Pennines and adjoining areas - Lowland Plains: Difference between revisions

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This encompasses the two lowland areas in the western and eastern parts of the region separated by the Peak District - Pennine Hills and coalfields areas. The western area extends from Macclesfield and Manchester westwards to Chester and then northwards to include Liverpool (Merseyside) through Preston and Blackpool to Lancaster. The eastern part stretches from Catterick in the north, southwards through York to Doncaster (South Yorkshire) and then Newark and Nottingham (P902241).
This encompasses the two lowland areas in the western and eastern parts of the region separated by the Peak District - Pennine Hills and coalfields areas. The western area extends from Macclesfield and Manchester westwards to Chester and then northwards to include Liverpool (Merseyside) through Preston and Blackpool to Lancaster. The eastern part stretches from Catterick in the north, southwards through York to Doncaster (South Yorkshire) and then Newark and Nottingham ('''Figure P902241''').


===Sedimentary Bedrock===
===Sedimentary Bedrock===

Revision as of 15:14, 6 October 2014

This encompasses the two lowland areas in the western and eastern parts of the region separated by the Peak District - Pennine Hills and coalfields areas. The western area extends from Macclesfield and Manchester westwards to Chester and then northwards to include Liverpool (Merseyside) through Preston and Blackpool to Lancaster. The eastern part stretches from Catterick in the north, southwards through York to Doncaster (South Yorkshire) and then Newark and Nottingham (Figure P902241).

Sedimentary Bedrock

Processing the Sherwood Sandstone into crushed rock aggregate, north Nottinghamshire. P596266.
A subsidence collapse feature formed by natural dissolution of gypsum, near Ripon. P223240.

A sequence of up to three sedimentary bedrock layers occurs in the Lowland Plains.

An uppermost mud-rich layer of sedimentary bedrock referred to as the Mercia Mudstone that was deposited in hot arid estuaries and coastal plains and is characteristically orange and red-coloured. The Mercia Mudstone is present in the northern part of Cheshire, parts of the Wirral and west Lancashire, and along the extreme southeastern part of the region in Nottinghamshire. The Mercia Mudstone is present at depths up to 1500 m in Cheshire, over 1000 m in west Lancashire and over 200 m in Nottinghamshire. To the west of the Pennines, thick beds of rock salt are developed beneath parts of north Cheshire and west Lancashire. These formed when the coastal plains, within which the Mercia Mudstone was laid down, dried out, allowing seawaters to become saturated with brine, from which the rock salt crystallised. These salt layers are up to 250 m thick, and have been mined in traditional ‘dry’ mines in parts of Cheshire near Northwich (at about 90 m depth), and in Lancashire near Blackpool (at 160 - 330 m depth). The salt has also been extracted as liquid brine by ‘solution mining’, a process where the mineral is dissolved underground and pumped to surface. The solution mined caverns are typically between 100 and 400 m below the surface. Specially designed and constructed solution-mined caverns are used for the storage of natural methane gas at depths between 300 and 730 m near Northwich in Cheshire.

The Mercia Mudstone is everywhere underlain by the Sherwood Sandstone (P596266). The orange and red coloured Sherwood Sandstone, which was deposited in mixed environments including large river systems and desert dunes, is present throughout the area and is the second most important aquifer in England. Water in this layer flows between the individual sand grains and within fractures in the rock and is generally low in natural minerals such as calcium carbonate making it suitable for agricultural or industrial use, and as drinking water. Where it occurs at depths greater than 500 m the water is commonly saline and so it is not generally suitable for drinking water. Due to the prevailing tilt of the strata, the Sherwood Sandstone thins towards older rocks in the coalfield and moorland areas. The Sherwood Sandstone is present at depths from surface of up to 200 m in Nottinghamshire, 400 m in North Yorkshire, and over 1000 m in Cheshire. However, the boundary between these rocks and older rocks in much of Merseyside and Lancashire is a faulted one, meaning in those areas this bedrock is present at depths from surface of over 1000 m. Hydrocarbons have been produced from the Sherwood Sandstone at several localities within this region. Small gas and oil fields are present in west Lancashire and parts of Nottinghamshire and South Yorkshire. Once the hydrocarbons have been extracted from these fields in the future they become targets for the underground storage of natural methane gas or carbon dioxide. Methane gas is also produced in this region from abandoned coal mines in northern Cheshire and there may also be a coal bed methane resource beneath much of the East Midlands.

Underlying the Sherwood Sandstone and occurring at surface on the eastern flank of the East Pennine Coalfield and Pennines is a layer of sediments up to 150 m thick that was deposited in deserts and shallow seas. The rocks include alternations of up to 50 m of limestones, mudstones and sandstones. This layer of bedrock thickens and tilts eastwards to reach a maximum depth of about 400 m beneath the easternmost parts of the region. Between Doncaster and Catterick, layers of up to 40 m of the mineral gypsum are present within these rocks, this was mined up until recently near Selby in North Yorkshire for use as a fertiliser and the main constituent of many forms of plaster. Subsidence hollows formed through the dissolution of gypsum is a feature of the Ripon area (P223240). Some of the limestone layers in this area are aquifers, corresponding to the Magnesian Limestone found in Northern England and provide minor amounts of water for local public supply. As with the coalfield areas, basement rocks are not encountered anywhere near the surface in the Lowland Plains, but do occur at several kilometres depth beneath these areas.