52 DK T. J. JEHU ON 



Inland this Upper Boulder Clay is often absent. When it occurs it forms a thin 

 covering only a few feet deep, and is often a mere rubble of boulders set in an earthy 

 matrix. It has evidently suffered much from weathering and denudation. 



This Upper Boulder Clay may be the product of another mer de glace which 

 followed the same course as that which produced the Lower Boulder Clay, and 

 which was not much less extensive than its predecessor; or it may be due to the 

 re-advance of one and the same mer de glace. Whether the two boulder-clays be the 

 product of two separate and distinct ice-sheets separated by a genial inter-glacial 

 epoch, or the product of one glacial epoch which was marked by an extensive 

 oscillation of one ice-sheet, it is impossible to decide on the evidence presented in 

 Western Carnarvonshire. The Drift deposits of this area can be accounted for on 

 either of these theories. 



The ridges and mounds of gravel and coarse grey sand which run from north to 

 south from the neighbourhood of Penygroes to that of Brynkir represent morainic 

 material re-arranged and re-sorted by fluvio-glacial action and under the influence of the 

 torrential waters which must have flowed over the surface at the time of the final 

 disappearance of the ice. They occur near the eastward margin of the mer de glace 

 which came from the north and passed over Lleyn. 



At the time of maximum glaciation the mountains of Lleyn must have been almost, 

 if not entirely, buried under the ice-sheet. On Y Gyrn Goch, south-west of Clynnog, 

 Drift was noted at a height of 1200 feet above sea-level, and a boulder of a foreign 

 granite at 800 feet. The highest peak of Yr Eifl (The Rivals) reaches an elevation of 

 1849 feet, and it is possible that the summit of this mountain stood as a " nunatak 

 above the surface of the ice. The upper part of Yr Eifl has not a glaciated aspect, but 

 of course this may be due to subsequent weathering. 



It is interesting to compare the sequence of glacial deposits in the Welsh area on the 

 eastern side of Snowdonia with that met with in Lleyn. Professor Kendall # gives the 

 succession on the coast east of Little Orme's Head as follows : — 



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. Boulder Clay with Welsh erratics and no shells. 



A similar succession is to be seen in the Vale of Clwyd. In these districts the Lowest 

 Boulder Clay (I) is entirely the product of Welsh ice, which spread without hindrance 

 seawards in the early stages of glaciation prior to the advance of the northern ice-sheet. 

 In Lleyn this lowest Boulder Clay of the eastern area seems to be represented by the 

 rock rubble or "Head" which underlies the Boulder Clay (2) laid down on the 

 advance of the northern ice-sheet. So west of Snowdonia the sequence is as follows : — 



* Wkight's Man and the Glacial Period, 1893, p. 148. 



