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| Urban Mersey Basin |
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| Habitat: Earth heritage (of national significance) |
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The rocks, fossils, minerals and landforms of geological interest, together with the natural geomorphological processes that continue to shape the landscape.
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Within the Urban Mersey Basin, the highest ground corresponds with the oldest rocks. These are the Lancashire Coal Measures which consist of a succession of sandstones, siltstones and coal seams. Fossils from different strata indicate periods of inundation in both marine and freshwater conditions. The coal formed from the accumulated debris of plants which grew in tropical swamp conditions.
The Coal Measures were laid down between 318 and 303 million years ago - the Upper Carboniferous Westphalian Period. They were subsequently folded and faulted as a result of earth movements towards the end of the Carboniferous. Today the Coal Measures underlie a plateau which varies between about 50 and 150 m altitude. In places coal is close enough to the surface to permit opencast extraction. To the west and south the coal measures are too deeply buried to have any direct influence on the landscape.
No further deposition of sediments occurred in the Natural Area until the Upper Permian, an interval of some 48 million years. Isolated outcrops of Permian rocks occur on the western fringes of the Lancashire Coalfield. Other examples, which include sandstones, mudstones and thin bands of limestone, underlie parts of the Manchester area and, at Stockport, a buried Permian ridge feature steepens the bed of the River Mersey, the increased flow rate once being important for early industry.
The western part of the Natural Area is underlain by Triassic sandstones and siltstones. Dry desert conditions prevailed throughout most of the Permo-Triassic and, in consequence, fossils are scarce. However, recent discoveries of reptilian footprints in sandstone on Hilbre Island confirm that potential exists for further such finds.
The sandstones were formed in a variety of circumstances; the Sherwood Sandstone series being deposited in riverine conditions; the Mercia Mudstone Group probably in a tidal, brackish environment, not unlike the Dee Estuary today. The harder coarse red Triassic sandstones outcrop along the Mersey Valley between Lymm and Runcorn and again in many parts of Wirral. They also underlie Edge Hill and Everton in Liverpool and form prominent cuesta features which overlook the Cheshire towns of Frodsham and Helsby. No rocks more recent than Triassic are known and it is assumed that terrestrial conditions have prevailed throughout the Natural Area ever since that time.
The glaciations of the Pleistocene caused extreme denudation of the Triassic sequence as a result of ice sheets extending southwards down what is now the Irish Sea. The glaciation led to most of the low-lying land being covered with ice-transported boulder clay. Many boulders and pebbles found in the Urban Mersey Basin are readily identifiable as having originated in the Lake District and Southern Uplands of Scotland. Many pre-glacial river channels became dammed by ice movement forcing rivers to cut new routes through the glacial deposits. Sands and gravels which accumulated in ice-dammed lakes occur in several places - one large deposit of this type underlies much of the land between Rochdale, Oldham, Manchester and Bury.
In the post-glacial period, with a warming climate and increasing vegetation cover, many glacially-formed lakes became swamp and fen and a succession of vegetation types which culminated in woodland progressed over much of the Natural Area. Wetter conditions following climatic change led to localised waterlogging which impeded the trend to woodland and, in some areas, reversed it. Bogs expanded and peat accumulated where trees had previously grown. Evidence of all these and other changes can be found today in the Mersey Valley mosslands.
The melting of vast ice sheets led to a rise in sea level which flooded what is now the Sefton Coast. Evidence of this is seen in the Hill House Coastline, a cliff feature in boulder clay which is today about 7 km inland. Isostatic uplift of the land, relieved of its burden of ice, took effect more slowly than the rise in sea level, but eventually overtook the sea level rise and elevated the coastline to its present level. Accretion of mud and sand continues today. Formed from material brought down the rivers of the region, the present salt marshes and sand dunes today provide a natural barrier against marine flooding of the low-lying agricultural land of Merseyside and West Lancashire.
The Natural Area contains nine sites which have been identified in the Geological Conservation Review as incorporating a range of key geological and landform features.
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