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

•* INTRODUCTION.
Carboniferous Rocks and New Red Sandstone; on their east lie
the gentler hills of the Coal-Measures of the Flintshire Coal-field
while the eastern side of the map is occupied by a portion of the
New Red Sandstone of the Cheshire plains, and by the intervening
estuary of the Dee.
The area is drained by the Clwyd, the Dee, and its tributary
the Alyn. The Clwyd, rising in the Silurian hills of central
Denbighshire, enters the Vale of Clwyd by a steep-sided breach in
the Carboniferous Limestone, near Llanfair, and flows thence, in
a channel excavated almost wholly in Drift, to the sea near Rhyl.
The Alyn, rising m and draining the- Vale of Llanarmon,«which
separates the Silurian from the Carboniferous Limestone hills,
flows north as far as Cilcain, but breaks abruptly through the
limestone-escarpment near thjfcplaggj; and. turns to the south-east
across the Coal-Measures by Mold and Hope. On emerging
from the deep gorge by which it traverses the limestone, the Alyn
falls into an old line of valley, which is prolonged right through
the Moel Fammau rangei into thV.Vafe of Clwyd, by way of
Nannercb, Caerwys, and BodfarJL This old valley it follows
southwards, receiving merely a small tributary from the northern
extension named above, whichr. is. drained by, the River Chwiler
(or Wheeler), a tributary of the Clwyd.
The watershed between the Alyn and the Clwyd follows the
summit-level of the Silurian hills as far as Moel Arthur. Thence
it turns north-eastwards to Moel Plas Yw, and follows the road by
Bryn-y-groes to the south side of Penbedw Park, where it crosses
the valley above mentioned. Its height above the sea in this
valley is 52,0 feet, but its exact position is so ill-defined that it
seems to be little more than chance whether the stream descending
from the south side of Moel Arthur turns to the north into
the Chwiler, or to the east into the Alyu. The water enters a
subterranean course at Siamber Wen, but reissues at Tardd-ydwr,
at 600 yards distance, and flows thence to the Alyn; but
the open-air course, which is still used in heavy floods, runs north
of this line and enters the Clwyd system. . During the flood of
August 1879, both channels were used, so that the water of the
stream reached the sea partly by way of the Dee, partly by way
of the Clwyd. From this point the watershed runs northeastwards
to Moel Ffagnallt and the chert-hills above Pentre
H*lkin.
The north-eastern corner of the map is occupied by the Triassie
hills of part of the peninsula of Wirral, of which about three
square miles, around Great Sutton and Capenhurst, drain into
the basin of the Mersey. The small streams flowing over the
western part of the peninsula make their way into the estuary of
the Dee, which opens out into a trumpet-shaped area of considerable
size in the map to the north. The contents and history of
this estuary are described in Chapter XII.
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