File:The Quarterly journal of the Geological Society of London (13204474205).jpg

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.