THE GEOLOGY OF PUFFIN ISLAND. 87 
the snowfields of the Lake district, of the south Scotch 
highlands, and of the north of Ireland, and together filled 
the Irish Sea by a great mer de glace that flowed south- 
wards, glaciating the plains of Lancashire, Puffin Island 
and Anglesey, on its route to its unknown termination. 
At the same time glaciers descended from the Snowdon 
group, through the pass of Llanberris and Nant Francon, 
and either ended by expanding over the plain between the 
hills and the shore, or joined the northern ice on the line 
of the limit of the northern drift, as shown in Strahan’s 
map.* Under the ice sheet the boulder clay and other 
deposits are supposed to have been formed as a ground 
moraine. One of the effects of the passage of this ice 
sheet over Anglesey was, according to Ramsay, the erosion 
of the parallel valleys of the Menai Straits and that 
between Pentraeth and Malldraeth Marsh, which are thus 
regarded as but large glacial grooves; Professor Seeley, 
however, demurs to this view,t and prefers to consider 
the former as a sort of cafion due to solution and denu- 
dation of the carboniferous limestone that once filled it. 
Succeeding this great ice sheet, a warmer interglacial 
period is supposed to have followed, during which the 
shell bearing sand of Moel Tryfaen, so well known since 
its original description by Trimmer,{ was deposited at the 
height of over 1350 feet above the present sea level. 
Colder conditions again prevailed, during which local 
glaciers filled the valleys of Cumberland and Snowdon. 
But recently a strong current of opinion has set in 
against this interpretation of our glacial deposits, led off 
by Mr. Belt’s series of letters to ‘‘ Nature”’ (vol. x., 1874), 
and by Mr. Goodchild’s advanced and suggestive paper on 
* Quar. Jour. Geol. Soc., vol. xlii., p. 370. 
+ Phillip’s Geol., edit. 1885, vol. i., p. 129. 
{+ Proc. Geol. Soc., vol. i., p, 331, 
