Geology of northeast Wales: Cenozoic
Tertiary Period: 65-2.6
(million years ago)
During Tertiary times Wales - as part of the Eurasian Plate - continued
to drift northwards from the Tropics towards its present location. By
this time, Pangaea had well and truly broken up: rifting that began in
late Mesozoic times progressively separated Eurasia from America, with
the fledgling Atlantic Ocean in between them.
During the first part of the Tertiary, a very warm climate prevailed
and deep weathering and erosion of the older strata was ongoing. Rocks
of Tertiary age are very localised in northeast Wales, perhaps
unsurprisingly
since the regime was of removal of (as opposed to the accumulation of)
sediment: however, small hollows and depressions in the weathered
surface of the Carboniferous limestone in the Halkyn area were filled
during Miocene times (later in the Tertiary) with sands and clays in
what are termed 'pocket deposits'. By this time, the climate was
cooling quickly and the conditions under which the deep tropical
weathering could occur had come to an end.
Quaternary
Period: 2.5-0 Ma
(million years ago)
The
cooling continued erratically to the end of the Tertiary and then Earth
entered the series of strong climatic oscillations of the Quaternary,
during which conditions switched repeatedly from glacial to
interglacial.
Several glaciations affected the northeast Wales district, but there is
only
good evidence of the last or Devensian Ice Age, which ended
approximately 17,000 years ago. Two ice sheets converged on the
area; from the west, the Welsh Ice Sheet from the Cambrian Mountains
and Snowdonia and from the north, the Irish Sea Ice Sheet. The Welsh
Ice Sheet covered the Denbigh Moors, and the Clwydian Range to a depth
of about 1 kilometre. The Irish Sea Ice covered the coast of
North Wales and flowed partly down the Vale of Clwyd and the Cheshire
Plain. The main convergence-zone for the two ice sheets was in
the Wrexham area.
The ice-sheets deposited huge amounts of what is termed "drift": till
(rock fragments in a clayey matrix), outwash sands and gravels
(deposited by meltwater streams). Sub- and post-glacial erosion
accentuated the pre-existing valleys such as the Dee, Wheeler and
Alyn.
The present day topographical features of the Denbigh Moors, Clwydian
Range, Vale of Clwyd and Wrexham were produced by glacial processes,
where many typical glacial features such as cwms, drumlins, eskers,
kettle holes and hummocky topography may be found.
Geological processes are still operating today, in the Holocene epoch
(beginning 11,780 years ago) of the Tertiary Period. The
landscape is being eroded by wind, water and ice. The sand dunes
at Gronant and the north Wales coast are constantly moving under the
influence of wind, waves and tides. There are frequent
landslides, especially on the coast and in river valleys as well as
rivers and seasonal floods changing the landscape. All of these serve
to demonstrate that geological processes are dynamic forces that have
shaped the earth without rest for over 4,500 million years - and will
continue to do so as long as Planet Earth continues to exist.
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Previous:
Lower Palaeozoic
(Ordovician
and
Silurian)
Upper Palaeozoic
(Devonian,
Carboniferous and Permian)
Mesozoic
(Triassic, Jurassic
and
Cretaceous)
Click
on any section to learn more!
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Above: Geological timescale, with the Mesozoic Era highlighted.
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Above: Hummocky
glacial drift landscape, Nannerch, between Halkyn Mountain and the
Clwydian Range.
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Above: The
limestone pavement here was formed after ice scraped off the covering
sediments and soils to reveal the Carboniferous limestone. Over time,
rain, which is slightly acid, has dissolved the rock into the intricate
forms, known as clints and grykes, that we see today.
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