GEOLOGY
Old Red Sandstone times were a natural prelude to those which brought about the formation of the limestone and limestone-shale of the Lower Carboniferous Series, and it is therefore necessary to a full knowledge of the latter that the main facts be at least outlined.
There is abundant evidence to show that a prolonged period elapsed after the formation of the Silurian, during which the deposits of the latter were subjected to considerable change and denudation. Only after prolonged erosion of their upturned edges, which formed part of a land surface, did a period of subsidence set in, and a series of depressions form, within which the red sandstones, shales, and conglomerates of the Old Red Sandstone were deposited. The character of these deposits clearly shows that they must have been accumulated not far from land, and the accepted belief is that the areas of subsidence, whilst in all probability connected with the sea at first, gradually became inland waters, passing in fact from a marine to a lacustrine condition.
The extensive development of the Old Red Sandstone deposits indicates also that a large continental tract must have existed around the areas of sedimentation from which the material was derived. The sandstone and conglomerates formed at the close of that period gradually gave place to calcareous muds and limestones, the latter showing that after a period of rest a slow and widespread period of depression had again set in. As subsidence went on the Old Red Sandstone lakes became once more merged into the sea, and as the movement continued the continental land surface also sank beneath the water, until marine conditions were established over almost the whole of England, Wales, and Ireland, and the southern half of Scotland, with the exception of a few island masses, one of which stretched from Leicestershire into Wales, occupying what is now St. George's Channel, and striking northward to the North of Ireland and the western coast of Scotland. As the sea area increased, beds of silt and mud took the place of pebbles and sand banks, to be overlaid in turn by purely marine deposits.
CARBONIFEROUS
The thick limestone beds which were gradually accumulated over the sea floor show that the water was clear and fairly destitute of material derived from the land. That these marine conditions were permanent for a long time is shown by the thickness of the Carboniferous Limestone, which in the neighbourhood of Clitheroe has been estimated at over 3,000 feet, without the base being seen. The waters of the carboniferous sea were tenanted with an abundant marine fauna, crinoids and corals predominating, the former to such an extent that great thicknesses of rock were built up almost entirely of the broken-up and commingled stems. Limestones of this character are well seen in the neighbourhood of Clitheroe, Whalley, and Whitewell, and also at the Salt Hill quarries. The corals grew either singly or in colonies, the latter often covering large areas with a thick layer of one species only. This was especially the case with forms like Lithostrotion, Syringopora, etc.
Brachiopods and pelecypods were well represented, and abundant evidence is furnished of shark-like fishes by the presence of teeth, spines, and scales. The boundaries of the Carboniferous sea are indicated by the intercalation of beds of mud and sand around the edges of the massive limestone, and by a thinning of the latter. It is by the careful mapping of these estuarine and littoral deposits that it has been possible to determine the main outlines of the sea area.
The formation of the thick limestone gradually began to fill up the sea-floor, and the materials brought to the sea margin by rivers, or derived from the eroded coastlines of the land, were carried farther and farther out until muddy and detrital deposits extended over the greater part of the sea-floor, and the formation of the Pendleside Group ('Yoredale Series')[1] began. The filling up still continued until large areas of the sea were cut off wholly or partially from the rest, and by the constant discharge into these of river waters marine conditions gave place to brackish, and the latter to fresh water, until, by the accumulation of sand and silt, the Millstone Grit Series was formed. At times, shallowing of the enclosed areas proceeded so far that vegetation extended from the land over the muds and sands, so giving rise to the thin coal seams occasionally found in the Millstone Grits. Subsidence still continued, but irregularly, so that a prolonged period of rest resulted in some lagoons becoming filled up and overgrown by coal forests, whilst very slow subsidence, and the continuance of shallow conditions, permitted the deposition of inshore materials, such as coarse sands, to be overlaid in turn by fine muds, when a greater subsidence caused the shore line to recede, and only finer water-borne material to be carried so far out. In this way arose the alternation of sandstones, grits, shales, and coals which make up the Lower Coal Measures.
The same process of subsidence followed by periods of rest brought about the formation of the Middle Coal Measures, only in this case, the land-derived waste was mainly deposited in the form of fine mud, probably owing to the general level of the land from which it was derived being so low that only the finer material could be carried in suspension by rivers. The existence of a low
- ↑ The name 'Pendleside Group' is here used in preference to 'Yoredale Series,' as the latter division at the typical locality in Wensleydale is considered to be on a lower horizon and equivalent to the upper portion of the Carboniferous Limestone. Hind and Howe, Quart. Journ. Geol. Soc. 1901, lvii. 376.
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