The Story of the Rocks

Limestone is the oldest rock of the Carboniferous period and was formwed 350 million years ago.  Limestone is formed from the bodies of dead shellfish, corals, sea lilies, and other sea plants which built up gradually on the sea bed and fossilised to become limestone.  350 million years ago Derbyshire was covered by a shallow tropical sea.  The water was shallow and there was very little current.   As a result the limestone formed in this area is very light coloured with lots of fossils.

Around 325 to 300 million years ago there was a huge river flowing from what is now Scotland and Derbyshire formed part of a huge delta.  Sediments of mud, sand and pebbles were deposited in this delta.  Coarser sand and pebbles formed the rock now known as Millstone Grit which outcrops in a horseshoe shape around the Peak National Park.   Fine grained mud and sand formed a rock called Shale.   This rock is easily eroded and so forms valleys such as Hope Valley and Edale Valley.

Behind the delta were huge swamps where giant ferns and mosses grew in a tropical forest.  Debris from the dead plants was buried in the delta and over time pressure from the sediments above turned these into coal seams.

Around 300 million years ago the sea floor was lifted by earth movements.   A gentle anticline was formed in the Peak District and the land took on a dome-like structure (see diagram above).  The rocks of the coal measures were eroded and gradually worn away.  This exposed the gritstones which were also eroded.  In the southern part of thePeak National Park the shales have also eroded exposing the limestone underneath.