50 



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



the trend of the striae is 30° W. of S. ; at Cefn Cadlan they run 38° 

 W. of 8., and at Gwern-cefn-y-garreg 42° W. of S. All these striae 

 are on mamillated surfaces of Millstone Grit. At Gwern-cefn-y- 

 garreg, their greatest height above the sea-level, as taken by aneroid, 

 is 1444 feet. 



The surface of the Millstone Grit here has been moulded into 

 elongated domes and hummocks (fig. 4), showing a bare bright surface 

 where they are not covered by peat or powdered rock. Wherever 



Fig. 4. — Bodies moutonnees, Givern-cefn-y-garreg ; Millstone Grit. 



its surface has escaped being crushed, it has been well striated, 

 grooved, and polished, the striae pointing direct to the two tabular 

 tops of the Beacons, distant 6 miles, giving a fall of 244 feet per 

 mile. A mile. S.W. of Gwern-cefn-y-garreg, on the right bank of 

 the M elite, 500 yards below Ystrad-fellte church, a recent slip of 

 Boulder-clay has exposed a well-glaciated surface of Carboniferous 

 Limestone. Where it has not been completely shattered, this rock 

 has taken a fine polish, is firmly striated, and slightly grooved. 

 This surface forms a steep slope in a bend of the river, which the 

 ice must have struck obliquely at an angle of 30°. All its projections 

 have been striated in a determinate direction, while the sheltered 

 ledges show striae running in various directions, often at right angles 

 to the true lines of glaciation. Tbe . author thinks that all these 

 striae were formed contemporaneously. The strike of the Carboni- 

 ferous-limestone beds here being nearly horizontal, and the bedding 

 even and definite, the stones, which the ice. was forcing obliquely 

 up the slope would be arrested at the points of junction between 

 the beds, and would have a tendency to slip along these lines of 

 weakness, and scoop out steps in the slope, as has actually been the 

 case. The direction of the grooves and coarse striae is less persistent 

 than that of the fine scratches, the former often describing a curve, 

 which increases with the steepness of the slope. These striae run 

 down to the brink of the river Mellte, and were evidently continued 

 below the present level of the valley- bottom. Their height above 

 the sea-level here is 740 feet, giving a fall from the top of the 

 Beacons, 7 miles distant, of 310 feet per mile. 



The furthest point west to which the author has traced these striae 



