One of the special features of the local limestone is the presence of fossils that tell us about life in the Carboniferous seas. Not all the beds are fossil-rich but in places fine examples of fossil corals, sponges, brachiopods (a type of shellfish), gastropods, molluscs, crinoids, bryozoans and algae can be found. You are most likely to spot corals and brachiopods. Look out for the curved shapes of brachiopod shells in cross-section and intricate patterns made up of many small circles or tubes which are colonial corals.
Limestone in a small quarry on the lower slopes of Knott, showing a close-up of a bed packed with fossils, including brachiopod shells and colonial corals. © M. Byron
Fossils may be scattered through the rock or concentrated in layers which would once have been flat-lying reefs and shell beds in a shallow sea. Some fossils are in ‘life position’, whereas others were tumbled around by the sea after they died.
Large colonial coral in its original life position on Ash Fell Edge. © M. Byron
Limestone bed packed with brachiopod shells in a small old working near Sunbiggin Tarn. © M. Byron
Many of the fossils have become impregnated with silica (SiO2, which is the same composition as quartz). This has made them insoluble and harder than the surrounding limestone. They stand proud of the rock and may be darker than the rest of the rock, making them particularly obvious in exposures and stone walls.
Fossil corals and sponges in Potts Valley. The silicified fossils are dark and stand proud of the limestone. © E. Pickett
Colonial coral standing out in a stone wall in Potts Valley. Local stone walls are an excellent place to spot fossils. © E. Pickett
A colonial coral in a stone wall near Smardale Bridge. © M. Byron
Some of the limestones contain layers and nodules of a light or dark grey rock called chert, which is also made of silica. It may derive from the remains of silica-secreting creatures such as sponges, although exactly how it formed is unknown. Good examples of chert can be seen in old limestone quarries at Knott.
A lens-shaped nodule of grey chert in limestone in a small quarry at Knott near Orton. © E. Pickett