Geology of the Peak District National Park

The Story of the Rocks
Limestone

Gritstone and Shale

Coal Measures

Volcanic Rocks

Uplifting, Folding and Wearing Away

Minerals
Shaping the Landscape of Today

Points to Consider

Geology of the Peak District National Park

The Peak District National Park landscape is strongly influenced by the rocks underneath the soil.

The rocks, and the soils formed from them, partly determine which plants will grow on the land and, in turn, which insects, birds and animals.  The rocks also affect people - where they live, the types of houses they live in and the jobs they do.

Geology of the Peak District National Park

Most of the rocks that now form the surface of the Peak District National Park were laid down in the Carboniferous period of geological time.

The three main rocks are:-

1. Limestone - in the south and centre of the Park, forming the White Peak
2. Gritstone - forming a horseshoe shape around the Park, forming the Dark Peak and South West Peak
3. Shale - a softer rock, interlayered and beneath the gritstone which forms broad valleys across the Park.
   

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The Story of the Rocks

Limestone

Limestone is the oldest rock of the Carboniferous period 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.

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 fossilised 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	Detail of Limestone Barn
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.

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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 delta advanced slowly southwards into the Carboniferous Sea and their deposits were laid down in successive layers covering the limestone.  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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Coal Measures

In the swamps behind the delta, plants such as giant ferns and club mosses grew to form a tropical forest.   Over time, layers of debris from the dead plants built up and were buried.   The pressure of the sediment above, together with chemical changes, eventually turned the debris into coal seams.  The coal measures are only found at the edges of the Peak District National Park but beyond the boundary form major coalfields.

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Volcanic Rocks

There are two types of volcanic rock in the Peak District:

Intrusions formed when hot liquid rock was injected from deep within the earth into the limestone.  The liquid rock gradually cooled and so developed largish crystals forming a black rock called dolerite.    Dolerite can be found at Tideswell Dale quarry, weathering to a rusty colour due to the iron it contains.

Extrusions formed when hot liquid rock was injected from deep within the earth, through the limestone, onto the sea floor of the tropical lagoons.  These bursts of lava poured out through volcanoes and quickly 'froze' on the surface forming a fine crystalline, glass-like rock known as basalt.  Basalt is very dark greenish blue rock and can be seen in Millers dale.

The Derbyshire name for these volcanic rocks is 'toadstone'.  This may be due to the colour of the stone or a reference to the age of the rock 't'owd stone' - the old stone.

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Uplifting, Folding and Wearing Away

About 300 million years ago, the sea floor was lifted as a result of further movement within the earth.  A gentle anticline (or upfold) was formed across what is now the Peak District.  The land took on a dome-like shape.  The folding caused faults (or cracks) to appear in the rocks - particularly in the limestone.

The rocks of the coal measures were eroded and gradually worn away.  The gritstones were then exposed and also became eroded.  At the most exposed part of the dome, the gritstone was completely removed to reveal the limestone underneath.  This formed the southern part of the Peak District, known as the White Peak.  The exposed gritstone edges can still be seen around the core of the dome, for example at Stanage Edge.

Where layers of gritstone and shale alternate and the shale is exposed to erosion, landslips can occur.    The shale becomes softer and allows slippage between the harder layers of gritstone.  These unstable rocks still cause landslips today such as the ones at Mam Tor and Alport Castles.

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Minerals

From the Permian period onwards, the faults and cracks created in the limestone became filled with a hot liquid which crystallised to form minerals.  It is thought possible that volcanic action inside the earth caused the formation and heating of these liquids, or another idea is that they came somehow from the surrounding sea water.

The veins of minerals (usually called rakes in Derbyshire) all show a similar pattern of bands of different minerals.  Galena (lead ore) and/or sphalerite (zinc ore) are in the centre of the vein, with barite and/or fluorite on either side and calcite next to the limestone.

A Lead Rake

Lead which is obtained from galena, has been mined in Derbyshire since Roman times or even before.  It was used for water pipes, gutters and on roofs.  The lead mining industry reached its height in the 18th century when there were thousands of lead mines in the Peak District.  Lead mining stopped towards the end of the 19th century, although there are still many remains of the old lead mining industry.

In the 20th century it has been the gangue minerals, or waste material that was thrown away during lead mining, that have become important.

Fluorite

Galena and Barite

Calcite

Fluorite is used in the chemical industry, the iron and steel industry and other applications.  (One of these is the 'fluoride' in your toothpaste).  Most British fluorite is mined in the Peak District.

Barite is used in paint and paper manufacture, and to lubricate the drills on oil rigs.

Calcite is used in the chemical industry, for the white lines on roads and for ornamental finishes to buildings.

Ornamental Rocks

Blue John Stone

A special form of fluorite is mined at Castleton.  It is called Blue John which comes from the French words for the two main colours in the mineral - bleu (blue) and jaune (yellow).  Many shops in Castleton sell jewellery made from Blue John.

Impure limestone was the basis of the Black Marble industry at Ashford and Bakewell.  Fireplaces and tables made from this marble were inlaid with many different minerals and fossils.

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Shaping the Landscape of Today

In Permian times, fine sands blew across the dry landscape and were trapped in hollows in the limestone forming pockets of silica sand, the basis of a local firebrick industry at Friden.

Erosion over many millions of years continued to remove the layers of younger rocks deposited over the uplifted land.

It was the alternating warm and cold periods of at least four Ice Ages during the last two million years that finally gave the landscape the shape that we now see.  Much of the land was frozen during this time.   A layer of boulder clay was left in some areas (west of Bakewell).   This was formed from the rocks and debris trapped under the ice of the glaciers in the early Ice Ages.  Fast flowing rivers fed by melting ice in the warm periods formed and deepened the valleys.  As the rivers became deeper and wider the dales of the present landscape were carved out of the limestone.  Some dales were formed, as caverns in the limestone collapsed.  Only a few of the limestone dales now have streams because the water now flows underground.  Cracks in the limestone widened into caves, as the land froze and thawed.  In some places, limey water coming to the surface produce tufa, a honeycombed rock.

A layer of fine sands and silt - called Loess - was blown from the north on top of the limestone plateau in some places.

The final layers of the younger rocks were carried away by erosion, leaving exposed the rocks of the Carboniferous period:- limestone, gritstone and shale.

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Points to Consider

How do you think the geology of the Peak District has affected the landscape we see today?

For more information on any of the subjects highlighted in this factsheet try searching the archive.

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