridgend was held back by the major escarpment
forming the southern margin of the coalfield. The tills, sands
and gravels in the north-west of the district have been inter-
preted as a complex kamiform morainic belt deposited by
Swansea Bay ice during the Margam Stage of the Devensian
glaciation (Bowen, 1970).

===================================

The above is interesting since the authors argue strongly that the Pencoed and Ewenny deposits (exploited in clay pits to serve at least fifteen local potteries) are late Devensian.  If they are right, this would extend the ice cover of the Late Devensian or LGM -- and this would perhaps reinforce the point I have been making about a complete Devensian ice cover of both Pembrokeshire and Gower.