42 J. W. E. DAVID GLACIAL ACTION IN SOUTH 



districts, the Glamorganshire and the Brecknockshire, according as 

 the erratics in each area were dispersed by Brecknockshire or Gla- 

 morganshire rocks. As the erratics of the Ely valley east of Peter- 

 ston have been derived partly in Glamorganshire and partly in Breck- 

 nockshire, the area occupied by them and the few foreign(?) erratics of 

 Chalk flints and one of quartz-porphyry may be termed " mixed." 



Brecknockshire Erratic district. — The district in which the erratics 

 are nearly all derived from Brecknockshire rocks extends from the 

 Beacons, twenty- miles south, to the Eglwysilan mountain, and east 

 and west from the Taff-fechan and Ehymney valleys to the Neath 

 valley, as far south as Glyn Neath, and the Aberdare valley and 

 the left side of the TafF valley as far south as Walnut-tree Bridge. 

 These erratics consist of blocks of Old Bed Sandstone, Carboniferous 

 Limestone, and Millstone Grit. The bottoms and sides of the Taff- 

 fechan and Taff-fawr valleys, between the Beacons and the northern 

 boundary-line of the Carboniferous Limestone, are strewn with an- 

 gular and rounded blocks of Old Red Sandstone : these erratics are 

 found to have crossed the area of the Carboniferous Limestone in a 

 southerly direction, and still further south, associated now with 

 erratics derived from that formation, to have invaded the area of 

 the Millstone Grit. 



South of the northern outcrop of the Millstone Grit, erratics of 

 that rock at once make their appearance, and from this point to the 

 Eglwysilan mountain form by far the largest proportion of the 

 Brecknockshire erratics. 



The further advance of the erratics to the south was evidently 

 considerably checked by the steep north scarp of the Coal-mea- 

 sures. Between Aberdare and Neath valleys the ground rises to 

 the south over 1000 feet in some places, in a space of one mile, so 

 that scarcely a single Brecknockshire erratic has found its way into 

 the Rhondda valleys. South of this escarpment erratics of Breck- 

 nockshire Carbonifereus Limestone entirely disappear. Erratics, 

 however, of Millstone Grit and Old Bed Sandstone are still met 

 with in the Aberdare, Taff, and Taff-Bargoed valleys, the Millstone- 

 Grit boulders being particularly plentiful in the Taff-Bargoed valley. 

 A very large group of these erratics has been found by Mr. Edwin 

 Corbett on the south side of the Eglwysilan mountain, 12 miles 

 south of the northern outcrop of the Millstone Grit. Thousands of 

 them lie inside the horseshoe formed by the mountain on its south 

 side. They consist of angular and rounded blocks of Millstone Grit 

 and Old Bed Sandstone, some of the largest being from 6 to 7 feet in 

 diameter, and weighing from 7 to 8 tons. The author noticed in 

 two places a perfectly angular erratic of Millstone Grit touching a 

 rounded block of the same rock, both being imbedded in what 

 appeared to be Boulder-clay. At Cwm Sarn, a continuation of 

 the Eglwysilan mountain to the S.W., there is a faintly grooved 

 surface of Carboniferous sandstone about 200 yards to the south of 

 these erratics. The trend of the grooves is 9° W. of N. and E. of S.; 

 while the mean lie of the longest axes of seven large blocks here in 

 position is 36° W. of N. and E. of S. 



