The rocks of the Abercrave Area formed during the Palaeozoic Era It began 550 million years
ago with the Cambrian Period |
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Cambrian Period (550-510 million years ago) was the
start of the Palaeozoic Era, but rocks of this age are only preserved in
Pembrokeshire to the West and Snowdonia to the North and
include the slates of Snowdonia. During this period the first invertebrates appeared in the seas of the Earth. These were mainly tube worms and corals, animals with a plant-like appearance, which are generically known as Crinoids. They had calcareous endoskeletons consisting of individual plates, each one of which was a single calcite crystal. Crinoids flourished in the late Cambrian Period and still survive today, usually in waters over 200 metres in depth. They are known as Sea-lilies, Feather Stars, or Comatulids. |
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| Crinoids, plant-like animals | A modern Crinoid | |||||
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The Ordovician Period (510-440 million years ago) saw the deposition of sedimentation in various depths of water in and around the Abercrave area. These ranged from the shallow water limestones and sandstones to deep water shaly mudstones containing graptolites and trilobites - two types of creature now extinct. This was a particularly geologically active period and to the West and North widespread volcanic deposits were being produced and the sea bed was "mobile" and rising. In the Ordovician Period the first vertebrates appeared in the seas. | |||||
Late Ordovician Life |
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The Silurian Period (440-410 million years ago) deposited further shallow marine sediments ending with a period of deltaic sedimentation (coarse gritty sandstone and conglomerates). Further North there was a deep sea trough into which sediments slumped and marked the edge of what we now call the continental shelf. The Silurian Period saw the development of the first land plants and the continued evolution of eel-like, jawless, cartilage fishes, of which the present day Hag Fish and Lamprey are descendants. | |||||
Hagfish ( Silurian Period) |
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The Devonian Period (410-360 million years ago)
saw what is now South Wales beginning to emerge from beneath the sea and the
local rocks show evidence of ripple marks and mud cracks
dating from this time. These appear to have formed in
shallow fresh water lakes as they contain fossil fish
adapted for fresh water. The extensive sandstone deposits in the Abercrave area suggest that the newly formed dry land appeared as sand dunes and continental desert-like conditions prevailed. The first seed plants developed on land at this time, whilst in the seas, the first bony fishes appeared. |
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Tetra pods ( Devonian Period) |
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The Carboniferous Period (360-290 million years ago)
began with rising sea levels and
the dry land vanished again beneath a shallow sea which
deposited more marine limestones . These limestones
contain the evidence of a sub-tropical climate with
masses of coral preserved just where it grew. Carreg
Cennen Castle to the West of Abercraf, sits majestically atop a
crag of this coral deposit, with a substantial cave
leading from inside the castle down into the depths of
the hillside. The Western hill-side of the Swansea Valley at Abercraf, known as
"Cribarth", is also limestone, but is composed
largely of the skeletal remains of Crinoids, which by
the time of the Carboniferous Period were common and widespread. As the sea retreated again the coastal flats became transformed into vast river deltas in which gritty sandstones accumulated, with inter-bedded shales on these temporary flood plains. Finally these coastal plains were colonised by vast, dense , tropical forests . The earliest amphibians began to take to the land at the beginning of the carboniferous period, whilst the lush forests with their cycle of growth and decay produced large peat deposits later in the period. |
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| Sphenacodontid ( Late Carboniferous Period) |
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The Permian Period (290-248 million years ago)
produced
deposits rich in gas,
oil, and coal from the decay of the organic remains of the tropical
Carboniferous forests. The Earth's crust was in a state
of great upheaval and unrest at this time, and the
greatest extinctions of all time occurred, with up to 95%
of marine life being terminated. Continents were heaved
up above sea level, mountain ranges thrust upwards and
the land masses drifted together as one great continent
named Pangea. |
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Lycaenops |
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| Home Page | Coal & Iron |
Lime stone and coal |
Later Periods |
Plate |
The Ice Ages |
Present Day |
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We thank Mr Mike Allen MSc for his contributions and the inspiration which made this site a reality. | |||||
| The Geology of the Swansea Valley web-site is produced by Cefn-yr-Erw Field Study Centre and Primate Sanctuary, Caehopkin, Abercrave, Swansea SA9 1UD. South Wales UK. Tel 01639 730276. | When visiting the Swansea Valley stay at Cefn-yr-Erw. All rooms have en-suite bathrooms. For more information please click here. | |||||