The Rocks

The rocks in Harehope Quarry were formed in the Carboniferous Period of earth history, around 350 to 300 million years ago. The quarry has exposed layers of limestone, shale, sandstone and coal.

In Carboniferous times the land that became Britain lay astride the equator and was periodically covered by tropical seas, huge river deltas and rainforests.

The tropical seas of the Carboniferous Period were full of life such as corals, sponges, crinoids and brachiopods. When these creatures died their remains accumulated on the sea floor and eventually were compressed and cemented to become the limestone seen in the quarry today.

Large rivers drained into these seas from the north and washed in mud and sand forming vast deltas. The layers of sand and mud settled on the sea floor and were compressed and cemented into shale and sandstone. Layers of sandstone and shale can be seen above the limestone in the quarry. The deltas built up above sea level into vast swampy areas and the relics of these are seen in the coal seams of the area and a very narrow seam is just visible in the top layers of the quarry face.

The repetition of limestone, shale, sandstone and coal is seen across the North Pennines and is the result of changing sea levels. When sea levels were low the deltas were built up and lush swamp vegetation grew. When sea levels fell the forests were drowned and the tropical sea returned.

This pattern repeated itself many times and the resulting repeating layers of rock are known as 'cyclothems'. A complete cyclothem is exposed in the face of Harehope Quarry.

Harehope Quarry is best known, however, for its exposures of Frosterley Marble. Frosterley Marble is a fossil-rich limestone that is found near the top of the Great Limestone and is exposed and polished by the riverbed at the western end of the quarry.

The fossils in the limestone are predominantly a solitary coral, Dibunophyllum bipartitum. When polished the white fossils contrast with the black of the limestone. It has been widely used as a decorative stone in local churches and private houses and more recently in a number of local sculptures.

The pillars in the Chapel of the Nine Altars at Durham Cathedral are made from Frosterley Marble, thought to have come from Harehope Quarry.

Harehope Quarry also has a small cave system called Jacob's Pot. This has formed as a result of the limestone being slightly soluble in rainwater. When rain or river water seeps down cracks in the limestone it gradually dissolves the rock. The cracks become larger over time and steams can sink into the cracks and flow underground. This creates tunnels and caves.

Jacob's Pot is a small cave system that is now blocked in places as a result of quarrying.