The hill is largely formed from a mass of
gabbroic-dolerite which is exploited for
roadstone at the large Criggion Quarry excavated into its western and northern sides. This rock was
intruded into the
mudstones of the surrounding Stone House Shales Formation in the form of a
laccolith during the
Ordovician period. An alternative interpretation is that the intrusion takes the form of a
sill. A separate intrusion of
andesitic magma forms Moel y Golfa whilst Middletown Hill and the lower summits to its northeast are formed from
tuffs and
volcaniclastic material which form the
Caradoc age Bulthy Formation. The hills are largely devoid of
superficial deposits though the low ground between the summits is mantled with glacial
till. The quarry was already operating prior to the construction of a railway link in 1866, a line which continued to serve it until 1959. A key product in earlier years were road
setts but crushed roadstone was the main output by 1912. It was operated as a single 200m high face before being benched in 1967. Output peaked at 350,000 tonnes a year in 1973. The presence of the minerals
chlorite and
epidote give the quarried rock its characteristic green colour. Material quarried in the upper part of the workings drops via a steep chute bored through the hill to a tunnel leading to the processing works below. ==See also==