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.
Carboniferous
Limestone capped with Millstone Grit
Disused
Millstone from the Millstone Grit
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.
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.
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.
