GLACIAL ACTION IN S. BRECKNOCKSHIRE AND E. GLAMORGANSHIRE. 39 



4. On the Evidence of Glacial Action in South Brecknockshire 

 and East Glamorganshire. By J. W. Edgeworth David, Esq. 

 Communicated by Prof. J. Prestwich, E.G.S. (Read Novem- 

 ber 15, 1882.) 



1. Introductory. 



Evidences of glacial action seem to have been very little studied 

 hitherto in this part of South Wales. Professor Ramsay, in the 

 fifth edition of his ' Physical Geology and Geography of Great 

 Britain/ merely mentions the occurrence of ice-scratched 

 erratics all along the low ground of Glamorganshire north of the 

 Bristol Channel, between Cardiff and Bridgend, and says that 

 Boulder-clay is common here and there all over South Wales. Mr. 

 A. Tylor has described glacial markings on the surface-rock 

 at Hirwain Common, near Myrthyr Tydfil*. No one, however, so 

 far as the author is aware, has as yet attempted to work out the 

 problems of these glacial phenomena in a connected manner. And 

 yet few parts of Great Britain offer equal facilities for such a study. 

 This is chiefly owing to the great development in South Wales of 

 rocks so favourable to retaining marks of glaciation as the Mill- 

 stone Grit and Pennant Rock, and to the fact that the length of 

 their area is more or less transverse to the direction of glaciation. 

 The Millstone Grit, cropping out to the north of the South- Welsh 

 coalfield, runs east and west, without interruption, except at Caer- 

 marthen Bay, from \\ mile S.W. of Abergavenny, in Monmouth- 

 shire, to within 2 miles of St. Bride's Bay, in Pembrokeshire. This 

 band of Millstone Grit, 76 miles long, and from \ mile to 2 miles in 

 breadth, probably shows glacial markings at intervals throughout 

 the greater part of its length. In Brecknockshire, even where it 

 must have been exposed for ages to subaerial waste, it still presents 

 a polished and striated surface. 



The Pennant Rock and sandstones of the Coal-measures often 

 exhibit grooves and stria? when their surface is protected by a thin 

 covering of turf or rubble ; striae may also be preserved in places 

 on the surface of the Millstone Grit at its southern outcrop, though 

 they are here unknown to the author. A careful examination of 

 such an extensive glaciated area might serve to connect the glaciers 

 of North and South Wales, while some light might be thrown on 

 the question of the supposed extension of the Scandinavian ice-sheet 

 into the Bristol Channel if glacial stria? could be discovered on the 

 Millstone Grit or Carboniferous sandstones of the Porest-of-Dean or 

 Bristol coalfields. Being unable, owing to an appointment abroad, 

 to continue such an investigation, the author communicates to the 

 Society the results of his work, though very incomplete, hoping they 

 may be of some use to future workers in the same field. 



The area embraced in this paper (see Map, p. 40) extends north 

 and south from the Brecknockshire Beacons to a line drawn between 

 Cowbridge and the mouth of the river Rhymney, and east and west 

 * Quart. Journ. G-eol. Soe. vol. xsiv. 



