Natural England - Derbyshire (including City of Derby)

Derbyshire (including City of Derby)

The geology and landscape of Derbyshire is dominated by Carboniferous rocks that form the central terrain of the Peak District.

Carboniferous limestone is the main rock type in the White Peak area. The dry stone walls, which characterise the area, are constructed from the limestone

These occupy a large, broad structural dome with the Carboniferous Limestone of the White Peak at its heart, so that younger rocks progressively outcrop to both the east and west and to a lesser degree north and south.

The core of the area comprises the massive Carboniferous Limestone, and which gives rise to the limestone plateau of the White Peak. This area demonstrates landforms typical of the weathering and solution of limestone such as sinkholes and steep-sided valleys. The Carboniferous Limestone gives way to the coarse sandstones and shales of the Millstone Grit, which form the upland area of the Dark Peak and the scarp and valley scenery along the eastern edge of the Peak.

The more industrialised and settled landscape of the eastern side of the County is located on the Upper Carboniferous Coal Measures. Although covered by much younger Quaternary glacial deposits, the underlying Carboniferous coals, clays and ironstones have been heavily exploited to form the basis of the once thriving coal industry. The Coal Measures are overlain by the marine limestones of the Permian Magnesian Limestone in the north-east of the County. There is another small outcrop of the Millstone Grit and the Coal Measures in the Swandlincote area which forms part of the Leicestershire-South Derbyshire Coalfield.

South Derbyshire is dominated by red sandstones and mudstones of Triassic age, which give rise to the lower, undulating ground to the south of the White Peak. The soft mudstones floor the Trent Valley, although there are no natural exposures. Large amounts of sand, gravel and till (or boulder clay) occur in the Trent Valley and the Erewash Valley and were deposited by Ice Sheets and meltwaters over the past 500,000 years.

Carboniferous

The oldest part of the Carboniferous (354-290 million years old) succession comprises the Carboniferous Limestone, which forms the solid geology of the so-called White Peak area and effectively comprises a broad area of outcrop between Castleton in the north and Matlock in the south. The Carboniferous Limestone was deposited in a sub-tropical sea. Two distinct environments during the Lower Carboniferous led to the formation of differing types of sediment being accumulated, which is reflected in the Carboniferous Limestone rock succession.

The main limestone type of the White Peak is a shallow 'shelf' limestone which comprises pale grey and thickly bedded limestones which give rise to the elevated plateau south of Castleton. In the south-west of the outcrop area, around the Manifold Valley, outcrops of the deeper 'basin' limestone occur, which is darker grey in colour and occurs in thinner, strongly folded beds. Around the margins of the shelf, reefs developed, forming a reef limestone. This type of limestone which is rich in fossils, particularly brachiopods is largely devoid of bedding, the hard fine-grained 'reef limestone' resists weathering and produces conical hills, known as 'reef knolls', for example at Thorpe Cloud and Wetton Hill.

During the Lower Carboniferous volcanic activity on the sea-floor produced lava flows or later intrusions into the rock sequence. This lava cooled to form layers of basalt, which are apparent by dark colour amongst the lighter limestones where they area exposed. These igneous rocks are known locally as 'toadstone'.

Towards the end of the Carboniferous, a period of uplift began, which saw the formation of thick deposits of more continental rocks. Around the fringes of the Carboniferous Limestone shelf, the change is marked by the incoming of marine shales, with thin limestones and sandstones (the Pendleside Series) which contain distinctive fossil brachiopods, corals and trilobites. These rocks give way to the more massive sandstones and shales of the Upper Carboniferous Millstone Grit Series (approximately 300 million years old). These outcrop all around the Carboniferous Limestone outcrop and form the distinctive upland 'gritstone' moors of the Dark Peak to the north and east and the higher undulating ground north-east of Derby. The hard "grit" sandstones of the Millstone Grit often form a series of harder edges or escarpments such as Stanage Edge and Froggat Edge along the eastern fringe of the Peak District. The Millstone Grit was deposited in a coastal environment where large river deltas were building out into the shallow marine waters.

Continuing deposition over the millennia led to the further building out of the deltas and the formation of an extensive low-lying, swampy land area in which the succeeding Coal Measures were deposited. The periodic flooding and building of the deltas along the coastline resulted in the deposition of a series of layers of coals (representing the compressed remains of the luxuriant swamp vegetation) interspersed with layers of shale, clay, sandstone and mudstone. These Upper Carboniferous Coal Measures outcrop in two areas in the County. In the east, they form the southern end of the Yorkshire-East Midlands Coalfield, outcropping in a broad band from south of Sheffield to Ilkeston in the south. South of Derby, the Coal Measures outcrop again in the Swandlincote area where they form the northern, exposed part of the Leicestershire and South Derbyshire Coalfield.

Permian and Triassic

The succeeding Permian (290-248 million years ago) Period is represented in Derbyshire by the Magnesian Limestone, which is so named because the rock contains the magnesium rich mineral 'dolomite'. This limestone was deposited in a relatively shallow landlocked sea extending from northeast England to Poland, known as the Zechstein Sea. The estimated average temperature during the period was approximately 23°C. In Derbyshire, the main Magnesian Limestone outcrop occurs from Alfreton, via Bolsover, to the north-east margin of the County, forming a relatively distinct ridge along its length of outcrop.

