The Sorby Natural History Society, SheffieldCovering a full range of natural history interests in Sheffield and the surrounding area, providing for both interested beginners and specialists.
Sorby Geological Group
The Sorby Natural History Society recording area is a classic geological area for the contrasting geology and scenery. Distinctive landscapes and scenery are formed by each major rock type from the Peak District in the west to Sherwood Forest and Humberhead Levels in the east. The main rock types are the early Carboniferous limestones of the White Peak, the late Carboniferous Millstone Grit and Coal Measures of the Dark Peak and adjacent areas, the Permian Magnesian Limestone, Triassic Sherwood Sandstone and Holocene sediments of the Humberhead Levels.
The Carboniferous limestones form the distinctive area of the White Peak, where much of the land is underlain by fossiliferous marine shelf and reef limestones deposited in the warm sea of the early Carboniferous. Small areas are underlain by contemporaneous basic igneous rocks, together with small pocket deposits of late Tertiary sediments. Mineralisation has altered the limestone to dolomite in some areas, while many faulted zones, locally known as rakes, are mineralised by calcite, fluorite, barytes and galena. This upland area scenery includes limestone gorges, swallow holes, caves and dry valleys. The soils developed in this area are calcareous, apart from the areas above sandy pocket deposits.
The Millstone Grit forms the moorland areas of the Dark Peak and southern Pennine margins. This distinctive area is underlain by the mudstones, shales, flagstones and sandstones of the Millstone Grit and lower Coal Measures, deposited by a major delta complex during the late Carboniferous. The dark mudstones of the Edale Shales pass upwards into alternating mudstones, sandstones and coarse sandstones known locally as gritstones. The thicker gritstones and sandstones form prominent edges and tors in the High Peak, including those around Kinder Scout, Stanage Edge and Derwent Edge. The higher parts of the Coal Measures contain more mudstones and thinner sandstones, together with coals, and form a ridge and valley scenery of relatively low altitude. Acid soils are developed in most areas, particularly above the upland sandstones.
The Magnesian Limestone, mostly dolomitic limestones deposited during the late Permian, forms a conspicuous low escarpment east of the Coal Measures outcrop. An number of gorges are cut into the limestone, including that at Creswell Crags. Calcareous soils are developed over most of this area.
The Sherwood Sandstone includes red sandstones and red mudstones deposited in a terrestrial environment during the Triassic. The poor acid-prone soils developed over the sandstones are partly covered by woodland, including parts of Sherwood Forest.
The silts and peats of the Humberhead Levels, deposited during the Holocene, underly the lowlying wetland areas of Thorne and Hatfield Moors to the northeast of Doncaster.
The Peak District is unusual for an area in northern England, as it was not covered by an ice sheet during the last, Devensian, glacial period. As a consequence, many periglacial sediments are preserved, including chert gravel in a few limestone areas and solifluxion deposits on most hillsides on areas underlain by the the Millstone Grit and Coal Measures. Landslips are a prominent feature of hillsides formed from late Carboniferous mudstones, including the large active landslip at Mam Tor. The sediments, including tufa, deposited in caves and near springs of the White Peak and Magnesian Limestone during the Pleistocene glacials and interglacials to the present day provide a record of the changing environment during this time. Today, most of the area is being slowly eroded, with the local rivers depositing alluvium on flood plains in their lower reaches.
The Geological Group is interested in studying the wide variety of rocks, sediments and soils that form the landscape and scenery of the Sheffield area and Peak District, together with understanding how these affect the local plant ecology and overall biodiversity. The fossil content of the sedimentary rocks includes brachiopods, crinoids and trilobites in the Carboniferous limestones, goniatites in the marine shales of the Millstone Grit, and plant macrofossils in the Coal Measures, all of which are of value in reconstructing past depositional environments. Areas with mineralisation and disturbed ground associated with landslips are often associated with the growth of specific plants.
The Sorby Natural History Society normally has one or two lectures and one or two field trips each year organised by the Geological Group, though other lectures and field trips often have a geological or geomorphological interest.