THE GLACIAL DEPOSITS OF WESTERN CARNARVONSHIRE. 47 



Astarte compressa, and Buccinum undatum ; and several species which are now 

 common in British Seas. Thus we have in the Lower Boulder Clay a mixture of 

 species belonging to different climates. These included shells and shell-fragments are 

 therefore evidently derived, and point to the fact that the mer de glace of which this 

 Boulder Clay is a product travelled over a sea-floor ere it reached this region. A part 

 of a Belemnite was also found in the Boulder Clay at Aberdaron and several pieces of 

 wood in that at Porth Nigel. 



This Boulder Clay is, as a rule, strikingly homogeneous and uniform in character 

 and has evidently been subjected to great pressure. But at places a fine lamination is 

 seen in the clay, especially towards the top. Where this lamination occurs the clay is 

 stoneless and sometimes sandy. It appears to be sometimes due to shearing move- 

 ments. Wherever land-sliding has taken place lamination occurs towards the top of 

 the Lower Boulder Clay. On the foreshore at Porth Nigel an unctuous stoneless clay 

 is seen which seems to result from the running or sliding downwards of Boulder Clay 

 from which the coarser material has been washed out, leaving the residue in a buttery 

 condition with a tendency to go on sliding down the foreshore. The lamination of the 

 stoneless clay in the upper part of the cliff at Porth Nigel may have arisen in some- 

 what the same way, for it is seen where slipping has taken place. 



The best exposures of the Lower Boulder Clay are met with in the coast sections, 

 especially towards the western end of the Lleyn peninsula, but throughout the district 

 there are indications that it underlies the Sands and Gravels, although these often form 

 the lower part of the cliffs further east. The Lower Boulder Clay is shown by a boring 

 to underlie the modern alluvium which forms the flats to the north of Dinas Dinlle. It 

 has been detected below the Sands and Gravels of the Dinas Dinlle mound, but no 

 further exposure is seen along this coast until we reach Gwydir Bay, where it 

 forms a conspicuous feature below the other Drift deposits and under the foreshore. 

 From Gwydir Bay westwards it appears in most of the bays, becoming more and more 

 prominent and forming greater parts of the cliffs of Drift as we pass towards the 

 extremity of Lleyn. It appears to have choked up most of the depressions, channels, 

 and inlets met with along the coasts of the western part of Lleyn, sometimes even to 

 the brim. These drift-filled depressions are now being rapidly eroded by the sea. At 

 the head of the bays the Boulder Clay often forms the entire cliff, and extends below 

 the modern beach to an unknown depth ; but towards the sides rock rubble is often 

 seen lying between the Boulder Clay and the rocky slopes where these come down to 

 the beach. At Porth Oer, Aberdaron, and Porth Nigel, cliffs formed of the Lower 

 Boulder Clay attain heights approaching 100 feet. At Aberdaron, towards the west 

 end of the bay, there appear to be inclusions of large irregular masses of sand in the clay, 

 and towards the east end some bedded sands and gravel appear in the Lower Boulder 

 Clay. The finest exposure of the Lower Boulder Clay in Western Carnarvonshire is 

 that at Porth Nigel, where it forms high cliffs for a length of 3 to 4 miles. Following 

 the coast on the southern side of the peninsula eastwards, the Lower Boulder Clay is 



