THE GLACIAL DEPOSITS OF WESTERN CARNARVONSHIRE. 33 



a kind of bedding or lamination can be seen which may possibly be due to a shearing 

 movement, for the clay has all the appearance of having been subjected to great 

 pressure. Boulders are fairly common but not abundant. They are distributed pell- 

 mell throughout the clay, not in clusters or patches but singly. The majority are about 

 the size of one's fist, and are subangular and beautifully glaciated. Far-travelled rocks 

 are common, and include chalk-flints, various south of Scotland granites, especially from 

 the Dalbeattie area, the Ailsa Craig micro-granite, a granite with reddish felspars, 

 Carboniferous Limestone, Millstone Grit, etc. Pebbles of the Ailsa Craig rock and of 

 the other Scottish granites were also picked up on the beach and had evidently been 

 derived from the cliff. Bits of wood were also found included in the clay. Marine 

 shells occur scattered irregularly in the clay, generally in a very broken condition, but 

 some complete shells were found. The forms identified are given below. The fore- 

 shore is for the most part covered by pebbles, but at some places it consists of buttery 

 dark blue clay evidently due to the running of the Lower Boulder Clay. 



The Intermediate Sands and Gravels. — These are somewhat inconstant in their 

 occurrence, but towards the eastern end of the bay they form the greater part of the cliff. 

 Usually they are seen to overlie the Lower Boulder Clay, but at places they rest 

 immediately on the modern beach, where the Lower Boulder Clay is not exposed. 

 Where the sands follow the clay in the cliff there is often much slipping. Sometimes 

 this series consists entirely of fine siliceous sand, sometimes of fine pebbly material, and 

 sometimes of coarse gravels. The sands and gravels often, but not at all places, show 

 stratification. These deposits vary much in thickness and are apt to die out when 

 traced laterally. At Porth Nigel no contortions were observed in the sandy layers. 

 Here and there the sands become largely ferruginous. No shell-fragments were noted. 

 The pebbles in the gravels are of rocks very similar in their origin to those found in the 

 Lower Boulder Clay. 



The Upper Boulder Clay is not very conspicuous in these sections. Where it occurs it 

 sometimes overlies the Intermediate Sands and Gravels, and sometimes rests immediately 

 on the Lower Boulder Clay. It is yellowish in colour, and much weathered, and tends 

 in places to become sandy and pebbly, or to pass into a rubble Drift. Ice-scratched 

 stones can be picked out, but are not so frequently met with as in the Lower Boulder 

 Clay. The included boulders contain erratics, but those from a great distance are not so 

 common as in the Lower Boulder Clay. Fragments of marine shells are rare, and those 

 obtained were very fragmentary. 



At the west end of the bay a little angular local detritus or " Plead " lies on the solid 

 rock where it slopes down to the beach. There is much slipping and overgrowth here 

 but the Lower Boulder Clay, which forms the base of the cliff of Drift, appears to overlie 

 this "Head." Towards this end of the bay the cliff attains heights approaching 100 

 feet, and consists from top to bottom of the massive Lower Boulder Clay. Further east 

 a somewhat sandy band runs along the face of the cliff 15 feet from the bottom. This 

 band has a thickness of 2 to 3 feet, and when traced laterally it has more of the nature 



TRANS. ROY. SOC. EDIN., VOL. XLVII. PART I, (NO. 2). 5 



