A HISTORY OF LANCASHIRE
of Thieveley, Flower Scar Hill, and Dulesgate (where the 'Feather-Edge' coal is seen at Banks Mill) to Shore and Littleborough.
In the Rossendale area, the lower bed of grit and the 'Feather-Edge' coal are present as a surface bed below the peat over the Brandwood, Cowpe, and Knoll Moors, whilst at Bacup the upper bed can be seen in Bankside Quarry resting directly upon the coal.
The conformation of Brandwood Moor, Seat Naze, and the flanks of Cribden have been largely dependent upon the occurrence of this grit. In the case of the former it forms a complete capping, and, judging from the abundant evidence of glaciation, has served to protect it during the glacial period. At Bury the 'Feather-Edge' coal is two feet thick, and is not overlaid by grit. It has been worked on Scout Moor, near Edenfield. Holcombe Moor is largely capped by this rock, and we also find it on Darwen Moor, Bunkers Hill, Tockholes Fold, and other places. Along the northern side of the Burnley coalfield the various members of the Millstone Grit series dip southwards, the Rough Rock having a strong dip. The same upheaval has brought up the overlying Lower Coal Measures, the seams of which were formerly termed 'Rearing mines.'
It must not be forgotten that the Millstone Grit series in Lancashire forms but a small portion of an extensive mass of sandstones and shales which spread over the high ground of the West Riding of Yorkshire and stretch southwards and eastwards into Cheshire, Derbyshire, and North Staffordshire. Taken as a whole, these irregular deposits of sandstones and shales are indicative of a lengthy period of subaerial denudation of older crystalline rocks of a granite texture; hence the prevalence of decayed felspar and mica in the sandstones, and also of a corresponding sedimentation along the borders of the old Carboniferous limestone sea. The labours of Professor Green and others have shown that the greatest amount of deposition took place over Lancashire and South Yorkshire. Outside this area the grits thin off, especially to the north and north-west. Dr. Sorby, from a study of the current-bedding which is so marked a feature of the sandstones, concluded that the material of the grits in Lancashire and Yorkshire was brought by currents flowing from north-east to south-west, and an examination of the mineral constituents led him to suppose that the main mass of the grits was derived from the destruction of a western prolongation of what is now Scandinavia, this prolongation, if we follow Professor Hull's view, being part of a continental land which stretched from Scandinavia over the north of Scotland and Ireland into the North Atlantic. It is quite possible also that some of the grits and shales on the southern side of these counties were derived from a central land area which occupied the middle of the old Carboniferous sea.
At this time, as during the deposition of the Pendleside ('Yoredale') Group, the north-easterly part of this sea had become landlocked, either by blocking up of its outlets or by upheaval of the sea floor. The enclosed inland sea, by the gradual spreading out over its floor of sand and mud brought by rivers from the north and east and south, became converted into a huge swampy marshland, enclosing large lagoons with communicating channels, and over these the Coal Measures were in turn deposited.
COAL MEASURES
The Lancashire Coal Measures, which were accumulated upon the substructure of grits and shales, are divided into the lower, middle, and upper series, but the boundary lines are purely arbitrary and drawn for convenience rather than as indicating any real change in the deposits or their contents.
Speaking generally, the Lower Coal Measures are specially marked by shales containing supposed marine forms of life, thick beds of grit, and but few and thin coals. The Middle Coal Measures form the productive measures, marine bands, with a single exception, being absent.
The Upper Coal Measures contain a few coal seams in their lower half, the upper beds consisting of red shales and thin limestones.
Topographical Features.—The topographical features of the Lancashire Coalfield are well marked. On the south and west it is bounded along a line of faulting by the low Triassic plain of Cheshire and western Lancashire. Along the northern and eastern sides it is shut in by a series of lofty moorlands covered by extensive peat deposits and overgrown with heather.
The flanks of the moorlands are deeply gashed by the narrow ravines called 'cloughs' (see p. 8), the sides of which, clothed with the bracken and other ferns, lodge a few hardy trees and shrubs. Here and there the ravines have vertical walls of massive grits or well-bedded shale.
At the base of the highest moorlands are low rounded foot-hills whose sides and crests are clad with trees or occupied by grazing farms. These hills consist of the upper members of the Millstone Grit, or of the Lower Coal Measures, in which grit rocks are a strong feature. The hill slopes are usually steep. Most of the mining of the Lower Coal Measures is done by means of 'adits' which pass into the sides of the hills or else by shafts which rarely exceed 100 yards in depth.
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