The evidence of the North Wales day presented an argument that the limit of the Devensian glaciation was on the north of the Lleyn, and that 'glacial' deposits to the south of the Clynnog Fawr/Bryncir end-moraine were therefore older than the last (Devensian) glaciation. In mid-Wales, particularly around Aberystwyth, there is plenty of evidence of periglacial activity, but the evidence for glaciation is limited. Indeed, what little evidence for direct glacial deposition that exists has been interpreted as evidence of glaciation prior to the Devensian. It was argued that the evidence from mid-Wales enabled a complete reconstruction of Quaternary glacial and interglacial events.
The tour today takes in both highland and lowland locations, although the most compelling evidence is from lowland coastal sites. The map below shows the sites mentioned.
The evidence I am presenting to you concerning 'day 2' is largely dependent upon the work of the late Dr. E. Watson and his wife Dr. S. Watson. The references I have given to you list a number of key papers written by one or more of the Watsons, plus one or two other key papers. It is critical that you read and understand their work as, despite being quite dated, the work has by and large withstood the test of time.
The Tal-y-Llyn valley is a superb example of a glacial valley. At the head of the valley there are thick accumulations of stratified talus (grèzes litées). Features that have been described as 'nivation hollows' can be found, and in one section through the talus an ice wedge pseudomorph can be clearly seen. The evidence here of considerable periglacial activity and the apparent lack of glacial landforms (apart from the valley itself) is repeated in the Rheidol and Ystwyth valleys further south.
Tal-y-Llyn valley. Cader Idris to the right
The evidence of the valley itself is worth considering. All researchers agree that the erosional glacial landforms of the British Isles are not the product of a single glaciation (the last) but have been reoccupied and modified by successive glaciations. The degree of modification of these landforms that was effected by the more recent episodes of glaciation is still not known, but the presence of glacial cirques in our mountains probably facilitates easier glaciation than might otherwise be the case. Therefore the fact that Tal-y-Llyn is clearly a glaciated valley is not, of itself, evidence of recent glaciation. Only the deposits, whether as landforms or sediments, are likely to tell us the story of Quaternary environmental change.
These valleys are two of the most attractive in the whole of Wales. The drive along the Aberystwyth mountain road in particular is well worthwhile, but this has nothing to do with the educational objectives of these pages. Do it once in your lifetime anyway.
The
Rheidol and Ystwyth valleys display considerable evidence of periglacial activity.
The Rheidol and upper Ystwyth valleys are notable for thick sheets of soliflucted
debris, and the Rheidol valley (left) displays classical periglacial asymmetry.
The
Ystwyth valley also shows considerable influence of periglacial activity, in
particular the development of what Watson described as thermocirques. Within
the valley there a number of landforms that can be described as cirques, but
they lack the over-deepened basin of the 'true' glacial cirque. The image right
shows one such example. This south-west-facing cirque has a thick tongue of
soliflucted debris in the valley bottom. It can be argued that the form of the
cirque does not suggest recent glacial activity (compare it with the image of
Cwm Idwal from 'day 1').
On the opposite side of the valley from the cirque pictured above, are two other cirque-like features. Both are difficult to capture as photographs, but are very important. The first cirque, Cwm Tinwen, is small, lacks an overdeepened basin but has a small accumulation of debris at the 'lip'. Watson believed that this small feature developed during the Loch Lomond stadial as a thermocirque or nivation hollow. The small ridge accumulated as what we would now call a pronival rampart. Cwm Tinwen is very small compared to the cirque above, and is also small when compared to the next image.
The
second feature (left and below, right), called Cwm Du, is also a north-east
facing cirque like Cwm Tinwen, but the front of the cirque is choked by a huge
accumulation of debris. Watson argued that the larger landform was also a periglacial
thermocirque, lacking the over-deepened basin of a true glacial cirque. It was
the nature of the debris on the cirque floor, and beyond the cirque that was
considered diagnostic of the origin of the landforms. Initially Watson argued
that the multiple ridges and thick accumulations of debris were the product
solely of pronival processes, but later (1977) suggested that the lowest deposits
in the sequence were deposited by a small glacier, which stagnated. The cirque
and associated debris are at least an order of magnitude larger than the Cwm
Tinwen, which is only a mile or so further up-valley. Watson argued that similar
features of similar size to Cwm Tinwen, that are active in the Ural mountains
today, took at least 10,000 
years
to be formed. Given that the Loch Lomond stadial apparently only lasted for
1,000 years and terminated 10,000 years ago, it follows that this periglacial
landform, at best, was formed between 20,000 and 10,000 years ago. Therefore
20,000 years ago this part of mid-Wales was not glaciated, but subject to intense
periglaciation. This accords with a Devensian maximum limit on the Lleyn peninsula.
