32 DR T. J. JEHU ON 



midway between its two sides <i slope of Drift is seen reaching heights approaching 100 

 feet. It consists of yellowish weathered clay with large boulders, and contains some 

 shell-fragments. Pebbly and sandy layers are seen here and there. At the sides of 

 the bay this Drift is separated from the solid rock by angular rubble or " Head." Some 

 of the big boulders in the cliff are well glaciated, and the beach below is strewn with 

 blocks derived from above. 



Porth Nigel or Hell's Mouth. — Situated on the southern side of the Lleyn promon- 

 tory, this bay opens out to the south-west. Its south-eastern side is bounded by the 

 high ground of Mynydd Cilan, which is made up of highly resisting hard gritty sand- 

 stones and shales of Cambrian age ; the north-western side partly by indurated shales 

 of Ordovician age, but chiefly by the greenstones which extend from Mynydd-y-Rhiw to 

 Penarfynydd. Between these sides the head of the bay is formed entirely of glacial 

 deposits and has an extent of about 4 miles, giving one of the finest sections of the Drift 

 to be seen in North Wales. These deposits stretch inland, covering the whole country 

 from Llanengan to Llangian and on north-westwards by Llandegwning to Bottwnog, 

 thence south-westwards as far as Mynydd-y-Ehiw. Erosion is still going on to a marked 

 extent at the head of the h&y. On the Geological Survey Map (l inch to the mile) it 

 is stated that the cliff consists of " stratified clays, sands, and gravel from 10 to 40 feet 

 high." This description requires considerable modification. At places the cliff attains 

 much greater elevations. The general succession is as follows : — 



4. Soil and Blown Sand. 



3. Upper yellowish weathered Boulder Clay, becoming sandy and pebbly in places. 

 2. Intermediate Sands and Gravels. 



1. Lower Boulder Clay — massive, tough, and homogeneous. (This Lower Boulder 

 Clay descends below the level of the modern beach to an unknown depth.) 



The cliff is highest towards the north-west end of the bay, where it attains elevations 

 approaching 100 feet, and shows almost vertical faces which present a striking appearance. 

 Here it consists almost entirely of the Lower Boulder Clay, which, indeed, is by 

 far the most prominent member of the various deposits exposed in this bay. The 

 Intermediate Sands and Gravels are not seen at the western end, but are well displayed 

 when we pass towards the eastern end. Taking the deposits from below upwards, we 

 have at the base — 



The Lower Boulder Clay. — This is a stiff, homogeneous clay, grey in colour on 

 the face of the cliff but becoming darker below the surface. It presents a massive 

 appearance, giving almost vertical faces which are traversed by fissures resembling 

 joints running from above downwards. These cracks are widened by the action of 

 rain and frost, and large blocks are thus wedged off and fall in masses on to the beach 

 below. The base of the cliff is reached by the waves at high spring tides, and the 

 clay is scooped out, leaving a smooth hollow over which the cliff above hangs. This 

 overhanging of the cliff shows the tough nature of the clay. Here and there traces of 



