632 STRATIGRAPHICAL GEOLOGY 



recently been found 13 that a great mass of ice passed down the 

 valley of the Towy, but that only its lower part followed that 

 valley westward, the upper ice - layers escaping southward over 

 the escarpment on the southern side. 



Much boulder - clay occurs in the track of this ice -sheet from 

 heights of 1 500 feet downward, that around Plynlimmon being 

 described as a stiff blue clay or till crowded with rock-fragments 

 of all sizes up to blocks of several tons weight. In South Wales 

 much of it is coloured red by the debris of the Old Eed Sandstone. 

 In Glamorgan it passes southward into coarse angular gravel, and 

 finally near the coast into roughly stratified gravels with rounded 

 but striated stones. The interest of the last is increased by the 

 fact that they overlie certain raised beaches so that the beach- 

 material with its marine shells and the marine sand of the caves 

 with the mammalian bones are thus proved to be of Glacial 

 age. 14 The shells are of recent species, but the mammals include 

 Elephas antiquus, Rhinoceros mercki, Bison prisus, Hycena spelcea, 

 and Ursus spelceus. 



In Pembrokeshire, however, there is quite a different set of 

 Glacial deposits. Those of Northern Pembroke between Cardigan 

 and St. Bride's Bay have been examined by Dr. T. J. Jehu, 10 who 

 found them divisible into (1) a lower boulder - clay, (2) inter- 

 mediate sands and gravels, (3) an upper boulder - clay. The first 

 is a tough bluish - grey clay containing boulders of Carboniferous 

 Limestone, of rocks from the south-west of Scotland and many Chalk- 

 Hints, together with many broken marine shells. It occurs in 

 patches up to 600 feet above sea-level. 



The sands and gravels frequently contain marine shells which 

 are generally broken, but some are perfect. Most of the species 

 are such as now live in the Irish Sea, but some are Arctic forms, 

 e.g. Trophon claihratus (common), T. scalariformis, Astarte compressa, 

 and A. arctica ( = borealis~); while the commonest shell is Pectun- 

 culus glycimeris. These marine sands occur up to 640 feet. 



The upper clay is of a yellowish-brown colour and is best seen 

 in the bays on the coast. The included stones are well scratched 

 and are mostly from northern sources. It is sometimes 40 feet 

 thick, but in places it passes into stony rubble. 



In South Pembroke the deposits are similar, 16 and near 

 Noltoii on St. Bride's Bay a mass of brown boulder-clay fills up a 

 valley and forms cliffs which are 50 to 60 feet high ; it includes 

 flints and many broken marine shells. Isolated transported 

 boulders also occur all over the country, and their disposition 

 shows that the ice-movement was from the north-west ; many come 

 from distant sources. 



