Recent expressions of opinion concerning the glaciation of west Wales have prompted this note on directions of ice movement. It has been known for many years that an extensive Irish Sea glacier was responsible for the last glaciation of the southern Irish Sea basin, having moved broadly southward across the Irish Sea from its source areas in Scotland and the Lake District of England. At the period of its maximum extent, the Irish Sea glacier swung towards the south-west on the east coast of Ireland, bounded by local Irish ice a few kilometres inland which prevented any marked westward incursion by the glacier. Most of the glacier ice was diverted along the coast to the vicinity of Wexford and thereafter the local ice appears to have been powerful enough to hold it some distance offshore (Reference Synge and StephensSynge and Stephens, 1960). Farther towards the west, however, the glacier encountered Iess resistance and was able to impinge upon the coast again in the neighbourhood of Cork Harbour. On the eastern shore of St George’s Channel a similar situation prevailed, with ice movement towards the south until at some stage there was a swing eastwards into the Bristol Channel (Reference West and RankamaWest, 1967). Overall, therefore, the Irish Sea glacier appears to have moved within St George’s Channel until it diverged as a piedmont glacier on reaching the oceanic south-western approaches of the British Isles.
Considerable difficulties are encountered when one attempts to interpret the influence of local ice on the directions of movement of the Irish Sea glacier. The presence of local ice in the Carnsore Point–Wicklow Head area in the west may have been matched by equivalent local Welsh ice in the Cardigan Bay sector. Probably the uplands of Wales acted as a centre of ice outflow during the last glaciation, and indeed the drift stratigraphy of the Cardigan Bay coast indicates a complex interplay of Irish Sea ice and Welsh ice (Reference WilliamsWilliams, 1927; Reference MitchellMitchell, 1960). The extent to which Welsh ice was able to hold Irish Sea ice offshore at the maximum of this glaciation is unknown; it is possible, however, that in conjunction with the Irish ice to the west it was powerful enough to effect a constriction in the Irish Sea glacier prior to its divergence southwards.
The evidence for directions of ice movement across Pembrokeshire is valuable in this context, although it appears to have escaped previous comment. Reference GeikieGeikie (1894) suggested that Pembrokeshire was over-ridden by ice from north-east to south-west. This was a logical hypothesis, assuming that there was no local Welsh ice in Cardigan Bay strong enough to cause any diversion of the Irish Sea glacier (Fig. 1). Geikie has been followed by many authors (e.g. Reference Pringle and GeorgePringle and George, 1948, p. 91–99; Reference MitchellMitchell, 1960) in showing an unimpeded passage of Irish Sea ice southwards across Cardigan Bay at the maximum stage of the area’s last glaciation. This ice impinged upon the coast of south Cardiganshire, but is not thought by these authors to have been diverted south-westwards by the orientation of the upland slope or by Welsh ice flowing off the uplands. Presumably, the forward movement of the glacier must have been balanced by ablation at its margin.
Fig. 1. Hypothetical directions of ice movement through St George’s Channel, assuming the absence of local Welsh ice from Cardigan Bay (in part after Reference GeikieGeikie, 1894).
C Cahore Point, Ca Carnsore Point, MB Morfa Bychan, NQ New Quay, SDH St David’s Head. Also marked is the glaciation limit as recorded by Reference GeikieGeikie (1894).
On the other hand, Reference HicksHicks (1885) and Reference JehuJehu (1904) suggested from the evidence of striations and transport of erratics that the Irish Sea glacier over-rode Pembrokeshire from north-west to south-east. Subsequent work has confirmed this and Griffiths (unpublished) has shown that at one stage ice movement was almost west–east. It appears, therefore, that the glacier ice was already diverging when it crossed the Pembrokeshire peninsula. The greatest constriction in the glacier must have been located, not between St David’s Head and Carnsore Point in Ireland, but farther to the north, possibly between Cardigan Bay and Cahore Point. In the east this constriction could have been caused by either:
-
a. a mass of impeded Irish Sea ice in the embayment, separated by a zone of shearing from the unimpeded glacier ice farther west, or
-
b. a mass of Welsh ice flowing from an ice cap over central Wales and Snowdonia.
The latter possibility has more to commend it, for there is every reason to suppose that the last glaciation of the Welsh uplands was contemporaneous with the Irish Sea glaciation. Indeed, evidence for an outflow of local Welsh ice before the incursion of Irish Sea ice at New Quay (Reference MitchellMitchell, 1960) and at the close of the Irish Sea glaciation in Lleyn (Reference SyngeSynge, 1964) indicates that Welsh ice must have been active in this area over a greater length of time than the Irish Sea ice itself. The contemporaneity of Welsh and Irish Sea ice in Cardigan Bay has not been doubted since the work of Reference WilliamsWilliams (1927), and has again been stressed by Reference BrownBrown and others (1967). In view of this evidence, the hypothesis of an ice-bounded constriction in the Irish Sea glacier seems entirely reasonable (Fig. 2).
Fig. 2. The position of the hypothetical constriction in the Irish Sea glacier during the last major glaciation of St George’s Channel.
-
Directions of movement of the Irish Sea glacier.
-
Directions of movement of local ice masses.
-
Zone of contact between Irish Sea glacier and local ice.
The age of this Irish Sea glaciation is still open to doubt. While it is just possible that the directions of ice movement referred to above date from different glaciations, most authors have referred them to one glaciation: this is the “Eastern General” glaciation of Reference Synge and StephensSynge and Stephens (1960) and the “Gipping” glaciation of Reference MitchellMitchell (1960). These authors have considered that this glaciation was the equivalent of the continental Riss glaciation. Together with Reference Watson and WatsonWatson and Watson (1967), Reference BowenBowen (1966) and Reference LewisLewis (1966), they have considered that Irish Sea ice of the Würm glaciation did not impinge upon the southern coast of Cardigan Bay, with the possible exception of a small area around the Teifi estuary.
Alternatively, the Irish Sea glaciation referred to above is here thought to be the equivalent of the European Main Würm glaciation. This is based upon stratigraphic evidence from Pembrokeshire and south Cardiganshire, and on a comprehensive series of radiocarbon dates for organic materials contained in fluvio-glacial deposits of the Irish Sea glacier. These dates indicate that the last glaciation of St George’s Channel probably occurred within the last 30 000 years (Reference JohnJohn, 1965; Reference BrownBrown and others, 1967).
If the Irish Sea ice which over-rode Pembrokeshire from north-west to south-east was Main Würm in age, then it follows that the hypothetical Iobe of Welsh ice in Cardigan Bay must have been contemporaneous. The coast of Cardiganshire could not have remained ice-free in these circumstances, and it seems possible that the problematical pseudo-stratified deposits at Marfa Bychan and New Quay may be the products of local Welsh ice which was at its most powerful during the Main Würm maximum. (For an alternative interpretation of the Morfa Bychan deposits see Reference Watson and WatsonWatson and Watson (1967).) Again, the presence of Irish Sea till above the local till at New Quay may indicate that the Irish Sea glacier achieved its greatest incursion of the south Cardiganshire coast towards the end of the glaciation.
There are many problems to be solved before the full story of ice movements on the shores of the southern Irish Sea basin can be told. Possibly glaciological criteria could be applied to the reconstruction of both the form and dimensions of Britain’s largest Pleistocene glacier. Concerning the age of the Irish Sea glaciation, the two viewpoints outlined above are clearly incompatible. However, it is hoped that the hypothesis of a lobe of Welsh ice in Cardigan Bay during the Main Würm will prompt some discussion during subsequent studies of glaciation in this area.
