Carboniferous times: 362 to 296 million years ago. Northern England on the equator: Tropical seas and forests.
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.
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.
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.
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.
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.
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 also 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 the Roman Wall, e.g. Housesteads and Steel Rigg. One of the dykes associated with the Whin Sill can be seen on Holy Island.
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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