84 DR T. J. JEHU ON 
what occurred to the south of his area. Speaking of the conditions which obtained 
in the northern part of the Irish Sea at the beginning of the Glacial period he says, 
“Along the shores an ice-foot probably formed in the winter and broke away in the 
summer into floes, which distributed their burden of rock-fragments broadcast over 
the sea-floor. This seems to be the explanation of the universally wide dispersal of 
the fragments from Ailsa Craig, which have been recognised in the drift almost all 
round the northern part of the Irish Sea basin, in Ireland and Wales, as well as in 
the Isle of Man. The sea-girt precipices of splintering rock in Ailsa would not fail 
to cast off a load upon an ice-foot below; and thus these fragments became strewn 
over the sea-floor almost as widely as the shells, and were subsequently carried by 
the ice-sheet into nearly every district to the southward where the shells were carried” 
(p. 370). This helps to explain also in a satisfactory way the occurrence of fragments 
from Ailsa Craig and of boulders from the north-east of Ireland, from the south-west 
of Scotland, and even possibly from the Inner Hebrides, in the drift, and on the beaches 
of northern Pembrokeshire. For the ice-sheet as it advanced would pick up any such 
fragments which had been previously strewn over the sea-bottom by ice-floes, and 
would carry them southwards on to the land as it carried the shell-fraements. 
LaMPLuGH estimates that in the neighbourhood of the Isle of Man the ice-sheet, 
at its maximum, must have attained an elevation of not less than 2000 to 3000 feet 
above the present level of the sea, and the general direction of the ice-movement 
was from north-north-west to south-south-east. He points out that “the West-British 
Ice-sheet probably attained its ultimate dimensions mainly from the accretion of 
snowfall upon its surface, and in only a minor degree from the inflow of tributary 
glaciers.” He calls attention to certain results which help us to understand the south- 
ward extension of the ice-sheet far enough to overwhelm Pembrokeshire. “As the 
British ice-sheets must always have received their increment principally from the 
moist Atlantic winds, it seems probable that, without any change of climate, the centre 
of greatest accumulation, and consequently of maximum glaciation, would tend to 
shift steadily westward and south-westward as the icy plateau rose higher in the 
path of the moisture-laden winds and compelled their earlier precipitation. This 
effect would, moreover, be accentuated by the obliteration of the open water in the 
sea-basins to the eastward. The West-British sheet might from this cause go on 
increasing, while its Hast-British and Pennine equivalents were already diminishing 
from lack of sufficient snowfall. ... For the above reason, the shrinkage of the 
ice-sheet covering the Isle of Man is likely to have commenced while the Welsh and 
Ivernian sheets were still increasing.” Although the ice in the southern part of the 
Irish Sea basin did not probably atta such a great thickness as the ice in the 
northern area did, nevertheless all the evidence goes to show that even as far south 
as Pembrokeshire it must have reached a considerable elevation. The presence of 
drift material at Pen Creigiau, at an altitude of over 600 feet above sea-level, indicates 
that the ice-sheet here was in all probability not much less than 1000 feet in thick- 
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