THE GLACIAL DEPOSITS OF NORTHERN PEMBROKESHIRE. 87 
believe that there had been a subsidence of 640 feet below the present level, so as to 
account for the marine deposition of the beds found at Pen Creigiau. Further, if the 
advocates of a submergence point to the presence of marine shells at places in the 
sands and gravels as a proof, on the ground that the shells must have been altogether 
destroyed if carried beneath the ice in the morainic débris, one asks then how they can 
account for the presence of fragments of shells—some large enough to be identified— 
in the tough blue boulder-clay underlying the sands and gravels? No one who looks 
upon that clay, as exposed, for instance, at the Cardigan brickworks, can doubt for a 
moment that it is the product of an ice-sheet. Not a trace of stratification can be seen 
in it, nor is there any character which suggests even the possibility of its being the 
result of marine deposition. And yet marine shells are seen imbedded in the clay. 
This, as has been already pointed out, is due to the fact that the ice-sheet, of which this 
clay is the bottom-moraine, travelled over a pre-existing sea-bottom. The fact that 
everywhere the shells are very broken and much rolled is hardly compatible with the 
view that they are now found in or near the positions in which the molluscs themselves 
lived. And it is worthy of note that the Lamellibranch shells obtained in these sands 
and gravels are never found with the two valves in apposition, as one might expect to 
find if they lie in ordinary sea deposits. Again it has often been pointed out that it is a 
significant fact that deposits of this kind only occur in glaciated areas, and that wher- 
ever broken shells are found, with them there also we find far-travelled erraties present. 
And this is to a marked extent the case in Pembrokeshire. 
Mr J. F. Buaxe (Geol. Mag., vol. x., 1893, p. 267) concluded that the shelly sand at 
Moel Tryfan had been pushed up in front of the advancing glacier, and that, as a result 
of this glacier meeting that which came out of the Bettws Garmon valley, the sand 
got pushed into a protected corner and was left there. But in northern Pembrokeshire 
the sands and gravels are found scattered in patches over a wide area, and are 
frequently well bedded. Here they are the products of the washing and re-sorting of 
infra- and intra-glacial detritus. This may have gone on partly under the ice, but it 
would no doubt take place to a great extent at the time of the melting of the ice-sheet, 
when large streams would issue from the margins of the glacier and re-arrange much of 
the superficial deposits left on the surface of the land. 
The Upper Boulder-Clay is so sporadic in its occurrence that it is difficult to draw 
any definite conclusions with regard to it. It may possibly represent a second advance 
of the ice-sheet after an interval of less severe glacial conditions. It is far more stony 
than the Lower Boulder-Clay, and in places passes into Rubbly-Drift. This Rubbly-Drift 
is very similar to that found by Lampiueu in the Isle of Man, and is probably “the 
remamé deposit of the ice-sheet modified by sub-aerial agencies.” At the time of the 
final disappearance of the ice, torrential waters must have overflown parts of the surface, 
and the rubble is probably to be attributed in part to the action of these waters. 
Morainic material would become mixed up with rock débris, formed by ordinary 
weathering processes, and the whole mass would be re-arranged, and in places sifted 
by the waters, 
