THE GLACIAL DEPOSITS OF NORTHERN PEMBROKESHIRE. 83 
it becomes somewhat difficult to separate them. But the distribution of the Upper 
Drift is not so wide; it is only met with here and there, and the true Upper 
Boulder-Clay is often replaced laterally by Rubbly-Drift. The more sporadic occurrence 
of the Upper Boulder-Clay is probably due in part to the fact that it has suffered 
more from denudation. 
The Lower Boulder-Clay is undoubtedly the product of an ice-sheet, and it has 
all the characteristics of a true ground-moraine. It is remarkably tough and homo- 
geneous, and shows no traces of stratification, and it has all the appearance of having 
undergone great compression. The included stones are often intensely glaciated, and 
are sub-angular rather than rounded in form. The fact that fragments of marine shells 
occur in the clay proves that the ice which gave rise to it must have travelled over 
a sea-bottom. On the other hand, the bits of woody matter sometimes seen embedded 
in the tough clay to a depth of 18 or 20 feet suggest that vegetation grew on the 
land bordering the sea, before the advent of the ice or during an interglacial period, 
and that some fragments of this found their way, by means of streams or otherwise, 
to the sea-bottom, where they lay in the path of the ice. Or they may have been 
derived from the remains of a submerged forest. The included erratic stones help 
us to follow the direction from which the ice came, and the occurrence of boulders 
from the south-west of Scotland and from the north-east of Ireland in the Lower 
Boulder-Clay and Drift as far east as Cardigan, and the discovery of fragments of 
marine shells in the Lower Boulder-Clay exposed at the brickworks near that town, 
make it clear that the whole of northern Pembrokeshire was buried underneath 
an ice-sheet coming from the north. The view held by Carviti Lewis, that the Irish 
Sea glacier (as he termed it) extended no further south than the extremity of Lleyn 
in Carnarvonshire, is shown to be inaccurate. And though Professor James GEIKIE 
makes the mer de glace which overwhelmed Anglesea flow down St George’s Channel, 
to a limit reaching beyond the south-west of Wales, he only indicates it as crossing 
the extreme west of Pembrokeshire at St David’s Head. But the facts just mentioned 
show that this mer de glace must have passed over a great deal more of Pembrokeshire 
than St David’s Head. It invaded northern Pembrokeshire along its whole extent, and 
even encroached on Cardiganshire to the east, and its trail is evident in the tough 
dark-blue homogeneous boulder-clay, with its northern erratics and the broken shells 
derived from the sea-bottom over which the ice travelled. How much further south 
this typical boulder-clay or ground-moraine extends is a point which must be left 
to future investigation. 
This mer de glace was of course the southward extension of that ice-sheet which 
filled the northern basin of the Irish Sea, and which has been described by Professor 
JAMES GEIKIE and other workers in Glacial Geology. ‘The latest results published 
are those of the investigations of Mr Lampiucu in the Isle of Man, and these have 
appeared in his Survey Memoir on the Geology of that Island. His observations on 
the Irish Sea Glacier are of great interest and importance, and throw light even on 
