386 MR. A. STRAHAN ON THE GLACIATION OP 



curvature; but more especially the development of undoubtedly 

 marine sands and gravels in association with Boulder-clays, prob- 

 ably of marine origin also. 



But the view of this ice-sheet having extended from the north 

 over this region and along the coast of ISTorth Wales has been put 

 forward by Sir A. Ramsay as a result of his observations on the 

 Isle of Anglesey *:,. The evidence adduced may be briefly stated as 

 follows : — The gently undulating plain is overspread by detritus 

 from the north ; the underlying rocks are often ice-smoothed and 

 marked with strise pointing directly to the mountains of Cumber- 

 land ; the Snowdon group of mountains sent down great glaciers 

 to the west, north-west, and north, but never across the Menai 

 Straits. The striations of Anglesey were produced by an ice-sheet 

 which overspread the region now occupied by the shallow sea of 

 Morecambe, Lancaster, and Liverpool Bays, and was of sufficient 

 power to stop the encroachment of the Snowdon glaciers. Mall- 

 draeth Marsh, the Menai Straits, and other valleys of minor note 

 run in precisely the direction of the striae, and were scooped out by 

 this vast glacier from the north-east. It may be added that if the 

 striation and configuration of Anglesey were produced by such an 

 ice-sheet, it would form a strong argument for that of South Lan- 

 cashire having been due to the same cause. 



But I would venture to suggest that it is not necessary to invoke 

 the assistance of this vast glacier to account for the form of the 

 ground in Anglesey. In the case of both the Menai Straits and 

 Malldraeth Marsh large faults have introduced strips of Carbo- 

 niferous strata among the older rocks ; such faults invariably con- 

 stitute lines of weakness, and the association of rocks so different as 

 the Carboniferous (especially the Coal-measures) and highly altered 

 schists invariably gives rise to a physical feature. These faults 

 date from an early period, and features due to the structure, and 

 corresponding more or less with those existing now, probably came 

 into existence long before the Glacial period. The drifts of the 

 island, as described, appear to be the continuation of those which 

 overspread South Lancashire and fringe the coast of North Wales, 

 and like them are probably marine. If, then, the features of the 

 island are due to the geological structure and the drift is a marine 

 deposit, the reasons for invoking the aid of an ice-sheet no longer 

 exist. 



While, however, there is a general agreement as to the marine 

 origin of the sand, gravel, and associated Boulder-cla3^s, there is an 

 equally wide-spread opinion that the tough and very strong base- 

 ment-clay, seen at Colwyn Bay, Bryn Elwy, Llanefydd, and other 

 places in Forth Wales, is the product of an ice-sheet. It has been 

 one of the objects of this paper to show that the striae of the border- 

 land belong to the later period of the undoubted marine drifts. So 

 far as I know, none of those herein described show any connexion 



* " How Anglesey became an Island," Quart. Journ. Geol. Soc. vol. xxxii. 

 (1876). 



