Prof. J. F. Blake—Shell-beds of Moel Tryfaen. 269 
in which there is very little, if any real, terminal curvature. These 
beds die out and disappear on the slope of the mountain to the west. 
They equally die out to the south, as seen in the Moel Tryfaen quarry. 
Towards the east they are not exposed, but the upper part of more 
clayey drift is thickening in that direction, and probably replaces 
the shelly. One or other of them is spread out in a fan-shaped 
mound, forming a feature in the upland valley, and leading on to 
Hafod-y-wern slate quarries in the Bettws Garmon valley, where 
perhaps 60 feet of drift with small foreign pebbles is seen sloping 
down towards the locally glaciated valley. On the eastern margin 
of this mound there is a great felsite erratic. 
In the Alexandra quarries the difference between the shell-bearing 
sands and the overlying Boulder-clay is well marked, the latter lying 
in hollows, or in festoons in the former. The Boulder contents of 
the two do not appear to me to be very different; in both the stones 
which may be, and probably are, of Welsh origin, are the largest— 
in the shelly-drifts they are immense—and in both there are smaller 
(3-4 inch) rounded stones of foreign granite—Criffell, ete. Under 
these circumstances it does not seem necessary to trouble about the 
overlying beds; anything special they contain can be easily derived 
from the shelly-drifts below. 
Such are the data we have to go upon, in addition to the well- 
known characters of the sands and shells. Now when asubmergence 
was assumed as the cause, the foreign pebbles that occur amongst the 
sand necessitated that the date of that submergence should be after 
the arrival of the ice-sheet, but if the sands were brought by the ice- 
sheet itself, they would be the first to come. Starting from the 
mountains of Scotland, or the Lakes, it would be the froné of the ice 
that would first reach the sea, clear it of its loose shells, and carry them 
forward to Wales. We may therefore safely say that the shell-drifts 
of Moel Tryfaen are the very earliest of the glacial deposits, other 
than local, in North Wales, and the shells themselves must have 
lived in pre-Glacial times. This is quite consonant with the occur- 
rence of the small rounded granite, etc., pebbles in these drifts; they 
are some of the pebbles of the old Irish Sea, and have suffered little 
injury at the hands of the ice-sheet. A terminal moraine is the sign 
of a retreating glacier, hence the long mounds on the west of Moel 
Tryfaen are the products, not of the advance but of the decadence of 
ice-sheet; the materials which should be contemporaneous with the 
shell-drift have been carried away south, and it is just because these 
shell-drifts have found a corner to rest in, out of the main route, that 
they have been preserved. 
How then did the material get separated from the rest of the 
moving mass? We might imagine that the sheet came pressing 
against the northern flanks of Moel Tryfaen, and split in two, leaving 
the lateral moraine stuff on the eastern side. In this case we should 
expect to find many angular masses of the conglomerate amongst the 
debris, and that the resisting front of the mountain should be greatly 
glaciated, neither of which is the case. If this be not the history, 
then the ice must have come either over Moel Smythaw, or along 
