THE GLACIAL DEPOSITS OF WESTERN CARNARVONSHIRE. 49 



ice-sheet filled Cardigan Bay and even invaded Pembrokeshire. It is impossible to 

 define accurately the eastern boundary of this ice-sheet. The northern ice met and 

 overcame the Welsh glaciers which moved towards Anglesey from the valleys of 

 Snowdon ; it invaded the foot-hills, such as Moel Tryfan, lying west of the Snowdonian 

 mass. Professor Kendall, in Wright's Man and the Glacial Period (1893), p. 171, has 

 pointed out that it was the proximity of Snowdon which really enabled foreign ice to 

 invade Wales in the Moel Tryfan region. He observes that " a glance at the map will 

 show that the ' radiant point ' of the Welsh ice was situated on or near Arenig Mawr, 

 and that the great mass of Snowdon stands quite at the periphery of the mountainous 

 regions of North Wales, so that it would oppose its bulk to fend off the native ice-sheet 

 and prevent it from extending seaward in that direction." Consequently, the northern 

 ice-sheet was able to overwhelm Western Carnarvonshire as far east as a line running from 

 Carnarvon to Cardigan Bay somewhere in the neighbourhood of Pwllheli. The western 

 slopes of the Snowdonian Mountains could only have given rise to small glaciers, and 

 these would unite and become merged into the glacial mer de glace which moved south- 

 westwards. South of Snowdonia a great glacier came down the Vale of Gwynant past 

 Bedd-gelert, reaching Cardigan Bay at Traeth Mawr, where it was joined by another 

 issuing from the mountains of Merioneth by the Vale of Ffestiniog. The united mass 

 would be large enough to fend off the northern ice and to spread out as a fan towards 

 Criccieth and Afonwen, and the volume of native ice would be increased by a smaller 

 stream coming down the mountain valley between Moel Hebog and Carnedd Goch. 

 The presence of the large volume of native ice in that neighbourhood explains the 

 absence of foreign erratics and of shell-remains in the Drift deposits of the coast sections 

 from Afonwen to Criccieth. The Sands and Gravels there seen are probably derived 

 from a Lower Boulder Clay, although this is nowhere exposed on this part of the coast. 

 We may assume that, if exposed, the Lower Boulder Clay in this part, like the Sands 

 and Gravels, would show a general absence of foreign erratics. 



The Intermediate Gravels and Sands form a very variable series and are somewhat 

 inconstant in their occurrence. They comprise both stratified and unstratified material, 

 but the former predominates, especially in the coast sections. The best exposures are 

 met with in the cliffs along the coast- line, the series being especially well developed at 

 Dinas Dinlle, in the stretch between Clynnog and Gwydir Bay, at Nevin and Porth- 

 dinlleyn, at the eastern end of Porth Nigel, at Llanbedrog, and east of Afonwen. The 

 sands are, as a rule, yellowish or yellowish-brown in colour, and are marine-like in 

 aspect ; they vary in texture, showing all grades from very fine sand to coarse gritty 

 material. The coarser sands pass into gravel and boulder gravel. 



Mr Mellard Reade's investigations proved that the sand contains a large propor- 

 tion of extremely well-rounded grains often possessing an extraordinary polish and 

 transparency. He speaks of these as "microscopic boulders," and states that the 

 contrast between these and the local grains is most remarkable. The sands and gravels 

 usually show bedding, although at places confused irregular masses occur, especially of 



TRANS. ROY. SOC. EDIN., VOL. XLVII. PART I. (NO. 2). 7 



