THE GLACIAL DEPOSITS OF WESTERN CARNARVONSHIRE. 53 



4. Boulder Clay with northern erratics and shells. 

 3. Sands and Gravels with northern erratics and shells. 

 2. Boulder Clay with northern erratics and shells. 

 1. Rock rubble or "Head." 



That the lowest Boulder Clay which is found to the east of Snowdonia, and which is 

 the product of native ice, does not occur also in Lleyn, is explained by the fact that there 

 are no gathering grounds for the production of glaciers of any size in that area, but the 

 severe climatic conditions would favour the accumulation of the rock rubble or " Head." 



The Drift which borders the coast of Lleyn has often a markedly terraced aspect. 

 This appearance is well seen in the neighbourhood of Clynnog, and again between Yr 

 Eifi (The Rivals) and Porth-dinlleyn. The upper surface of the Drift in these places 

 looks remarkably even as seen from a distance, and lies at a height of 100 to 150 feet. 

 A similar terraced aspect was noted at Llanbedrog, but here the surface is only about 

 40 feet above sea-level. 



Where the Drift of the coast shows this more or less even surface it is found to consist 

 for most part of the Sands and Gravels, and this may be taken to indicate a submergence 

 of the land to an extent of 150 feet or more. But the character of the shell-remains and 

 the form of the boulders in the boulder gravels militate against the theory that these 

 sands and gravels are ordinary marine deposits ; and where the cliffs are rocky there are 

 no indications that the land stood lower relative to the sea than it does at present. It 

 is more probable that these stratified Drift deposits were accumulated between the margin 

 of the retreating ice and the land area. The present coastal plain would at that time be 

 under fresh water, due to melting of the ice and to the ponding of the land drainage. 



Along certain parts of the north-west coast of Lleyn, notably at Porth Colman, a rock- 

 shelf or platform is seen at a height of 10 to 15 feet above the level of the present beach, 

 which simulates an old shore platform such as that which has been described by Messrs 

 Wright and Muff on the south coast of Ireland.* The surface of this platform is clear 

 of Drift for a few yards inwards from its seaward edge ; and it slopes gently upwards until 

 it disappears under the Drift — usually the Lower Boulder Clay (fig. 2). This platform 

 cannot be traced back to any well-marked pre-glacial cliff, nor is it overlaid anywhere 

 by any undoubted shore-deposits. At its inner end, where it passes under the Boulder 

 Clay, the surface often has a glaciated aspect. In early Glacial times the shore-line stood 

 further out to sea than it does at present, and the land descended in a gentle slope to 

 sea-level. The surface underwent severe glaciation and was buried under Drift. Since 

 that time there has been some subsidence, and the cliff has also been cut back by marine 

 erosion and the foreshore lowered. Where the land slopes gently to form a low cliff, the 

 Drift has been cleared away for some distance from the edge during stormy weather 

 and high spring tides, when the waves would be dashed against the face of the cliff to 

 considerable heights. 



* Scientific Proc. Roy. Dublin Soc, vol. x. (N.S.), part ii. (1904), p. 253. 



