Tropical Seas, Swamps and Deltas
Around 335 million years ago, warm tropical Carboniferous seas covered the area. They were full of life and as these creatures died, the calcium carbonate from their shells and the sea was deposited on the sea-bed to form the horizontal layers (beds) of Carboniferous limestone (the Danny Bridge Limestone Formation of the Great Scar Limestone Group). These beds are only exposed in the south of the area, in the centre of the Greenhow Anticline, where they are currently worked at Coldstones Quarry, and were formerly worked at Duck Street Quarry (now designated a geological Site of Special Scientific Interest).
The tropical paradise did not last. By 330 million years ago mud and sand was periodically being washed into the tropical sea from huge river deltas. Over millions of years changes in sea level produced repeated cycles (cyclothems) of alternating beds of limestones, mudstones, sandstones and coals, forming the Yoredale Group (Alston Formation). Exposures of the Yoredale Group are confined to the Greenhow Anticline and, further north, inliers where rivers and/or meltwater erosion or faulting have exposed the sequence.
As time progressed, the extent of the river delta systems became greater, and thick beds of sandstones and mudstones were laid down forming the Millstone Grit Group. Preserved now as the gritstone moors, these rocks dominate Upper Nidderdale. The youngest of these sandstone rocks is around 316 million years old.