Geology Reference
In-Depth Information
Overlying the Cambrian the 'Ordovician' period is followed
by the 'Silurian', both named after ancient Welsh tribes. The
Ordovices were the inhabitants of North Wales and Anglesey
who resisted the advancing Roman army until they were almost
wiped out, and the Silures were the inhabitants of the Welsh
borderlands. These rocks, widely exhibited in these parts of
Wales, are renowned for the trilobites they contained - enor-
mous woodlice-like creatures who scavenged the sea bottom
and frequently shed their skins, leaving us to match body with
head, millions of years later. Trilobites were a fantastically suc-
cessful group, spreading in their di┬Čerent forms all round the
world.
Above the Silurian lie Old Red Sandstone rocks, which, as the
name describes, are represented by an enormous thickness of
red sands and muds deposited in arid, desert-like conditions.
The seas that lapped against this desert were first investigated
in detail where they occur in Devon, southern England, and so
this geological age is called the 'Devonian'. At the end of the
Devonian the sea invaded the desert from the south, gradually
spreading northwards over what is now Britain. Lush vegetation
proliferated and in river deltas where land met sea huge man-
grove-like swamps developed. Trees shed their leaves and finally
died there, the debris building up year upon year, eventually
forming the thick coal seams we see today in the 'Carboniferous'
period.
At the end of the Carboniferous desert conditions formed
again and prevailed throughout the 'Permian' and 'Triassic', this
time depositing the 'New' Red Sandstone, typified by huge
fossilised sand dunes. However, sand dunes are almost devoid
of fossils, so are very di~cult to position accurately in the
geological column. At the end of the Permian the most massive
of all mass extinctions occurred, with almost 95 per cent of all
species disappearing in about 2 million years. It was the end for
the trilobites, and most marine life was severely decimated. On
land proto-mammals, amphibians and reptiles disappeared
 
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