BRECKNOCKSHIRE AND EAST GLAMORGANSHIRE. 49 



well adapted for gravelling drives and footpaths, for which purpose 

 it is actually employed. Boulder-clay has been forced into the 

 shattered strata to a depth of 12 feet at least. Ice-scratched stones 

 are often to be found mingled with this debris, as shown in the 

 accompanying section (fig. 3). 



4. Grooved and striated BocJc-surfaces are exhibited, under favour- 

 able circumstances, by three kinds of rock in this area — the Carbo- 

 niferous Limestone, the Millstone Grit, and the Carboniferous sand- 

 stone, especially the Pennant rock. The Millstone Grit retains glacial 

 markings on its surface even after long exposure to the weather ; 

 the Carboniferous sandstone shows grooves or striae, if its surface 

 has not been long exposed, or is sheltered by a thin covering of 

 Boulder-clay ; the Carboniferous Limestone quickly loses all trace of 

 glaciation on exposure. 



The furthest point east at which the author has seen striae is at 

 Craig-y-gaer, 5 miles west of Abergavenny, somewhat beyond the 

 prescribed area of which this paper treats. The mamillated sur- 

 face of the Millstone grit is here very faintly striated, the trend of 

 the striae being about 70° E. of N. and W. of S. The author was 

 unable to determine from which end the striae started ; if from the 

 east end, they may have been produced by the Scandinavian ice-sheet ; 

 if from the west, they may be due to a local glacier nearly conforming 

 to the trend of the Clydach valley. Going westward, the next point 

 at which the author has seen striae is at Twyn-cilog, three miles 

 north of Rhymney. Here their trend is about 37° E. of 8., the ice 

 which caused them having come from the north-west ; the surface 

 of the Millstone Grit here is mamillated ; the height of the striae 

 above the sea-level is about the same as that of those at Twynau 

 Gwynion, 1| mile west, viz. 1840 feet ; surface of Millstone Grit at 

 Twynau Gwynion mamillated, trend of striae 36° E. of S., pointing 

 in the direction of the Brecknockshire Beacons. 



This is the highest point above the sea-level at which striae are 

 known to the author. They are 7 miles distant from the Beacons, 

 2910 feet high, giving a fall of 153 feet per mile. 



Proceeding westward, from Gwern-cefn-y-garreg, the striae gradu- 

 ally swing round so as nearly to converge on the top of the Beacons. 

 At the E. side of Cefn-cil-sannus their trend is 12° E. of S. ; the sur- 

 face of Millstone Grit is crushed, mamillated, grooved, striated, and 

 polished ; greatest height above the sea-level of striae, as taken by 

 aneroid, 1278 feet ; fall from top of the Beacons 233 feet per mile. 

 On the west side of Cefn-cil-sannus, the trend of the striae is 8° E. 

 of S.; at the east end of Mynydd Penmailard, first 3°E. of S., then, one 

 mile further west, N. and S. The respective heights of these two 

 last points above the sea-level are 1095 feet and 1199 feet, as taken 

 by aneroid ; fall from the Beacons of highest point 228 feet per 

 mile. Between these two points there are several angular slabs of 

 Old Red Sandstone, the largest measuring 4 ft. X 2 ft. 6 inches 

 x 2 ft. 6 inches. There are a great number of angular and rounded 

 blocks of Old Red Sandstone on the eastern slope of this mountain. 



A mile and three quarters further west, on the same mountain^ 

 Q.J.G.S. No. 153. e 



