is an appropriate term in Central Wales, as it describes the great
range of geological, geomorphological and associated features in the
region. We have a rich heritage in our rocks and landscape which tells
the fascinating history of the evolution of the region spanning more
than 700 million years of geological time. This evolution extends from
the late Precambrian, through the Palaeozoic, Mesozoic and Tertiary,
into the more recent glacial events of the last 2 million years and on
to the present day.
Left: fossil graptolite, 3cm in
length, of early Silurian age (about 440 million years old), from
Central Wales. Our fossils are just one part of the geodiversity of
of Central Wales is made up of Ordovician and Silurian marine
sedimentary rocks, deposited in the Lower Palaeozoic Welsh Basin when
Wales, as part of Avalonia, lay beneath a shallow sea on the SE side of
the great ocean of Iapetus. On its far side lay Scotland and Northern
Ireland, both part of the continent of Laurentia.
The rocks laid down in the Welsh Basin are dominated by sequences of
sandstone, siltstone and mudstone. Many of these sequences are referred
to as turbidites, because they were deposited from turbulent,
sediment-laden submarine currents, which flowed off the shallower shelf
areas onto the deep floor of the basin.
Volcanic activity took place locally, notably in the Welshpool and
Builth Wells areas. This activity produced basaltic lava flows and
dolerite intrusions, some of which have been quarried for building
stones and aggregates.
geologist Bob Mathews examining sandstone bedding-planes with current
markings, typical of those seen within turbidite sequences, at New Quay
on the Ceredigion coast.
Welsh Borderlands were situated on the margin of the Welsh Basin during
Lower Palaeozoic times. This part of the basin was a shallow, warm
shelf sea, teeming with life. Fossiliferous limestones, deposited in
this sea, are now exposed in several areas hereabouts and are today
quarried for aggregate and lime. They contain the remains of its
inhabitants, such as trilobites, brachiopods, bivalves and corals.
The deeper water turbidite-dominated sediments occurring farther to the
west are less obviously fossiliferous, but shale bands contain remains
of free-swimming and drifting organisms such as graptolites, which sank
to the sea-bed after they died.
Left: a view of part of Strinds
Quarry, near Kington in the Welsh Borderlands. Light-coloured Silurian
limestones here lie unconformably on darker Precambrian sandstones and
rocks of the Welsh Basin were deformed by the Late Caledonian (or
Acadian) earth-movements as Iapetus closed and Avalonia collided with
Laurentia, some 410-400 million years ago. One result was the folding
of the Lower Palaeozoic rocks; another was the development of a strong
cleavage in mudstones, which locally provided the raw material for the
Regional scale folds in Central Wales include the Berwyn and Plynlimon
domes, whilst trains of small folds are commonly seen in road-cuts and
coastal sections. These structures record the deformation that
accompanied the shallowing and final silting-up of the Welsh Basin in
Above: folded sedimentary rocks, of
Silurian age, exposed at Hendre Quarry in Ceredigion. The folding
records an episode of continental collision that brought an end to
marine conditions in Central Wales during Lower Devonian times.
Brecon Beacons, the Silurian-Devonian Old Red Sandstone records the
environmental change from marine to terrestrial conditions as the Welsh
Basin silted up. The environment was arid but it also featured braided
streams flowing into lakes in which primitive fish lived. Sedimentary
rocks of this age, exposed in southern Powys, contain fossils both of
the fish and also examples of some of the earliest known land plants.
The distinctive red soils of the area indicate that the underlying Old
Red Sandstone was deposited in desert environments in which
iron-bearing minerals were readily weathered to reddish oxides such as
Left: Duncan Hawley, of Swansea
University, with a slab of Old Red Sandstone showing a zigzag trail
made by a primitive fish on the bed of an ancient lake.
rocks cap the Black Mountains and also occur in the extreme NE of our
region, where they are succeeded by Permo-Triassic desert sandstones. A
thick sequence of Jurassic marine sedimentary rocks occurs beneath the
seabed of Cardigan Bay, where a sedimentary basin existed during
Mesozoic and Tertiary times. The existence of this basin was confirmed
by drilling in the late 1960s.
Onshore, however, there is a stratigraphic gap in Central Wales which
spans much of the Mesozoic and Tertiary.
Island and the Lleyn Peninsula, seen across Cardigan Bay from the
Ceredigion Coast. A large part of the Bay is floored by Mesozoic and
Central Wales Orefield, situated in N Ceredigion and NW Powys, was
worked for lead, zinc, copper and silver from Bronze Age times until
the mid 20th Century. The mineral veins were formed during several
phases of hydrothermal activity from Devonian to Permian times.
