On the Millstone-grit of North Staffordshire and the adjoining parts of Derbyshire, Cheshire, and Lancashire

Abstract

Summary We have now traced the Millstone-grit from the borders of Lancashire and Yorkshire, where it contains five thick beds of massive gritstone, and reaches a thickness of 2800 feet, to the borders of the North Staffordshire Coal-fields, where only two of these beds are left, and where the whole thickness is not more than 200 or 300 feet. We find the Rough Rock (the uppermost bed, with one exception) present everywhere, and keeping pretty much the same thickness throughout, but losing altogether its coarseness and massive character in the south. The Haslingden flags, the second bed, maintain their thickness and character unchanged, until they thin away somewhat suddenly, about five miles south-west of Buxton. The Third Grit runs through the whole of the district; but from a thickness of more than 400 feet, which it reaches in Lancashire, it lessens down to about 100 in the neighbourhood of Congleton, and still further south seems to be on the point of dying out altogether. Though this bed becomes, without doubt, finer to the south, it keeps more than any other a certain massiveness of structure to the last. The little coal on the top of the Third Grit is one of the most persistent beds in the series. Seldom reaching more than a foot in thickness, it is found here and there over the whole country. The passage of the fourth or Kinder Scout rock from an enormous mass of gritstone and conglomerate, a thousand feet thick, into two beds of finer gritstone with a shale between, north of Buxton, has been pointed out, as also the further change which the latter undergo into still finer sandstones, and their total disappearance in the Biddulph and Rudyerd basins. These general results are embodied in the set of vertical sections given in fig. 10. It is well known that the Millstone-grit is wanting in the South Staffordshire Coal-field, and is only feebly represented in Leicestershire; a still further thinning away of the series must therefore take place, below the New Red Sandstone, between these places and the country under notice. One other point calls for notice. On the top of each of the gritstone-beds there are traces, more or less distinct, of coal: the Featheredge coal on the Rough Rock, the little coal found near Buxton on the Second Grit, the coal just mentioned on the top of the Third, and the very thin seams of coal which, accompanied by Gannister or Gannister-like rock, often lie on the top of the Kinder Scout Grit in Lancashire. The rough thick-bedded grits, and the finely laminated shales that lie between them, must, of course, have been deposited under very different circumstances, sand these coal-beds seem to point out that in the intervals during which the changes were being brought about, each grit-bed became in turn, for a time, almost a land-surface. In the Yoredale series we have seen that the upper group dies away altogether. The Quartzites must also partake in the general thinning out, for they are wanting, most likely, in Leicestershire, and certainly in South Staffordshire; but in the just described country they do not show any very marked falling off. The lowest group is also wanting in South Staffordshire, and is only 30 or 40 feet thick in Leicestershire; but it is too sparingly shown in the district under consideration to allow of its change of thickness being traced with any certainty.