Rocks and minerals of the Peak District National Park, England.
The underlying geology gives the area many of its special qualities and is one of the reasons why the Peak District was designated as the UK's first National Park in 1951. The rocks and minerals of the Peak District are also an important resource, for example limestone is used in the chemical industry, for agricultural purposes, and in the construction sector for cement and aggregate.
The Peak District National Park has eight distinctive landscape character areas:
Dark Peak; Dark Peak Western Fringe; Dark Peak Yorkshire Fringe; South West Peak; Eastern Moors; Derbyshire Peak Fringe; Derwent Valley; White Peak
Where did the rocks come from?
Formed in the early part of the Carboniferous Period (between 360 and 326 million years ago), limestone is the Peak District's oldest exposed rock. At this time the land that is now the Peak District lay just south of the equator and was covered by a shallow tropical sea. The sea was warm and full of nutrients, providing an ideal habitat for many marine species. Corals grew to form reefs encircling clear tropical lagoons in which prehistoric sea plants and creatures lived and died. Many sea creatures, such as sea lilies (crinoids), brachiopods and bivalves, had a hard outer skeleton (exoskeleton) of calcium carbonate. When these creatures died their bodies would sink to the bottom of the sea floor and the soft inner parts would rot away leaving the exoskeletons. Over a period of around 30 million years, the calcium carbonate turned into limestone.
Reefs made from corals 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 stands. There is reef limestone in Middleton Dale and in the Dove Valley where Thorpe Cloud and Parkhouse Hill stand as isolated reefs. Crinoids tended to grow on the edges of the reefs, so reef limestone is usually surrounded by limestone containing many crinoid fossils. Looking like nuts and bolts in the rock, the fossilised stems are known as Derbyshire Screwstone.
Limestone has many uses--as building stone and aggregate (crushed stone) for roads or concrete; to make cement (with shale); and, as burnt lime or pure calcium carbonate, in the chemical industry. Most current quarrying and mineral extraction operations are carried out under old planning consents, which were given before the area became a National Park.
The national park's gritstones and shales were laid down in the middle Carboniferous Period (around 326-316 million years ago) when the northern part of the Peak District was covered by a huge river delta flowing down from what is now the Scottish Highlands and Northern England. The river carried sediments of mud, sand and pebbles which were deposited on the bed and at the front of the delta as it flowed into the shallow sea.
Fine grained mud and sand sediments formed shale and siltstone. Coarser sand and pebbles eventually became gritstone, also known as Millstone Grit because millstones were made from it. As the delta advanced slowly southwards, deposits of mud, sand and pebbles were laid down in successive layers on top of the limestone. The alternating layers of shale and sandstone seen in the face of Mam Tor overlooking Castleton are a result of sands cascading down the front slope of the delta beneath the surface of the sea (turbidites). Later layers of gritstone, such as the Kinderscout Grit which forms the Kinder plateau and the Chatsworth Grit (found around Baslow and the Chatsworth estate) were coarser.
The gritstone outcrops now form a horseshoe shape around the northern fringe of the Peak District National Park, and their sharp edges (scarps) can be seen around Curbar and Calver in the east, Edale in the north, and the Roaches and Ramshaw Rocks in the west. Non-marine fossils are rarely seen in the gritstone rock, but marine fossils can be found in the shale layers of the turbidites, showing that the area was once close to or beneath the sea.
In the swamps behind the delta, plants such as giant ferns and mosses grew to form a tropical forest. Over time, layers of debris from the dead plants built up and were buried. Pressure and chemical changes eventually turned the debris into seams of coal covering the whole of what is now the Peak District and the land on either side. Around 300 million years ago, movement from deep underground caused the area to bulge upwards, and subsequent erosion wore away the coal measures to reveal the underlying gritstone and limestone. The coal measures on the eastern and western fringes of the Peak District were relatively thin compared with those in the surrounding lowlands. They were largely worked out by the end of the 19th century.
Geology and people
The geological resources of the Peak District have provided its inhabitants with a livelihood from the very earliest times.
Gritstone was quarried in the Iron Age to make hand-powered stones to grind grain ('querns'). Later, gritstone was quarried to make millstones for use in water, wind and steam mills, crushing stones for mineral extraction and as grindstones for the Sheffield edge tool industry. There are more than 1,000 discarded millstones scattered in parts of the Peak District National Park, and the millstone is the Park's symbol.
Gritstone is used as a building material because it can be easily shaped. Chatsworth House is built from gritstone, as are many other buildings in the Peak District. Gritstone was often used to make lintels around doors and windows, gateposts and water troughs.
Limestone is also an important construction material. It was quarried by the Romans, who used it for building stones and mortar. Since the 17th century, the Peak District has been a major producer of lime and limestone, employing thousands of people. Dark limestone, containing fine-grained organic material and other impurities, was the basis of the black marble industry at Ashford and Bakewell from the late 17th until the early 20th century. When polished, the stone turns a deep glossy black.
Bronze Age farmers mined copper, one of the main constituents of bronze, sometime between 2,000 and 1,500 BCE, as well as small quantities of lead for ornaments and ritual objects. The Romans mined lead on a much larger scale, with Roman lead ingots ('pigs') from the Peak District found as far away as Normandy. Several local lead mines were mentioned in the Domesday Book of 1086. Lead was used for water pipes, gutters and on roofs, and later, for lead shot, leaded windows and in paint.
In the 20th century it was the waste material (gangue minerals) from lead mining, notably fluorspar, barite and calcite that became important. Today, the Peak District's spectacular landscapes attract millions of visitors every year. Thousands visit the show caves at Castleton and the limestone crags and gritstone edges draw rock climbers from around the world. Many sites are Regionally Important Geological and Geomorphological Sites (RIGS), and some are also designated as Sites of Special Scientific Interest (SSSIs) for their geological value.
Compiled by Henricus Peters Editor
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|Title Annotation:||Earth Sciences|
|Date:||Mar 22, 2014|
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