THE GLACIAL DEPOSITS OF WESTERN CARNARVONSHIRE. 29 



steeply to Bardsey Sound. The south-west side of the island consists of lowland rising 

 gently towards the higher ground. This lowland is covered by a skin of Drift. The 

 whole island has the appearance of having been moulded by ice during the Glacial period, 

 but, as Ramsay observes,* "the mammillated roches moutonnees have since been 

 roughened by weather." The island as a whole may be regarded as an example of the 

 phenomenon known as " crag and tail," the crag facing the north-east, from which direc- 

 tion the ice-sheet came. 



At Porth Solfach, on the island, the low cliff is composed of yellowish-brown clay 

 showing a depth of 6 to 7 feet. Though weathered near the surface, the clay gets 

 stiffer below. Bits of shells in the clay were too fragmentary for identification. 

 Amongst the included boulders were found chalk-rlints, the Ailsa Craig micro-granite, 

 Dalbeattie granite, a picrite, a lava closely resembling that of Borrowdale in Cumberland, 

 Millstone Grit, Carboniferous Limestone showing crinoid stems. Some of the boulders, 

 especially those of the limestone, were well glaciated. 



Again at Henllyn beach, in Bardsey, pebbles were picked up of Carboniferous Lime- 

 stone, chalk-flints, and of a reddish-streaked rhyolite. These were probably derived 

 from the Drift of the low cliff adjoining. 



Returning to the mainland, the coast is rocky as far as Aberdaron, but the cliffs are 

 interrupted by an inlet at Porth Mendwy, where a stream comes down to the sea. Some 

 river-like gravels and sands are seen at the sides of this inlet. 



At Porth Pistyll, 1^ miles to the south-west of Aberdaron, the cliff is steep 

 and rocky, but above, at a height of about 100 feet, bluish-grey boulder clay of ex- 

 cessive toughness was noted — the deposit attaining a thickness of 4 to 5 feet. 

 The exposure is a very small one. This stiff clay is capped by weathered yellowish 

 rubbly clay. 



Aberdaron Bay. — Situated on the southern side of the extremity of Lleyn, this bay 

 is excavated in beds of Bala age let down by faults into the hard schistose rocks of pre- 

 Cambrian age which bound them on either side. The sides of the bay are rock-bound, 

 but the cliff between the two arms consists entirely of Drift deposits and is described on 

 the Geological Survey Map (1 inch to the mile) as composed of sand, clay, and gravel. 

 The bay is over a mile broad, but the cliff is interrupted opposite the village where a 

 stream debouches. The height reached by the cliff varies considerably up to elevations 

 of over 100 feet. At the south-western end of the bay the rocky cliff descends with a 

 steep slope to the beach and is followed by rough rock rubble or " Head." This 

 " Head " is overlain by more rubbly material mingled with Boulder Clay, the matrix, 

 however, becoming sandy or gritty in places. To the west of the wooden pier the 

 " Head" is well defined for 8 or 10 feet from the base of the cliff. It dips eastwards at 

 a high angle and disappears under the beach level. The material above this well-defined 

 " Head " is still rubbly, with big angular boulders of the neighbouring rock, but it 

 becomes more and more clayey as it is traced up the cliff face, and the included boulders 



* Survey Memoir, " The Geology of North Wales," 2nd ed., 1881, p. 212. 



