The geology of the neighbourhoods of Flint, Mold, and Ruthin ...

UPPER SILUHIAN BOCKS.
CHAPTER II.
UPPER SILURIAN ROOKS.
Wenlock" Shale.
The whole of the Moel Fammau raDge is composed of rocks
belonging to the Wenlock Shale, and consisting principally of
roughly cleaved shales with fine-grained sandstones occasionally
interstratified. The direction of the cleavage is almost constant
through the whole length of the range; in Moel-y-parc it
averages E. 35° S., bending on the south and east sides of Pen-ycloddfau
to about E. 30° S. and E. 25 S. Near Pen-machno it
ranges E; 15'— 20° S., and on Moel Fammau E. 10°—16° S.
(except locally on the north side of Moel-y-Gaer, where it points
in two or three places E. 10°— 16° N., perhaps owing to faults).
South of Foel Fenlli it resumes the direction E. 20° S., or, south
of Moel Gyw, of E. 20°—35° S. The cleavage therefore crosses
the range obliquely.
Eegarding the district as a whole, we might consider the strike
of the rocks to be nearly parallel to the direction of the range,
and to be crossed obliquely by the cleavage; but locally the
strike and cleavage are generally parallel, for the strata have been
bent into a number of small synclinal and anticlinal folds, the
axes of which run strictly parallel to the cleavage, This coincidence
of direction leads to the supposition that the folds and
cleavage were contemporaneously formed, as a result of compression
by forces acting from the north-north-east, and southsouth-west
and, as we have elsewhere shown, at a time when the
Wenlock Shale was deeply buried under newer Silurian Rocks
(Ludlow Beds, &c.)* At the same time, upheaval, due to the same
force?, led to the denudation of the newer Silurian strata and to
the preparation of the floor of old rocks, over which the Carboni-
ferous group was subsequently spread. At a comparatively late
date the district was traversed by faults and gentler folds, by
which the Carboniferous rocks, and the Trias also in part,' were
affected, and by which such striking features in the configuration
of the country as the Vale of Clwyd and the Moel Fammau
range were produced. The approximate parallelism of the trend
of this range to the
accidental.
strike of the strata composing it is therefore
The description of the Wenlock Shale will be conveniently
taken from north to south. On the north side of the Wheeler
valley flags are exposed in the deep strike-ravine of Coed-cochion,
* The Geology of the Coasts adjoining Rhyl, &c. (Mem. Geol. Survey. Explanation
of Quarter-sheet 79 N.W.), 1885. See also Denudations of North Wales, Proc.
Chester Soc. Nat. Set. No. 3, p. 38, 188S.