South Yorkshire (Barnsley, Sheffield, Rotherham and Doncaster Districts)
The South Yorkshire area is underlain by rocks of Carboniferous age which are tilted gently to the south-east so that the oldest part of the succession occurs in the west. The moors to the west of Sheffield are formed in the shales and hard coarse-grained sandstone beds of the Millstone Grit.
The moorlands of the Dark Peak are dominated by Millstone Grit, formed during the Carboniferous.
These rocks give way to the shale, mudstone and sandstones of the overlying Coal Measures which underlie Sheffield and Rotherham. The Coal Measures are in turn overlain by the Permian Magnesian Limestone which occurs as a narrow north-south ridge from north-east of Worksop to just east of Doncaster. To the west, the low-lying moors at Hatfield and Thorne and the catchment of the River Don to the north of Doncaster are founded on the Triassic Mercia Mudstones.
The youngest geological material is that left by the melting of ice over 10,000 years ago during the closing stages of the last Ice Age of the Quaternary. This material, called till (or boulder clay), is still soft and consists of sticky clay containing boulders and pebbles.
Carboniferous
Carboniferous rocks (354-290 million years old) comprise much of the solid geology of South Yorkshire and belong to two main series; the Millstone Grit and the overlying Coal Measures. The hard ‘grit’ sandstones and intervening shales of the Millstone Grit outcrop west of Sheffield and Barnsley and form the north-western corner of the Dark Peak moorland. These rocks were deposited as sediments in a coastal environment where large river deltas were building out into the shallow, tropical marine waters that covered much of Britain at this time. Continuing deposition over the millennia led to the further building out of the deltas and the formation of an extensive low-lying, swampy area in which the succeeding Coal Measures were deposited.
The Millstone Grit is characterised by rolling moorland tops being punctuated by rocky tors and vertical cliff-like faces of 'gritstones', known as edges. These can be up to 20 metres high, one of the more prominent, being the 19km edge that runs between Derwent and Stanage.
In Upper Carboniferous times, the periodic flooding and building of the swamp deltas along the coastline resulted in the deposition of a series of coal layers (the compressed remains of the luxuriant swamp vegetation) interspersed with layers of shale, clay, sandstone and mudstone. These sediments now form the Coal Measures, which overlie the Millstone Grit and form the solid geology underlying the major conurbations of Sheffield and Rotherham. Fossils within the various layers of the Coal Measures include mainly plants, but also marine shells and animals that lived in brackish water conditions indicating that there were repeated advances and retreats of the shallow sea over the deltas throughout the Upper Carboniferous.
The Lower and Middle Coal Measures contain the once economically important coal seams, together with fireclay and ironstone. The overlying Upper Coal Measures are mainly barren of workable coal seams and are largely composed of red marls, mudstones, siltstones and sandstones.
Permian
To the east, the Coal Measures are overlain by rocks of Permian age. At the base of the sequence are sandstones known as the ‘Yellow Sands’ which represent the remnants of Saharan-like desert sand dunes. The Yellow sands are overlain by cream to buff coloured limestones know as the Magnesian Limestone which forms a west-facing escarpment. The Magnesian Limestone is tougher than the gently folded Coal Measures rocks it rests on, and so erodes more slowly resulting in the formation of an elevated ridge which forms a distinct barrier between the industrial coalfields to the west and the low-lying Humberhead Levels to the east. The Magnesian Limestone was deposited in a relatively shallow landlocked sea extending from north east England to Poland, known as the Zechstein Sea. The estimated average temperature during the period was approximately 23°C. Due to the landlocked nature of the sea and high temperatures, gypsum and anhydrite (collectively known as evaporites) were produced as the sea water gradually evaporated and today form extensive deposits beneath the floor of the North Sea. Within the Magnesian Limestone the remains of fossil reefs (often including numerous fossil algae, bivalves, gastropods and bryozoans) which grew on the edge of the Zechstein Sea can be found. The Magnesian Limestone has been extensively quarried, particularly along the valley of the River Don where it cuts through the Limestone ridge to the west of Doncaster.
The unique chemistry of the Magnesian Limestone (the limestone is high in Magnesium carbonate, when compared with other limestones) gives rise to soil conditions which support an interesting combination of plants, with grassland species typical of the Chalk of southern England occurring along with rare plants, such as thistle broomrape, which is not found away from the Magnesian limestone outcrop.
Triassic
Rocks of Triassic age (248-205 million years old) in South Yorkshire are represented by mudstones and sandstones that underlie the area to the north-east of Doncaster. The Lower Triassic sandstones of the Sherwood Sandstone Group outcrop from Doncaster eastwards to Thorne and can be seen in the cutting at Doncaster railway station. During the Lower Triassic all the continents had moved together to form the supercontinent of Pangaea (‘all Earth’) and in the area occupied by Britain, arid, desert conditions predominated. The red and buff sands of the Sherwood Sandstone were deposited on a large desert plain across which large braided rivers intermittently coursed.
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.
There is evidence in the South Yorkshire area for two main glacial phases (the Anglian and Devensian) with an intervening temperate (Ipswichian) phase. During the last glaciation, ice did not reach as far south as Sheffield, but a large lobe of ice pushed down the Vale of York, along the line of the Trent, as far as Doncaster and deposited large amounts of till, or boulder clay and sands and gravels. Ice flowing down the east coast blocked the Humber Estuary, leading to the creation of a large glacial lake, known as Lake Humber. The sediments formed in Lake Humber consist largely of laminated clays with marginal sands and these now underlie the low-lying land of Thorne Moors which originally formed part of the lake bed. It is thought that the lake eventually disappeared through silting up rather than by drainage.
Geological Highlights:
- The geology of the South Yorkshire area has been paramount in the industrial development of the area. Coal was mined in quantity in South Yorkshire from the medieval period onwards and was one of the major sources of power behind the global industrial revolution of the 18th and 19th centuries, and the early decades of the 20th centuries. Coal was mined extensively around Barnsley, Rotherham and Doncaster. The industry steadily declined during the second half of the 20th century and there are now only a few deep mines in production in the Doncaster and Maltby area.
- Sheffield has been famous for the making of steel since at least the fourteenth century when one of Chaucer's pilgrims is described as carrying a “Sheffield Thwitel in his hose” (a thwitel is old English for knife). The proximity of iron ore, from the Pennines, streams and rivers for power and suitable grinding stones from the Upper Carboniferous Millstone Grit made Sheffield an ideal centre for steel making. In the sixteenth century Sheffield began to increasingly specialise in making cutlery with the arrival of Flemish immigrants and the following century in 1624 a Company of Cutlers was established. Steel making improvements continued in Victorian times particularly with the development of the Bessemer (high temperature) process of making steel in the 1850s. The next major event in the history of steel making was the making of Stainless Steel which was pioneered at Sheffield in 1903 - although it was developed in Germany and the USA at around the same time.
- Hatfield Moors, 10 km east of Doncaster, is a remnant of a once extensive lowland raised bog which once occupied the Humberhead levels. For the most part it lies close to sea level and occupies part of the former area of the glacial Lake Humber. Lowland peat bogs are a nationally rare habitat with none occurring in lowland England in completely unmodified form, most having been cut for peat. Hatfield Moor is the second largest remaining raised bog in lowland England and supports wetland and heathland habitats that are important for breeding bird and insect populations.
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.
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