The cyclothemic nature of the Coal Measures has long been recognised and studied. At least 40 such cycles are known from the Pennine Coal Measures Group of County Durham with the main lithologies following one another in each cyclothem, in an ascending order of mudstone at the base (overlying the coal at the top of the underlying cycle), siltstone, sandstone, seatearth, coal . The nature and origin of the cyclothems have been much discussed, and it appears that no single explanation will suffice. Such cyclicity is a natural reflection of the interplay of sedimentary processes, and the only external mechanism needed to produce them is continuous subsidence. However, it is generally accepted that the periodic changes in sea level leading to marine flooding events are related to global glacial events, in this case the late Carboniferous glaciation of southern Gondwana which at that time lay over the South Pole. Some activity along the Stublick–Ninety Fathom fault system continued during the Westphalian but, in contrast to circumstances earlier in the Carboniferous, contemporaneous fault activity is not considered to have been a major influence on sedimentary patterns. Only local fault-controlled effects have been proposed and these include deposition of the Langsettian of Cumbria, and the stacking of channel sandstones in the Durham Coal Measures.


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