Carboniferous (362-296 million years ago)
The Carboniferous Period began when what is now Britain had drifted northwards close to the equator. Crustal extension formed "blocks" and "troughs" in the landscape such as Alston and Askrigg in the North Pennines. 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 Ninety 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, which can be 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.
Changing sea levels resulted in a variety of sediments such as limestones, mudstones, siltstones, sandstones and coal in a cyclical sequence known as a cyclothem. Such successions can be seen in Wensleydale and Wharfedale. It was during this time of upheaval that the Whin Sill, an igneous rock, was intruded. It is beautifully exposed near Middleton in Teesdale at High Force, and along parts of Hadrian's Wall, such as Housesteads and Steel Rigg. One of the dykes associated with the Whin Sill can be seen on Holy Island. It makes for strategic building sites for castles such as Bamburgh and Lindisfarne, as it is more resistant than the surrounding rock.
In the Upper Carboniferous (Namurian and Westphalian times) most of Northern England was covered by deltas. This swampy environment led to the repeated burial and preservation of plant material which was turned 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 effect of Carboniferous geology on past industries is very much in evidence. Limekilns for fertiliser can be seen, for example in Hudeshope Beck, Middleton in Teesdale. Black fossiliferous Frosterley "Marble" from Frosterley near Stanhope can be seen in Durham Cathedral. Evidence of past leadmining, can be seen in the Swaledale area in Arkengarthdale, Gunnerside Gill and Reeth. Today, Blue Circle in Weardale use limestone to make cement.
The Whin Sill intrusion at Howick Bay.