Today, the most important mines are being conserved by several
organisations. The contribution of mining to our cultural legacy is
being publicised and promoted through the Ceredigion Uplands Mining
Heritage Project, `Spirit of the Miners'.
Left: Cwmystwyth Mine in eastern
Ceredigion has a long history extending back to the Bronze Age, over
3500 years ago, when copper mining took place. The mine saw many phases
of working, with lead, silver and zinc being extracted during its most
productive phase in the 19th Century.
mineral veins of Central Wales contain a lot more than ores of copper,
lead, silver and zinc. There are compounds of cobalt, nickel, antimony,
arsenic, cadmium, barium and even traces of gold. These include some
internationally rare mineral species among the more familiar ones.
Weathering of the metal ores has led to the formation of many brightly
coloured secondary minerals, for which the mines have long been visited
by amateur and professional mineralogists researching the processes
that led to their formation.
Above: orange crystals of the
secondary lead molybdate mineral, wulfenite - just one of many
interesting minerals found at minesites in Central Wales.
legacy of the Central Wales mining industry is the pollution resulting
from acidic, metalliferous minewaters. The pollution occurs because
sulphide minerals tend to be unstable in the presence of air and
moisture. The oxidation process creates sulphuric acid, which then
digests other minerals, resulting in the mobilisation of a range of
Tackling the various aspects of pollution at mines is a difficult job
which is largely undertaken by the Environment Agency.
Left: the entrance to No. 9 Adit at Cwmrheidol mine in 1992. Ochre
(yellow iron oxide) and highly acidic minewater are present, both
having formed by the weathering of marcasite, an unstable iron sulphide
which is abundant at this site.
landscape is thus the product of many processes acting over many
millions of years. One of the most significant events to affect the
landscape was a series of ice ages. Approximately 2.4 million years
ago, the Earth's climate began to fluctuate between cold intervals
(glaciations), some of which lasted up to 100,000 years, and
interglacial periods during which temperatures were similar to or even
warmer than those of today.
During the glaciations, large ice-caps formed on the Welsh mountains
and glaciers occupied the valleys. The glaciers carved deeply into the
rocks to give the landscape its now familiar appearance.
Above: winter on Plynlimon - a
landscape created by glacial ice which left the area about 14,000 years
the region there are `humps and bumps' in the landscape,
geomorphological features that record erosion and deposition by
glaciers during the Quaternary Period, such as the moraine ridges and
cwms of the Brecon Beacons. Cryogenic structures, such as pingos formed
by ice pushing up through soil and melting to leave depressions, are
found near Tregaron and elsewhere. These glacial and other periglacial
(cold-climate) features are all part of our Geodiversity, as are the
traces of Quaternary rivers, lakes and the soils which formed in that
period and have evolved through to the present day.
Left: Quaternary geomorphology
specialist Rick Shakesby, of Swansea University, on fieldwork in the
Brecon Beacons, studying morainic ridges which can be seen in the
we don't stop there: many of our rocks have been found to be useful for
building castles, mansions and churches in Central Wales, so
Geodiversity merges with archaeology and cultural activity.
The identification of the source of the stone from which an important
building has been constructed is a factor to be considered when
undertaking restoration projects. One of the roles of the Welsh Stone
Forum is to undertake such work throughout Wales.
Above: Tim Palmer and Richard Cave
examining a rock sample at Powis Castle, near Welshpool. The rock face
beside them consists of the Powis Castle Conglomerate, of Silurian age,
from which the castle above was constructed.
to be overlooked is the human face of Geodiversity in Central Wales.
Two of our `old girls' are Gertrude Elles and Ethel Wood who, more than
a century ago, used graptolite fossils to unravel the time sequence of
Lower Palaeozoic rocks in Wales and to establish principles now applied
globally. Another is Nancy Kirk, recently departed, who brought
graptolites to life with intricate and elegant models and her use of
simple and inelegant language! Among our `old boys' are Murchison who
put the Silurian on the map nearly two hundred years ago, and O.T.
Jones who first made sense of the Welsh Basin (`geosyncline') and
placed the Central Wales Orefield on record in many publications and on
the basis of enormous amounts of fieldwork over 40 years in the first
half of the 20th Century.
Left: a geological
field party on Hanter Hill, where there are exposures of Precambrian
igneous rocks dating back to over 700 million years old - the oldest
rocks in Wales!