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The Carboniferous Period
began when what is now Britain
had drifted northwards close to the equator. During Lower Carboniferous (Dinantian) times post-Caledonian crustal extension broke
up the eroded roots of the Caledonian mountains into a series of more
upstanding "blocks" and subsiding "troughs" or basins.
This north-south tensional stretching is believed to be the result of a subduction event to the south of Britain which
led eventually to the Variscan Orogeny.
The blocks and troughs are due to variations in density of the underlying
crust.
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Carboniferous
Limestone capped with Millstone Grit
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. The photo below is a
view of the North Craven fault as seen from Crummackdale.
The fault line is along the horizon, centre and top right.
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The Carboniferous
"blocks" and "troughs" still influence the landscape of Northern England today. Furthermore, it is their
general dip or tilt to the east which has resulted in the high ground of the Pennines.The North Pennines consist of two blocks known
as the Alston (to the north) and Askrigg (to
the south) Blocks.
They are
separated by the east-west trending Stainmore Trough
along which the present day A66 runs. The
southern margin of the Askrigg Block is defined by
the Craven Faults, and its western margin by the
Dent fault. The Northern margin of the Alston Block is defined by the Stublick-Ninety Fathom Fault and its western margin by
the Pennine Fault.
Landscape features formed as a result of these faults can be seen from
Selset reservoir (Lunedale
Fault), on the A66 from Brough to Penrith (Pennine Fault), and Giggleswick
Scar near Settle (South Craven Fault). The 90 Fathom Fault can be seen at Cullercoats near Tynemouth,
but it does not form a landscape feature
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During Lower Carboniferous
times warm shelf seas encroached on the old land surface. The result in many
areas was an unconformity of horizontal marine limestones
overlying folded and eroded Lower Palaeozoic rocks, e.g. as seen at Thornton
Force and White Scar cave, both near Ingleton, as
well as Arcow quarry near Austwick.
Lower Carboniferous Limestone of the Great Scar Limestone Group is well
displayed around Malham, Horton in Ribblesdale and Ingleton.
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Photo of Arcow Quarry
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Millstone fashioned from the Millstone Grit
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However, deposition of
marine sediments did not occur evenly over Northern
England. Furthermore, changing sea-levels due to subsidence
and/or worldwide sea-level changes, as well as advancing and receding deltas
resulted in a variety of sediments such as limestones,
mudstones, siltstones, sandstones and coal in a cyclical sequence known as a cyclothem. These sequences are best displayed in Wensleydale, e.g. Penhill near Leyburn and Buckden in Wharfedale where the hillsides are characterised by a
series of stepped features due to limestones and
sandstones which are more resistant to erosion being interbedded
with less resistant mudstones and siltstones. They are known as Yoredale cyclothems, Yoredale (derived from the River Ure)
being the old name for Wensleydale.
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During Upper Carboniferous
(Namurian) times sea-level continued to vary but
tended to be lower resulting in sediments from northern deltas spreading in a
southerly direction. The advancing deltas produced an interbedded
sequence of shales, coarse sandstones and
occasional limestones. The latter, however are not
as predominant as in the Lower Carboniferous. The coarse sandstones are known as Millstone Grit since millstones used to be
made from them. Because many of the rocks are poor in nutrients, they give a
distinctive bleak landscape, e.g. the moorlands of the South Pennines, such
as Saddleworth Moor along which the M62 passes, and
in the North Pennines, along the A66 over Stainmore.
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The Whin
Sill forming the top of High Force Waterfall
WS = Whin Sill, S= metamorphosed sandstone, TBL=
Tyne Bottom Limestone.
Photo: courtesy, Derek Teasdale.
Annotation: from Northumbrian Rocks & Landscape,
Yorks. Geol. Soc.
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Eventually, during Westphalian times, most of Northern
England became a vast swampy deltaic area of river channels, lakes
and coastal mudflats. It was this environment which led to a repeated burial
and preservation of plant material which were to turn into the seams of the
Coal Measures. One of the best exposures of the Coal Measures is on the coast
between Whitley
Bay and Seaton Sluice.
The area is in fact an SSSI (Site of Special Scientific Interest). A fossil
tree from the Coal Measures can be seen in the parish churchyard at Stanhope,
Weardale.
It was during this time of upheaval that the Whin
Sill, an igneous rock, was intruded, and which is beautifully exposed near
Middleton in Teesdale, also at High Force, and
along parts of Hadrian's Roman Wall, e.g. Housesteads
and Steel Rigg. One of the dykes associated with
the Whin Sill can be
seen on Holy Island.
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The effect of Carboniferous
geology on past industries is very much in evidence. Limekilns for fertiliser
can be seen e.g. in Hudeshope Beck, Middleton in Teesdale and building stone, e.g. black fossiliferous Frosterley
"Marble" from Frosterley near Stanhope as
seen in Durham Cathedral. Evidence of past leadmining,
e.g. hushes and mine entrances, can be seen in the Swaledale
area in Arkengarthdale, Gunnerside Gill and Reeth.
Today, Blue Circle
in Weardale use limestone to make cement.
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Looking west at part of Hadrian's
Roman Wall built on the north facing cliff formed by the Whin
Sill which is dipping to the south.
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Figure to the
left shows position of the continents during Early Carboniferous times. From
Cocks, L.R.M. & Torsvik, T.H. 2006.
European geography in a global context from the Vendian
to the end of the Palaeozoic. In Gee, D. G. & Stephenson, R. A.
(eds). European Lithosphere Dynamics.
Geological Society, London,
Memoirs, 32, 83–95.
The British
Isles can be seen just south of the equator.
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