The Peak District National Park - Fact Zone 10. Geology in the National Park

	The Park
	Tourism
	Land Use
	Conservation
	Design
	Farming
	Castleton
	Dovedale
	Upper Derwent
	Geology
	Minerals
	Nature
	Burbage Valley
	Erosion
	Bakewell
	Population
	Langsett
	The Rangers
	The Pennine Way
	The Goyt Valley
	Longdendale

THE STORY OF THE ROCKS

Limestone

Limestone is the oldest rock of the carboniferous and was formed around 350 million years ago. At this time, Derbyshire was covered by a shallow tropical sea. What is now Britain was then quite close to the equator. The bodies of dead shellfish, corals, sea lilies (crinoids) and other sea plants gradually built up on the sea floor and fossilised over time to become limestone.

Palaeography of the Peak District in Lower Carboniferous times

In the lagoons the water was shallow with little current. The limestone formed in the lagoons is almost pure calcium carbonate, very light coloured, with crinoid and brachiopod fossils (see below).

Reef limestone in Dovedale

Reefs were mostly formed from corals and they became pure white limestone. Reefs were mostly found in a fringe or barrier around the lagoon. A good example of a barrier reef is the hill behind Castleton on which Peveril Castle was built. There is reef limestone in Middleton Dale and in the Dove Valley, where Thorpe Cloud and Parkhouse Hill stand as isolated reefs. Crinoids grew on the edges of the reefs, so reef limestone is usually surrounded by limestone containing many crinoid fossils. The fossilised stems look like nuts and bolts in the rock and are known locally as Derbyshire screwstone.


Screwstone	Limestone Cottages, Sheldon
Limestone has a great variety of uses. It is used as building stone; as aggregate (crushed stone) for roads or concrete; to improve agricultural land; to make cement (with shale) and (as burnt lime) in the chemical industry. The manufacture of a great number of materials uses limestone at one stage or another.
	Detail of Limestone Barn

Gritstones and Shales

Around 325 to 300 million years ago, Derbyshire formed part of the huge delta of a river flowing down from what is now the highlands of Scotland. Sediments of mud, sand and pebbles were deposited in the delta.

Gritstone edge

Fine grained mud and sand lying under the water formed a rock called shale. It is easily removed by erosion and so forms valleys such as the Hope Valley and the Edale Valley.
Coarser sand and pebbles rose as sandbanks, which eventually became the sandstone known as Millstone Grit; so-called because millstones for grinding grain were made from it.

Over the years the deltas advanced slowly southwards into the Carboniferous Sea and their deposits were laid down in successive layers. The alternating layers of shale and sandstone seen in the face of Mam Tor are a result of sands cascading down the front of the delta (turbidites). Later layers of gritstone, such as the Kinder Grit (forming Kinder Plateau) and the Chatsworth Grit (found around Baslow and the Chatsworth Estate) were thicker and tougher.

The millstone grit outcrops in a horseshoe shape around the Peak District National Park and the sharp edges (scarps) of the outcrop can be seen around Curbar and Calver in the east, Edale in the north and the Roaches and Ramshaw Rocks in the west. Marine fossils can be found in the shale and sandstone layers, showing that the area was close to the edge of the sea.

Fossils found in the Peak District





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