Nick o’ Pendle
Nine members and friends assembled in the quarry at the Nick o’ Pendle to examine the Pendle Grit; the oldest of the Millstone Grits and the only one which is a turbidite. It is a large submarine channel and fan system.
Initially moving to slightly higher ground gave a better view across the Craven basin into which the sand was deposited. A large river bringing down material eroded from a mountain chain to the north approached the basin from the direction of what is now the Askrigg Block. An earth tremor or earthquake would be enough to tip deposits of sand off the palaeo-coastline down the slope into the basin picking up debrites as it descended the slope. The basin, caused by extensional tectonics, was heavily faulted to the north hence the channel and fan system. The material was sorted as it flowed so that grain size of the sandstone is larger close to the source than it is downslope. The outcome was that it formed thick amalgamated beds.
Unlike deltaic deposits of Millstone grit there is no cross bedding, no pebbles, and fossil remains are rare.
Our group then moved down into the small quarries at the Nick to look at features that identify the rock as a turbidite . In the quarries at the Nick we saw a loaded base bed, an example of a bulbous loading structure, plus casts of mud or shale clasts, a megaflute showing which direction the current was moving in, and ripple forms which also shows this.
We then made our way across to Wiswell Moor Quarries for some lunch and a chance to examine similar features on a rather larger scale.
Wiswell Moor Quarries
At Wiswell Moor quarries (there are three) undulose bedding planes are exposed as are scours, some exposed in cross-section, and flutes. Also concretions which in the Pendle Grit are formed by iron carbonate. Finally some coalified plant fragments were identified.
Carboniferous Geology- Bowland Fells to Pendle Hill, Ed by Paul Kabrna. Ch. 4 Pendle Hill – a turbulant past, by Ian Kane, and Excursion 5, Pendle Hill and Clitheroe, page 157.
Sedimentology and Architecture of the Pendle Grit, by Ian Kane.
Images and text, J. Hutchinson