Rocks of Triassic (248-205 million years old) age occupy the southern end of the County, from a line between Ashbourne and Derby southwards, although exposure is limited due to a thick covering of Quaternary till (or boulder clay). During the Triassic, dry arid, desert-like conditions predominated and were responsible for oxidation of iron compounds within the rocks, leading to the characteristic red colouration of the deposits are now represented by the red sandstones, breccias and mudstones that occur in South Derbyshire.

Earth movements at the end of the Carboniferous had resulted in uplift of the land and the formation of mountainous areas in the Midlands and other parts of Britain. Erosion of these over millions of years produced the thick sequences of sediments over the plains and basins adjacent to the mountains. East of Ashbourne, overlying the Millstone Grit, the Triassic sequence is represented by the Sherwood Sandstone Group which comprises red sandstones with horizons of pebbles deposited by desert flash floods. To the south the Triassic succession of the overlying Mercia Mudstone Group, which comprises red and green mudstones, sandstones and thin horizons of gypsum and rock salt, probably represents wind-blown dust that settled in shallow salt-lakes and sun-baked mudflats on the extensive flood plain. The arid conditions under which these rocks were deposited are indicated by the occurrence of numerous layers of salt, which formed through the evaporation of mineral-rich water under the intense desert sun.

Tertiary (comprising Neogene and Palaeogene)

The Carboniferous Limestone outcrop in north-west Derbyshire between Longnor and Wirksworth contains hollows, filled with silica sand, clay and pebble bands, known as ‘pocket deposits’. These deposits contain fossil plants indicating that they are of Late Tertiary (Miocene (approximately 20 million years old)) age and represent the remnants of continental, river deposits that once covered the area, but which have subsequently been removed through erosion.

Quaternary

Over the last two million years the climate of Britain has varied tremendously with periods of temperate climate interrupted by repeated advances and retreats of glaciers and ice sheets. Collectively these periods have become known as the Ice Age (we are still in one of the temperate phases) and the actions of the ice sheets have been instrumental in forming the landscape we see today.

Various deposits of Quaternary age occur throughout Derbyshire. Till (or boulder clay), which formed in and beneath glaciers and ice-sheets occurs in the main valleys of the Trent and the Erewash and in some locations on the limestone plateau (e.g. east of Bakewell) and the Peak District generally. During the glacial phases, it is likely that local glaciers developed over the Peak, although ice only probably covered the area during the Anglian and Wolstonian glacials, approximately 450,000 and 300,000 years ago. Ice did not reach Derbyshire during the last glaciation, the Devensian, but the area would have been subject to an arctic, tundra-like climate. During this period, the natural caves in the Carboniferous Limestone would have provided shelter for animals including bears and hyenas.

The erosive action of earlier glaciers would have deepened the river valleys and rounded the rocky summits of the Dark Peak. Seasonal meltwaters and meltwater following glaciation would have would have assisted in carving out the limestone caverns of the White Peak and deep gorges such as Winnats Pass near Castleton. Deposits from one of the temperate periods (the Ipswichian, approximately 120,000 years old) have been recorded at Boulton Moor. Here organic material, channelled into gravels has yielded fossil mammals including hippopotamus.

During the final phase of the Devensian glacial, cold icy winds deposited large amounts of fine silt, or loess, across the limestone plateau of the White Peak. This forms a rich, stoneless loam, up to 1m thick, which in places supports pasture in more upland areas (e.g. around Chelmorton, south-east of Buxton).

Geological Highlights:

  • Mam Tor, near Castleton, is well known for the large landslide (500 m long x 250 m wide) that is evident on its eastern face and which has closed the road since 1979. The geology comprises Lower Carboniferous limestone overlain by the Upper Carboniferous Edale Shales and Mam Tor Beds. A number of factors have contributed to the development of the landslide, including the structure of the rock sequence, the chemical composition of the shales and the geological history of the area. One major factor is that during the last glacial period the rapid erosion of valleys and action of ice in the area caused significant ground disturbance which may have initially triggered landsliding. The main slip surface is in the Edale Shales and the slide, which has been active for at least 3,600 years, is still inherently unstable. The main trigger for movement is when groundwater levels rise after heavy rainfall, lubricating the slip surface. Present day measurements show that there is an average movement rate of the whole mass of about 20 cm/year and with the central region moving faster at almost 1 m/year. Studies suggest that the landslide will remain unstable for a few more thousand years.

  • The Lower Carboniferous Limestone of the Castleton area is famed for the mineral Blue John that occurs here and nowhere else in Britain. Chemically, Blue John is a variety of fluorspar (calcium fluoride) and occurs in veins or in nodules that line the walls of cavities in the surrounding Carboniferous Limestone. It was first used by the Romans, 2000 years ago when the natural caverns in the Castleton area were opened up to mine lead and other ores. There are a number of references to the stone in Roman literature and ornaments made from it were highly valued.

  • The millstone grit of the Dark Peak area weathers to produce a coarse, gravelly soil which, because of its free-draining nature and the high rainfall of the region leads to the development of acidic, nutrient-poor soils. More usually however, the gritstone is overlain by blanket peat which reaches its greatest depth on the plateaux. Blanket peat stretches the length and breadth of the Dark Peak with natural breaks only on the steep slopes below the Kinder Scout plateau. It has formed since the last glaciation and peat development has been more substantial here than elsewhere in Britain, probably because deforestation occurred here earlier than elsewhere.

Local sites

The following localities represent, in part, the geology of this county. Each locality has a grid reference, a brief description of how to get there and a short summary of the geology you are likely to find. All the localities listed are openly accessible.