Geology and Mineral Resources
Drift Geology
The County's drift geology comprises all those sediments laid down over the last two million years, compromising material such as fluvial sand and gravel, glacial material including glacial sand and gravel, boulder clay, blown sand and beach deposits. In addition thick deposits of blanket peat cover the higher upland fells.Map showing the Drift Geology of County Durham
The following is a map showing the drift geology of County Durham. Please click on the thumbnail below to see an enlarged version. Please note that the image may take a short time to load due to the increased file size (29.7Kb).
Solid Geology
The County's solid geology comprises the underlying bedrock. County Durham's solid geology is widely recognised as one of the most interesting and diverse in the Country with rocks representing most of the major divisions of geological time. Whilst the overlying drift geology is relatively complex, the composition of the County's underlying geology is well known.Map showing the Solid Geology of County Durham
The following is a map showing the solid geology of County Durham, please click on the thumbnail below to see an enlarged version. Please note that the image may take a short time to load due to the increased file size (30.1Kb).
The solid geology of the County consists principally of a succession from west to east of Carboniferous and Permian rocks, with the Permian giving way to Triassic rocks in the south east of the County; all the strata dip gently towards the east. The Great Whin Sill, an igneous complex consisting of a number of sills and associated dykes, underlies much of the county but is best exposed in Upper Teesdale and to a lesser extent in Weardale. The Whin Sill has been extensively quarried for crushed road aggregate.
The rocks of Carboniferous age may be divided into three main geological groups - the Carboniferous Limestone series, the Millstone Grit, and the Coal Measures.
The Carboniferous Limestone series is represented by alternative layers of sandstone, shale and relatively thin limestone bands, which outcrop in the upper reaches of West Durham. The Carboniferous sequence also plays host to an ore field consisting of numerous mineral infilled fractures and although it is mainly developed in the Carboniferous Limestone and Millstone Grit to the west of the County, in places it extends to the coal measures in the east. The orefield consists of two distinct zones, with an inner zone centred on Weardale dominated by fluorspar, and an outer zone in which barytes is the dominant mineral. Lead ore occurs and has been exploited throughout the ore field, but the richest deposits occur within the outer fringes of the fluorspar zone. This ore field is unique in that it carries economic deposits of the rare mineral witherite.
Although generally eroded away in the upper reaches of west Durham, the Millstone Grit increasingly forms the fells and ridges between the main Dales to the east of a line between Blanchland and Middleton in Teesdale. The Millstone Grit series is represented by three groups of sandstone beds separated by shales, with a few thin coal seams.
The succession eastwards continues with the Millstone Grit being overlain by the Coal Measures, the division broadly corresponding with the easterly extent of the open moorland of west Durham. The Coal Measures extend to the east of the Wear Valley, where they are overlain by Permian rocks. Southwards the strata are thrown into strong undulations beyond the Butterknowle Fault and end in a sharp uprise beneath the Magnesian Limestone. These boundaries to the east and south mark the extent of the 'exposed' part of the Durham coalfield; the concealed part of the field extends beneath the Permian strata eastwards across the County and continues beneath the North Sea. The Coal Measures consist essentially of beds of sandstone, mudstone and shales with numerous coal seams of varying thickness. Ironstone and fireclay, or seat earth are sometimes found in conjunction with the coal seams. Over the lowlands of the Wear valley and its tributaries the Coal Measures are covered by glacial drift deposits which give rise, from time to time, to deposits of sand and gravel and brick clay.
The eastern edge of the Wear lowlands is marked by the outcrop of Permian rocks in the form of a bold escarpment running in a north - south direction between Pittington and Ferryhill and then turning south-westwards, with the outcrop gradually disappearing to the south of Shildon. At the base of the Permian can be found Basal Yellow Sands which are orientated east-west and outcrop north of Ferryhill. Above the Basal Yellow Sands lies a thin bed of marl slate, followed by deposits of Magnesian Limestone which consists of a variable mixture of the minerals dolomite and calcite. Where the mineral dolomite is the principal constituent and scarcely any free calcite is present, the deposit is referred to as dolomite rock. The occurrence of such deposits of dolomite is mainly confined to the lower beds of the Magnesian Limestone, the outcrop of which within the County is restricted to the lower slopes of the escarpment between Ferryhill and Pittington. Throughout the remainder of the Magnesian Limestone series the rock ranges through varying degrees of dolomitised limestone to pure limestone. Eastwards from the escarpment, the Magnesian Limestone is extensively covered by glacial drift deposits, which occasionally give rise to deposits of sand and gravel and brick clay.
Further Information
For further information on the either geology, mineral resources or minerals extraction in County Durham contact either the Planning Policy Team or the Planning Development Control Team:Planning Policy Team
Telephone 0191 3834114
Email: Planning Policy
Planning Development Control Team
Telephone: 0191 3835698
Email: Development Control
