In a time 100 million years before the dinosaurs, there was only one continent – Pangaea – where huge rivers carried sediment from granite mountains, through tropical forests to a swampy delta emptying into a warm sea. This sediment, together with marine life and vegetation, built up in layers in the delta, and over millennia were buried deeper and deeper until pressure forged them into hard bands of sandstone, mudstone shale, and coal – collectively called Millstone Grit. Geologists have given each band of the Pennine Millstone Grit a specific name (Fig 1 ).
Eventually, Pangaea split-up, the land mass floated off colliding and breaking up until today’s continents emerged and the delta’s rock bands were pushed up to form the Pennine Hills and The Chevin. Now, under much of the Park is a 25m (82ft) band of Guiseley Grit showing ripple marks from the warm sea. Whereas, beneath Great Brow and along the bottom of the Park, is a band of mudstone shale formed when the sea flooded the delta leaving a layer of fossilised Goniatites – an ancient squid type creature with a coiled shell. (Fig 2 and Fig 3)
As time passed, wind and water eroded the layers of Millstone Grit at different speeds, creating stepped hills. Rain soaked through the Guiseley Grit layer and seeped out of the hillside when it hit the non-porous mudstone shale, creating springs. Then, between 30,000 – 13,000 years ago, a great ice sheet 250m (820ft) thick covered the whole of northern England to just south of Bradford. Ice from the mighty Wharfedale glacier spilled across the watershed to join the Airedale glacier carving out the Millstone Grit to form the Guiseley Gap. (Fig 4) When the climate warmed and the glaciers slowly retreated they deposited the debris carried along in the ice over the millstone grit – giving a layer of glacial till (Fig 3)- which is why the Park gets so slippery and muddy.
With thanks to West Yorkshire Geology Trust for their input.