And this was not the only evidence from mid-Wales that supported this view.
Morfa Bychan is an enigmatic site, which to this day still provides opportunity for debate concerning the genesis of the Quaternary sediments found at the site. Morfa Bychan is a coastal exposure of thick sedimentary sequences. The cliff runs approximately north to south, exposing a long cross section of sediment, many tens of metres high. The site is only a few miles south of Aberystwyth.
The
Quaternary sediments of Morfa Bychan can be divided into three distinct units.
At the base is a yellowish coloured diamict, termed the 'Yellow Head'. The thickest
(middle) unit is termed the 'Blue head'. The uppermost unit is another diamict,
called the 'Yellow head'. There is a layer of gravel between the blue and brown
head units, which is just about visible in the two photos. In the picture left)
the yellow head is the unit immediately above the group of eager undergraduates.
To the right of them is solid rock (turbidites). The blue head is the greyish
central unit, best shown on the furthest left part of the cliff. Brown head
caps the lot.
(Reminder: 'Head' is a quarrymens' term for unconsolidated rubble found above (at the head of) solid rock. It represents frost shattered debris and colluvium. The geomorphologist might use terms such as soliflucted debris, talus, colluvium or periglacial diamict. The old quarry-men had a more practical vocabulary than we do).
Depicted
right is the brown head.A relatively thin unit, the blue head beneath is also
claer. No-one doubts that the Brown and Yellow head units represent accumulation
of soliflucted material under periglacial conditions. The fabric of the two
units clearly indicates that the clasts are oriented east-west (ish) such that
the debris flowed downslope, under the influence of gravity, toward what is
now the sea. The fabric is entirely expected, it concords with local topography.
The clast lithology within each unit is uniform - typical of head deposits.
The
problem is with the blue head. Only the sediment colour separates the three
units. Their fabric is identical. But the clasts within the blue head (and only
this unit) are striated, yet only a single lithology is present - there are
no erratics. There are two possible scenarios. Firstly, the yellow head accumulated
during the early Devensian, as material was weathered locally. Then, during
the Devensian maximum, solifluction brought an ancient till down from a position
higher up the slope. Finally, during the Loch Lomond stadial, the brown head
was deposited. It was also weathered from local bedrock. It naturally differs
from the brown head as it has been less weathered over time. The striated clasts
of the blue head were inherited from an older glaciation, and could even have
formed under periglacial transport conditions.
A second scenario would accept an explanation for the brown and yellow heads, but would argue that the lowest part of the blue head (at least) is in fact an in situ glacial diamict. Demonstrating that this is true is difficult, since the fabric of all three deposits is similar. It is known that the general trend of glaciation should be north-south in Cardigan bay (direction of movement of the Irish sea glacier). However, closer to the Welsh uplands it might be expected that the fabric should concord with east-west trend of the major valleys. (Think back to Wood Lane and Aberconwy - which was the most powerful ice stream?).
A further twist is provided by recent research. Ballantyne and Harris (1994) and Harris (1996, 1998,2001) suggest that the blue head is paraglacial in origin. The fabric we see today results from immediate post-glacial sub-aerial re-working of glacially transported debris. It is possible, but Watson (1996) disagrees. In summary, the story has yet to be told to universal satisfaction. The one thing that everyone agrees is that the Morfa Bychan sequence represents a depositional history of the Devensian cold period only. Unlike the next site.
Just a few miles further down the coast from Morfa Bychan is Llannon. This site is a low lying area of low relief. Although not well exposed there is a complex sequence of alluvial gravels and diamicts. The diamicts at the base of the sections are separated from the overlying alluvial gravels and boulders by what has been termed an interglacial soil.
The upper gravels and display evidence of intense cryoturbation. Mitchell (1960, 1972), and Watson (1976, 1977), argue that the lower diamicts predate the last interglacial period, partly because of the presence of the 'interglacial soil'. The soil has yielded pollen, and one critical species has been identified from the soil - Abies. Consult your notes from the earlier section of the course for the significance of this. Unfortunately the 'soil' did not (apparently) yield a great deal more environmental information. What is this soil? What is its significance within the stratigraphy at Llannon? I'll leave you to ponder this.
