Cheshire

The form and landscape of Cheshire, a low, flat plain with a series of small prominent sandstone ridges, is reflective of its geology. The geology of Cheshire is dominated by Triassic rocks such as these cross bedded fluvial sandstones at Dee Cliffs, Farndon. The Cheshire Plain extends from the broad Mersey Valley, in the north, to the Shropshire Hills in the south. To the west the plain is bounded by the hills of the Welsh borders, and to the north-east by the foothills of the Pennines. Apart from a small outcrop of Carboniferous rocks in the north-east corner of the County, the solid geology of Cheshire comprises Triassic mudstones and sandstones that were deposited on a wide desert plain. These rocks are throughout Cheshire overlain by Quaternary glacial deposits, largely consisting of till (or boulder clay), with local deposits of silt, peat, sand and gravels. Rising up from the Plain are a number of small sandstone ridges and scarps formed from the Lower Triassic Sherwood Sandstone, such as the northern end of an outcrop which runs through central Cheshire between Malpas and Tarporley (the Peckforton Hills), while in the east the sandstone gives rise to Alderley Edge.

Carboniferous

The oldest rocks occur in the north-east of the County and are of Carboniferous age (354-290 million years old). Here, the Carboniferous rocks comprise marine shales, with thin limestones and sandstones (the Pendleside Series) which contain distinctive fossil brachiopods, corals and trilobites. These rocks outcrop from east of Congleton Edge up to Macclesfield and then into the Goyt Valley. Some hard quartzites in the Pendleside Series, known locally as Crowstones, have contributed to the formation of the higher moorland at Bosley and neighbouring hills. The succeeding massive sandstones of the Upper Carboniferous Millstone Grit Series (approximately 300 million-years-old) outcrop on either side of the Goyt valley and form the more elevated tracts of moorland. Cloud Hill, a striking feature near Congleton, is capped by one of the large sandstone units within the Millstone Grit. The sandstones were deposited in a coastal environment where large river deltas built 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 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 Coal Measure sediments outcrop in a narrow tongue from Macclesfield northward to Stockport.

Triassic

The Triassic (248-205 million years old) period in Cheshire is represented by red mudstones and sandstones that underlie virtually the entire County. These rocks were deposited under arid, desert conditions. The Lower Triassic sandstones of the Sherwood Sandstone Group form low, but prominent ridges in the east and central part of Cheshire. The Sherwood Sandstone Group consists largely of red, yellow, and brown sandstones that often show colour mottling. Pebbles are scattered through much of the sequence and include the well known Chester Pebble Bed, which is exposed in the banks of the River Dee and underlies the eastern side of the County. The smoothness and roundness of the pebbles within the Chester Pebble Bed indicates that they were transported by a large and powerful braided-river system, probably on the margin of an arid, desert mountain range. The mudstones of the overlying Triassic Mercia Mudstone Group probably represent wind-blown dust that settled in shallow salt-lakes and sun-baked mudflats on the extensive flood plain. These rocks occur in the central and eastern half of Cheshire, but are rarely exposed. 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. The commercial extraction of salt, by dissolving it and pumping it out of underground deposits from the Mercia Mudstone, has caused subsidence flashes (water pools), in particular around Northwich and Middlewich in Cheshire.

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 Cheshire, with the most extensive deposit being till (or boulder clay), which is formed in and beneath glaciers and ice-sheets. During the last glacial advance some 20,000 years ago, ice invaded from the Irish Sea area and deposited till, sands and gravels over much of Cheshire. Till with large amounts of gravel and boulders derived from the Lake District and southern Scotland formed at the edge of and under this ice sheet and now thickly blankets the northern and central part of the County. Deposits of glacial till, sands and gravels, particularly in the south of the County north of Whitchurch have given rise to an undulating landscape which includes a number of ponds and lakes, known locally as mosses and meres. These areas of open water and peat have developed in natural depressions in the glacial drift left by the ice sheets which covered the area. These natural sites form, together with similar features in North Shropshire, a network of wetland habitats of national importance.

Geological highlights:

Within the Mercia Mudstone Group there are many beds of rock salt, mostly thin; although two are much thicker than the others, being from 20m to over 30m thick. Thin beds and veins of gypsum are also common in the mudstones. Evidence for the exploitation of these deposits of salt in Cheshire dates back to at least 600 B.C. When the Romans arrived they increased production through evaporation in lead pans. The Anglo-Saxons called a saltworks a wich, and any place in England where the name ends in “wich” at one time produced salt (e.g. Nantwich). By the end of the 17th century salt was produced from brine pits in Nantwich, Northwich and Middlewich. Not until the 18th century were deep seams of rock salt, the source of the brine, discovered during prospecting for coal. The discovery and extraction of the rock salt gained economic importance over the next few decades and led directly to the construction of canals and the development of refineries and associated industries along the Mersey. Copper and lead mining is known to have taken place at Alderley Edge since the 1690s. Firm evidence of earlier mining has not been found yet except for some crudely shaped stones found during the nineteenth century which are thought to be Bronze Age hammers. The Romans may have also mined the ores, as evidenced by the discovery of roman coins in one of the abandoned shafts. Mining continued sporadically through the 18th and 19th centuries but ceased in the 1920s. In total there are at least 12km of workings of which over 9km are currently accessible. Many of the mines are owned by the National Trust and have been leased by them to the Derbyshire Caving Club which maintains access and continues to explore the network for areas of mining that have been closed for centuries. The Meres and Mosses of the north west Midlands form a nationally important network of open water and peatland sites. These have developed in natural depressions in the glacial drift left by the ice sheets which covered the Cheshire-Shropshire plain some 20,000 years ago. The origin of most of the hollows can be accounted for by glaciation but a small number have been formed at least in part by more recent subsidence resulting from the removal in solution of underlying salt deposits. Wetland habitats such as reedswamp, fen, carr and wet pasture occur around the ponds (or meres). As the basins have, over time, become infilled with vegetation, peat has formed leading to the formation of peat bogs or mosses. Rostherne Mere, which occupies a deep hollow in glacial drift to the south-west of Altringham, is one of the deepest and largest meres. Within the central and eastern part of the Cheshire Plain some pools in the surface glacial till have formed as a result of subsidence due to the solution of underlying salt deposits. The water in these pools, such as those at Sandbach Flashes, varies from freshwater, to highly saline. Inland saline habitats are extremely rare and are of considerable interest because of the unusual associations of plants and animals. Due to the differing age, depth and water chemistry the flashes show considerable variation in their plant and animal communities. The more saline flashes are fed by natural brine springs and contain a range of species tolerant of brackish water and support saltmarsh vegetation around their margins. Information taken from naturalengland.org.uk
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