Geology of northeast Wales: Upper
Palaeozoic
Devonian Period: 416 - 359
Ma (million years ago)
Devonian strata do not crop out at surface anywhere in north east
Wales. However, outcrops elsewhere in Wales consist of reddish
sedimentary rocks deposited in river basins and lakes in a frequently
arid environment, giving us an idea of what Wales, including this
district, would have been like back then. Indeed, recent research has
suggested that a considerable thickness of Devonian strata was present
over much of Wales, but in most places it has since been eroded away.
Carboniferous
Period: 359 - 299 Ma (million years ago)
During the Carboniferous Period, Wales lay in the equatorial region,
with a similar latitude to the Amazon river basin of the present day.
The oldest Carboniferous rocks, the red 'Basement Beds' now called the
Ffernant Formation, were deposited in a fluvial environment and these
sediments are unconformable (an interruption in sediment deposition,
which can last millions of years) with the underlying Silurian rocks.
The Basement Beds are only present in a thin strip along the western
side of the Vale of Clwyd, near Colwyn Bay in the north and Eglwyseg in
the south.
Following deposition of the terrestrial Basement Beds, the overlying
Carboniferous limestones were deposited in a tropical, shallow-marine
environment. Corals, both colonial and solitary, brachiopods,
crinoids, goniatites (similar to an ammonite) and bivalves testify to
the abundant sea life. Occasional palaeosols (fossil soils) reflect
periods of emergence and subaerial exposure. Tectonic activity,
mainly faulting, at the same time gave rise to topographic highs and
lows, which accounts for the localised thickness variations of certain
beds across the district.
Over time the sea became shallower and more quartz-rich sand was
present in the system as a river-delta environment grew out into the
sea. The Pentre Chert was deposited at this time in the north of
the district: this unusual, bedded, marine deposit was thought to have
been the formed from the spicules (needles) of sponges and tiny
radiolarian tests (silica shells of tiny organisms). There were
periodic marine incursions onto the river deltas, which flooded the
land and left behind telltale marine fossils, mainly goniatites, which
are found in layers known as Marine Bands.
By Upper Carboniferous times, the marine influence waned and delta-top
environments predominated, with many cyclic sequences of mudstone,
siltstone, sandstone, seat-earth (a coal swamp fossil soil) and coal
that together make up the Coal Measures. The youngest Carboniferous
rocks consist of unfossiliferous red mudstones and sandstones that were
deposited on alluvial plain and lake environments, and marking a
transition towards the more arid conditions of the Permian.
Permian
Period: 299 - 251 Ma
(million years ago)
During
the late Carboniferous and early Permian, there was significant
faulting and uplift followed by erosion associated with the Variscan
Orogeny. The orogeny came about when continental collision brought
together the major continents on Earth to form a single supercontinent
called Pangaea, in which Wales found itself fairly close to the centre.
Still close to the equatorial belt, but now cut off from the sea,
desert conditions prevailed throughout the Permian and succeeding
Triassic periods.
*******************************************************************************************
Previous:
Lower Palaeozoic
(Ordovician and
Silurian)
Next:
Mesozoic (Triassic, Jurassic and
Cretaceous)
Cenozoic (Tertiary and Quaternary)
Click on any section to learn more!
|
|
Above: Geological timescale, with the Upper Palaeozoic Era highlighted.
|
Above: A cut and
polished sample of Lower Carboniferous limestone with fossilised
corals, from Cefn Mawr Quarry, near Mold.
|
Above: Fossilised
plants in Upper Carboniferous mudstone, from Brymbo, near Wrexham.
|
|
|
