THE GLACIAL DEPOSITS OF NORTHERN PEMBROKESHIRE. va 
emerging from underneath the boulder-clay, which becomes thinner in this direction. 
At places the rock shows a hummocky surface, marked with glacial striz, which run 
from north-west to south-east. At the base of the small promontory called Trwyn 
Hwrddyn, on the north side, a rubble of very coarse fragments lies between the solid 
rock and the drift. Here the drift shows a rough sort of stratification, and has 
much sandy and pebbly material intermingled with boulder-clay. 
Fig. 5 shows a section in Whitesand Bay seen further south. This is also about 
20 feet in height, but here it consists more of a rubbly-drift. The matrix is earthy and 
sandy, and is choke-full of small flakes of slaty and other rocks, which have a rude 
kind of arrangement, especially towards the lower part. Boulders of larger size occur 
here and there, and consist for most part of grit and conglomerate and slaty rocks, 
with some blocks of the local gabbros and diabases. Pebbles of white and yellowish- 
white quartz are common. The top is covered by loose yellow sand, probably wind- 
blown, and just underneath this are traces of a pebbly bed. Though rock does not 
appear at the bottom just at this spot, it crops out on both sides a short distance away. 
Drift of a similar kind is seen in sections, and capping the cliffs at other places on 
the western coast of the promontory. 
Boulder-clay is seen at Porth-lisky stuck full of stones, many of which are smoothed, 
polished, and striated ; and a boulder of the St David’s Head gabbro, measuring roughly 
3x2%x2 feet, lies on the beach below. A stony boulder-clay or a more rubbly drift 
is seen at various places on the south coast, the best exposures being above Caerfai 
Bay and at Caerbwdi Bay. At Caerbwdi the cliff is over 20 feet in height, but the 
base is hidden by talus: the matrix is here rather sandy, and streaks and pockets of 
rather fine sand are seen here and there. The included stones are often pebbly, but 
some are sub-angular and ice-marked. They are made up almost entirely of rocks 
found in the neighbourhood. Near the top flaky fragments are very plentiful, and 
these are derived from the local purple flagstones and slates. A big boulder of the 
coarse gabbro from St David’s Head lies at the base of the cliff. 
No good section is seen at Porth-y-Rhaw, but the drift caps the hills and cliffs 
to the south-east. 
Very similar sections are seen on the north coast at Abereiddy and above Traeth 
Llyfn. At the latter place a rubble of big boulders is seen; most of these are of local 
igneous rock, very iron-stained and decomposed. Towards the top the section is freer 
of big boulders, and is full of little flakes of sedimentary and cleaved rocks. 
One of the finest sections on the coast is seen at Aber-mawr, west of Strumble Head. 
At the northern end the rock is seen capped by 10 to 15 feet of stony-drift. As traced 
southwards the drift thickens to about 40 feet, then tails off rapidly. Where thickest 
the lower part shows some tendency to a rough kind of bedding, and is full of small 
flakes and little stones, more or less pebbly. This passes above into a rubbly clay, full 
of boulders of all sizes, most of which are angular and sub-angular, and derived from 
rocks of the locality. The cliffs on Strumble Head in many places are covered by a 
