of the Elan, a tributary of the Wye, for the supply of water to
Birmingham. The group of heights of _South Wales_, running on the
whole from west to east, marks the outcrops of the Old Red Sandstone
and Carboniferous strata which lie within a vast syncline of the
Silurian rocks. The Brecon Beacons of Old Red Sandstone are the
highest (2907 ft.), but the Black Mountain bears a number of
picturesque summits carved out of Millstone Grit and Carboniferous
Limestone, which rise frequently over 2000 ft. Throughout Hereford,
and in part of Monmouthshire, the Old Red Sandstone sinks to a great
undulating plain, traversed by the exquisite windings of the Wye, and
forming some of the richest pasture and fruit lands of England. This
plain formed an easy passage from south to north, and since the time
of the Romans was a strategical line of the greatest importance, a
fact which has left its traces on the present distribution of towns.
Around the western and northern edge of the Old Red Sandstone plain
the underlying Silurian rocks (and even the Cambrian and Archaean in
places) have been bent up so that their edges form hills of singular
abruptness and beauty. Of these are the Malvern Hills, east of
Hereford, and in particular the hills of Shropshire. Wenlock Edge,
running from south-west to north-east, is an escarpment of Silurian
limestone, while the broad upland of Long Mynd, nearly parallel to it
on the north, is a mass of Archaean rock. The Wrekin, the Caradoc and
Cardington Hills are isolated outbursts of pre-Cambrian volcanic
rocks. The outer rim of the Welsh area contains a broken series of
coal-fields, where patches of Carboniferous strata come to the surface
on the edge of the New Red Sandstone plain. Such are the coal-fields
of Flint in the north, the Forest of Wyre and the Forest of Dean,
close to the Severn, on the east. The great coal-field on the south is
a perfect example of a synclinal basin, the Millstone Grit and
Carboniferous Limestone which underlie the Coal Measures appearing all
round the margin. This coal-field occupies practically the whole of
Glamorgan and part of Monmouth, and its surface slopes from the Black
Mountain and Brecon Beacons to the sea as a gently inclined plateau,
scored by deep valleys draining south. Each chief valley has a railway
connecting a string of mining villages, and converging seaward to the
busy ports of Newport,