Geology of northeast Wales: Lower
Palaeozoic
Ordovician Period: 488 - 443
Ma (million years ago)
The oldest rocks in the district are of middle to late Ordovician
(Llandeilo, Caradoc and Ashgill) age. The outcrop comprises:
* Strata in the Glyn Ceiriog and the Teirw valleys forming part of the
northern limb of the Berwyn Anticline (a fold in the form of an arch);
* The southern limb of the Llangollen Syncline (a fold in the form of
an upside-down arch);
* Two small inliers (areas of older rocks surrounded by younger rocks)
on the northern limb of the Llangollen Syncline;
* A small area in the Conwy Valley south of Llanrwst.
These Ordovician strata consist of
mudstones and siltstones, with occasional volcanic tuffs (a rock formed
from volcanic ash) and dolerite sills (dark grey igneous rock injected
as a magma into the surrounding rocks). The mudstones and siltstones
contain fossils such as trilobites and graptolites (tiny, floating
colonial animals that evolved rapidly and are hence used in
biostratigraphy to date Lower Palaeozoic sedimentary rocks) and they
were deposited in a deep marine basin. The tuffs were deposited
from clouds of ash that were sent airborne during explosive volcanic
eruptions that occurred in the adjacent Snowdonia volcanic province.
Silurian
Period: 443 - 416 Ma (million years ago)
Silurian strata crop out extensively in two prominent areas:
* The Denbigh Moors
* The Clwydian Range
The outcrops consist of mudstones and siltstones that were deposited in
a deep marine environment within a small sedimentary basin called the
Denbigh Trough, which formed part of the larger Welsh Basin. These
rocks also contain graptolites.
The Ordovician and Silurian strata were folded, squeezed and faulted by
the Acadian Orogeny (mountain building event) during early Devonian
times. The incredible pressures involved in this process in turn
created an important natural resource in parts of northeast Wales -
slate - a metamorphic rock
whose formation is a complex process.
The first step in this process involved thick layers of mud
accumulating at times on the Ordovician and Silurian sea-floor. The
were progressively buried in time to become beds of mudstone. Mudstones
contain abundant clay minerals - which belong to the mineral family
known as sheet-silicates. Mica is another sheet-silicate and is
well-known for being easily split into thin sheets. Sheet-silicates
split easily due to their chemical structure, which consists of atoms
of aluminium, potassium, silicon, oxygen and hydrogen arranged in
layers. Such minerals, crystals of which may easily be split into many
thin layers, are said to have a perfect cleavage.
When those layers of mud accumulated on the Ordovician and Silurian
seabed, the microscopic grains of clay minerals were arranged randomly.
However, under the great squeezing pressure that prevailed during the
Acadian Orogeny, the clay minerals all recrystallised in one direction,
at right angles to the direction from which the pressure was coming. As
a result, the mudstone now contained countless sheet-silicate crystals
(with their perfect cleavage) all aranged in one orientation. The
result was a rock that could itself be split into thin sheets - in
other words, slate.
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Next:
Upper Palaeozoic (Devonian,
Carboniferous and Permian)
Mesozoic (Triassic, Jurassic and
Cretaceous)
Cenozoic (Tertiary and Quaternary)
Click on any section to learn more!
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Above: Geological timescale, with the Lower Palaezoic Era highlighted.
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Above: A sample of
Middle Ordovician dolerite, a crystalline igneous intrusive rock, from
Hendre Quarry, Glyn Ceiriog, near Llangollen.
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Above: A
fossilised graptolite of Lower Silurian age, about 3x actual size.
Graptolites occur in Ordovician and Silurian mudstones
across northeast Wales.
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