THE GLACIAL DEPOSITS OF WESTERN CARNARVONSHIRE. 23 



nant the stiff bluish-grey Lower Boulder Clay is exposed, in which shell-fragments can 

 be seen. It forms the lower part of the cliff for about 10 feet above the modern 

 beach. This Boulder Clay is followed above .by sands and gravels up to a height of 

 about 70 feet. 



Again, to the south-west of Careg-y-llam, opposite Pistyll Church, deposits of a 

 similar nature occur, but the cliffs are much grassed over. As seen from a distance, 

 these Drift deposits form a well-marked terrace with a somewhat flattened top, border- 

 ing the higher land. 



The cliffs of Porth Nevin and Porth-dinlleyn are described on the Geological 

 Survey Map (1 inch to the mile) as being composed of sand and boulder clay. Both 

 bays have a horseshoe or crescentic shape and open out to the north. They are ex- 

 cavated for the most part in Drift deposits overlying green quartzose and chloritic schists, 

 but the north-east end of Nevin Bay is carved out of the Lower Silurian (Ordovician) 

 sedimentary rocks. As seen from a distance, the cliffs seem to be bounded above by a 

 terrace which extends inland to the base of the hills with a very even surface. Along 

 the greater part of this north-west coast the superficial deposits have this remarkable 

 flat-topped aspect. 



The smaller bay at Porth Nevin has an extent of about a mile. The cliff attains 

 heights of over 100 feet, but the sections are much obscured by talus formed of debris 

 which has slipped from above, and the cliff face is often grassed over. The cliff for 

 the greater part of its length is made up entirely of sands, gravels, and laminated 

 silty clays. At the two extremities of the bay the bed-rock is seen at the base, 

 and here the sands and gravels rest immediately on the solid rock. Where, as opposite 

 the middle of the bay, the sections are deeper and the bottom of the Drift deposits not 

 seen, the typical stiff dark greyish-blue Lower Boulder Clay appears underneath the sands 

 and gravels. This clay contains shell-fragments. Many of the included boulders, 

 amongst which the commonest consist of Carboniferous Limestone, are remarkably well 

 glaciated. 



Where the bed-rock is visible, the sands and gravels are at some places seen to be 

 separated from it by broken-up or shattered rock material, a foot or more deep. 

 Fragments of marine shells are plentiful in the sands, but these are generally too 

 imperfect for identification. This shelly sand is often stoneless, but at places it 

 includes irregular masses, pockets, and seams of coarser gravelly material. Sometimes 

 the gravel is so coarse that it would be more accurately described as a boulder gravel, 

 and this occasionally swells out to form considerable portions of the cliff. The sands 

 are stratified, the gravels sometimes show bedding, but occasionally are tumultuous, 

 showing no definite arrangement. Contortions can here and there be observed in the 

 sandy beds. 



Above the sands and gravels are buff-coloured laminated silty clays free from stones. 

 These at places form the upper part of the cliff, but elsewhere in the same bay the cliff is 

 capped by gravel or by a brownish-yellow weathered clayey rubble. 