The
upper gravels represent fan deposition during the Devensian. Nobody disagrees
with this. Exactly when during the Devensian is another matter. Watson suggested
that the deposition of the thickest gravel sequences was during the early or
middle Devensian. The gravels were then intensely cryoturbated in the harsh,
ice-free environment of the Late Devensian. The image left shows examples of
'festoons', picked out by the lighter colours. Watson also recognised a younger
stage of alluviation and further cryoturbation.
The diamicts below the gravels are also interesting. There are two distinct units of different provenance. The uppermost diamict is described as an 'Irish Sea Till', the lower as a 'Welsh Till'. This is the first time in quite a while that we find evidence of the interaction of Welsh and Irish Sea ice. Clearly the two ice masses were co-eval prior to the deposition of the gravels above. Sorry, no photos - whenever I visited the site the diamicts were poorly exposed.
Within
the Cledlyn valley are some of the best examples of pingo ramparts to be found
in Britian, although they are not easy to photograph. Watson mapped and described
the occurrence of the Cledlyn pingos in some detail. He observed that the density
of pingos was very high, particularly when compared to the Yukon, Alaska. In
Alaska the density of pingos beyond the limit of the last glaciation is similar
to that found in mid-Wales. Within the Yukon limit (moraines dated to 7,000
years ago) the density of pingos is much less. Therefore to achieve the high
density of pingo distribution found in mid-Wales requires a time period greater
than 7,000 years. They therefore cannot have formed during the Loch Lomond stadial,
but must have formed, at the latest, during the Devensian maximum. They were
certainly not destroyed by glacial ice. Therefore mid-Wales was ice-free during
the Devensian. Simple isn't it?
This 'stop' will be re-visited on 'day three'. I will present one interpretation of the site here, and a different one 'tomorrow'. Poppitt sands is important because it is the first time since the Lleyn that we have come across a raised beach, and this time there is no doubt about landform. (see note 1)
The
raised beach deposit is a superb example of an unconformity (see photo). A horizontal
deposit of rounded pebbles rests upon folded beds of Aberystwyth grits (Ordovician,
if I remember rightly). The beach deposits must have been left behind when sea-level
fell at the end of an interglacial period, but which interglacial?
The
raised beach is overlain by a thick accumulation of head, which in turn is overlain
by a diamict, interpreted as Irish Sea till. If mid-Wales was not glaciated
during the Devensian cold period then the glacial diamict must represent an
older glaciation - the Wolstonian. The raised beach is therefore Hoxnian in
age. (Note: the above is a rather simple presentation of the case argued by
Mitchell (1960, 1972), and little direct mention of Poppitt sands is used by
Mitchell or other members of the 'Irish School' (Bowen, 1974). The site does,
however, illustrate the ideas well. Mitchell used a number of arguments to date
raised beaches, undoubtedly Poppitt would have been described as Hoxnian in
age.)
SUMMARY
The diverse range of landforms and sediments of mid-Wales can be interpreted to suggest an ice-free environment for the western part of mid-Wales during the Devensian. The dating of events is a problem, and thus chrono-stratigraphies have to be inferred. You need to ask yourselves whether you are convinced by the evidence before considering the literature in detail. I throw in the following for you to consider.
Essential reading: McCarroll, D. 2001. 'The Glacial Geomorphology of West Wales' IN: Walker, M.J.C. and McCarroll, D. The Quaternary of West Wales Field Quide. QRA, London. It's in the LRC.
Note 1: Since I wrote this things have, natch, changed. The raised beach deposit at Poppitt Sands is now questioned, in that Etienne et al. (2001) argue that the gravels are fluvioglacial, and that the platform is a glacially eroded platform. Probably. Perhaps. Well at least it throws the cat amongst the pigeons. In terms of the story I am trying to tell, for now go with what I say above. Etienne et al. is in the west Wales field guide.This web site is all that remains of what was a hastily thrown together field course. Inspiration for the course came from a field course I was invited to attend by Dr. Geoff Thomas. I am grateful to him for filling in the gap in my knowledge between the Gower and Lleyn peninsulas. And for a stunning friday night tour of some of the less respectable hostelries of Aberystwyth.