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The Carboniferous 363 to 290 Ma
uk map 1 By the Lower Carboniferous the area to the
north of Devon and Cornwall was no longer a
continent of arid highland but was slowly
being invaded by the sea. Fault movements
associated with the advancing plate collisions
created troughs and highs. This produced
islands surrounded by tropical seas which
favoured the deposition of limestone. To the
south the advancing micro-plate of Normannia
continued to push up a mountain range
characterised by thrust faulting and nappe
structures. Ahead of this linear mountain chain
was a subsiding trough where deep marine
sediments were deposited.
Today, collectively, these rocks are called the Culm Measures. As the compression continued into the Middle Carboniferous and the trough deepened ahead of the advancing mountains, turbidite deposition reached a maximum in Devon and Cornwall, forming the Crackington formation. To the north, Scotland was mountainous land, over England swamps formed as the great invasion of the land by plants took place; coal-producing forests laid down carbon rich material in delta swamps. In northern England the famous Millstone Grit was laid down.
globe 3 By Late Carboniferous times the South West of
England was part of a mountain chain, thrust up from
the sea by immense pressures. Now the continents
were all joined to form the super continent Pangaea.
Earthquakes would have shock the ground, just as
India suffers today, as massive sheets or rafts of rock
re-adjust themselves one against the other, building
mountains
breccia as it was

Now,
Breccia of late carboniferous/permian
age resting on a buried landscape of Devonian Rocks, Torbay Devon.

Then,
Devon circa 280 million years ago?
Now land locked, these semi-arid mountains were eroded quickly to form the breccias and sandstones of South and East Devon. these rocks are known collectively as the New Red Sandstone Series. This erosion would continue from Late Carboniferous to Triassic times.
uk map 3 Volcanoes would also have been active, but
unlike the earlier volcanoes of the Devonian
and Early Carboniferous these were the
explosive type characteristic of mountain
chains. Like Mount St Helens, volatile release
of gases and magma would throw volcanic
bombs far and wide. These bombs can be
found in the early breccias around Torbay.