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In periods when land and sea level remained stable the sand and mud would accumulate to form low-lying swamps. A warm, humid climate provided ideal conditions for the growth of dense forests in these swamps. Over many years the debris of dead leaves, trees and decaying vegetation accumulated on the swamp floor. Bacterial action prevented the breakdown of the vegetation to form humus and instead it absorbed water like a present day peat bog. The weight of the dead vegetation still falling to the ground compressed the organic debris into dark brown peat. In deep stools a slimy mud containing plant debris, spores and algal material was formed.
Flooding took place frequently, with mud and silt being deposited over the thick piles of vegetation. These layers sealed off oxygen from the vegetation and effectively prevented any further bacterial decay of the peat. Occasionally the land level would vary enough for the sea to gradually spread over the land, carrying marine organisms which can today be identified in a number of well defined marine bands. Each transgression of the sea over the coal swamp left behind a characteristic marine fauna, including marine bivalves and goniatites.
These periods of alternating sinking, flooding, swamp formation and coal forest growth form a cycle known as cyclotherm. The complete cycle does not always occur, but when it does it follows the pattern below :
coal, seatearth, sandy shale, siltstone, shale, shale with non-marine bivalves, shale with marine fossils.
This rhythmic pattern was repeated many times in Lancashire, and each time the earlier layers of peat were buried deeper below the surface. The weight of the overlying sediments compressed the peat and squeezed the water out of it. The temperature increased with depth and various volatile compounds containing oxygen were lost, resulting in an increase in the percentage of carbon. A layer of peat 40 metres thick would form a seal of coal 2 metres thick. This compression converted the peat to brown coal (lignite), bituminous coal (household coal) and finally anthracite. The lake sediments formed dull, soft cannel coal, like that mined near Wigan, and boghead coal made mainly of algal material.
The sequence of coal and cylic sediments in Lancashire is traditionally called the Coal Measures (Westphalian) and is over 1700 metres thick, but the coal seams only make up about 4% of this thickness. Because it varies so much from area to area it is very difficult to subdivide - correlation of the coal seams is only possible over short distances. The variation in the forms of certain non-marine bivalves, which are associated with the coal deposits, is used in the correlation of the various levels of the Coal Measures (Westphilian) - 'Marine Bands' are also used for this purpose. Another marker horizon is he tonstein - thin clay-rich beds formed under the water. The earliest attempts to divide the Coal Measures used fossil plants, but today this is supplemented by examination of microspores.
Towards the end of the Carboniferous period conditions became less suitable for coal formation. Earth movements took place, folding the rocks into arches and hollows (anticlines and synclines).
In the mid-Pennine area, which was formed at this time, thick deposits of relatively soft sedimentary rocks (including the Coal Measures) were pushed up to form a high land mass. However, the upper layers of the deposits, including the coal seams, were subsequently weathered away, exposing a high ridge of underlying sandstones and grits. The previously uniform coal deposits across the country were now divided into two seperate synclines, or hollows, dipping away from either side of the Pennines. These synclines now form the Lancashire and Yorkshire coalfields.
At the southern edge of the Lancashire coalfield the Coal Measures dip down to the south and disappear beneath the later rocks of the Cheshire plain, reappearing in the North Staffordshire coalfield. There is a broad syncline beneath Cheshire where the coal is too deep to mine at present.
A major feature of the Coal Measures in the Salford region is the large number of faults in the rocks where movement has taken place in response to tension in the earth's crust. There are five major faults in Salford, all aligned in a north-west to south-east direction. The largest is the Irwell Valley Fault where the maximum distance the rocks have moved downwards is about 1000m to the north east.
This movement has taken place over a long period of time and has continued in recent times,
causing minor earth tremors.
To log this earthcache please answer the following question's via my geocaching E-Mail.
1) Name the different type's of coal's that are produced.
2) What is the minimum and maximum thicknes of a coal seam.
3) How many years ago were the coal seam formed.
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Coordinates are in the WGS84 datum