86 DR T. J. JEHU ON 
eastwards to the neighbourhood of Caer-bwdi Bay. In this connection it may be 
mentioned that Mr J. Harris, in a report on erratics in South Wales, which appeared 
in the British Association Reports, 1898, refers to some boulders found at Pencoed, 
near Bridge-end, Glamorganshire. Microscopic sections of some of these were prepared 
and sent to petrologists for examination. The result of this is given as follows :— - 
“One was identified with the gabbro of St David’s Head; a felsite bore some 
resemblance to the pre-Cambrian rocks of Pembrokeshire; two or three acid rocks, 
brecciated felsites, and tuffs are very like those of the Lleyn promontory.” From 
these data it is concluded that the transport of boulders was from the west or 
north-west. If one of the boulders found near Bridge-end is accurately identified as 
belonging to the St David's Head gabbro, it is a most remarkable fact. It is 
hardly safe to draw any conclusion until it has some further corroboration. 
The more or less loose materials covering the bottom of the sea, which existed 
before the advance of the ice, would become incorporated into the lower layers of the 
ice-sheet. And as the ice was very thick, and moved onward slowly, it would exert 
a great pressure over its bed, with the result that much of the rocky floor would be 
torn away, and much of the material ground up and pulverised to form the typical 
ground moraine. The shell- banks which occurred on the sea- bottom would be 
destroyed, and the marine detritus would be carried forward under the ice or in the 
ice. This accounts for the presence of shell-fragments at places in the Lower 
Boulder-Clay. And the most natural explanation of the shelly sands and gravels 
is that they represent the material of a sea-bottom, carried onwards and upwards to 
their present position by an ice-sheet, and re-arranged by fluvio-glacial action. 
That is to say, they are remamniés derived from the bottom-moraine of an ice-sheet 
which had travelled over a sea-floor. Similar sands and gravels have been found 
at many other places on the west side of the island, and they have given rise to 
much discussion—notably those found at Moel Tryfan in Carnarvonshire. The Pem- 
brokeshire series differ from those of Moel Tryfan in that they are found overlying 
the well-marked stiff Lower Boulder-Clay. The most remarkable feature in connection 
with the Moel Tryfan beds is the great elevation at which they are found—1350 feet 
above sea-level. In Pembrokeshire the greatest height at which they have been met 
with is 642 feet at Pen Creigiau, four miles south-west of Cardigan. The mode of 
origin of such sands and gravels has been one of the most vexed questions in 
Glacial Geology. Some writers, such as Macxrntosu, ‘I. M. Reape, and others, have 
argued that the sands and gravels represent marine deposits laid down in place 
during a great submergence. It is admitted, even by the opponents of that theory, 
that a partial submergence took place during Glacial times, but to what extent is 
not known, and there is no evidence to show that it meant a sinking of the land 
in Carnarvonshire to as much as 1350 feet below its present level. And the partial 
subsidence which is allowed is generally thought to have diminished towards the south. 
In Pembrokeshire no evidence can be seen along the coast which would lead us to 
