The last British Ice Sheet: A review of the evidence utilised in the ...

The maximum southerly extent of this Celtic Sea ice lobe has been mapped on the Scilly Isles using
stratigraphic evidence for a Devensian glaciation (Scourse, 1986, 1987, 1991a,b; Scourse and Furze, 2001).
Specifically, solifluction breccia overlying and enclosing organic layers has been observed to overlie a raised
beach deposit. Radiocarbon dates and palynological evidence from the organics record a tundra grassland
between 34 ka and 21 ka BP. The breccia is overlain by till (Scilly Till) in the northern Scillies and by loess
on the southern isles, the loess having been dated by thermoluminescence at 18.6 ka BP (Wintle, 1981).
Earlier work by Mitchell and Orme (1967) identified a southern limit for the glacial deposits that impinged
upon the north coasts of the northern Scillies. They proposed a Gipping (Wolstonian) age for the till based
upon comparisons with stratigraphic sequences in southwest England. Bowen (1969, 1973b) later argued that
the till had been soliflucted after having been deposited originally in the Wolstonian. These age estimates
were based upon “the interpretation of the number and age of the stratigraphically juxtaposed raised beach
units” (Scourse, 1991b) on the isles and therefore are less secure than the age derived from large numbers of
absolute dates on the sub-till breccia. Characteristics of the Scilly Till are summarized by Scourse (1991a,b;
Hiemstra et al., in press). It contains abundant siliceous sponges and Miocene glauconite-bearing micritic
limestone derived from the Jones Formation lying offshore to the north (Pantin and Evans, 1984), indicating
that it is derived from the southern Celtic Sea. The continuation of the till offshore has been assessed by
Scourse (1986) and Scourse et al. (1990, 1991, 2000) based upon samples collected and analysed previously
by Pantin and Evans (1984). The characteristics of “till-like” materials in the Devensian Melville Formation
(Cameron and Holmes, 1999) lying in the southern Celtic Sea have been interpreted as the deposits of an ice
stream emanating from the Celtic Deep to the south of St George’s Channel and flowing southwestwards
towards the shelf edge. A change in depositional processes at approximately 49°30΄ is thought to represent
either a grounding line position or a change from proximal to distal glacimarine deposition (Scourse, 1986;
Scourse et al., 1991). The “till-like” material in the Melville Formation is correlated with the Scilly Till by
Scourse et al. (1991), implying that the Dimlington Stadial ice in the Celtic Sea was considerably more
extensive than the previously accepted limits. This more extensive coverage has gained support from more
recent work in southern Ireland (see below). In the Irish Sea, St George’s Channel and north Celtic Sea
contain two widespread deposits linked to the whole Devensian Stage but also documenting Dimlington
Stadial glaciation (Cameron and Holmes, 1999). The Western Irish Sea Formation and Cardigan Bay
Formation include glacigenic sediments relating to the last glaciation. Specifically, the Upper Till Member of
the Cardigan Bay Formation is interpreted as a basal till and correlated with the Irish Sea Drifts of southern
Ireland by Wingfield (1994, 1995). The Western Irish Sea Formation includes waterlain sediments and is
sub-divided into Formation B, the uppermost of which is coeval with the Upper Till Member of the Cardigan
Bay Formation, and Formation A, predominantly deglacial glacimarine sediments and the infills of deep
glacial incisions (Wingfield, 1994, 1995). The very patchy nature of the Upper Till Member in the Celtic Sea
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