Yordale Rocks. Millstone Grit. 105 Fig. 28. Comparative Sections of the Millstone Grit and Yoredale Rocks. 2500 I SHEFFIELD JE III IV V BELPER Plan to show the position of the sections. 13 Mountain Limestone. 3 2 limestones of the lowest division of the Yoredale Rocks. At Kirk Langley, however, in the extreme south of our country, we find sandstones which are strikingly like the Staffordshire representatives of this group, and it looks as if the area over which these beds were originally deposited, or over which they reached their greatest thickness, was a strip of country ranging in a north-westerly and south-easterly direction from this village towards Macclesfield. Along such a belt we find the representatives of the group present in force, except where they have been removed by upheaval and denudation; to the north of it, on the eastern side of the Penine anticlinal, there are only a few doubtful representatives of the Yoredale Sandstones, local in their occurrence, and wanting in any marked mineralogical character; on the west of the anticlinal we do not find, on the north of the belt marked out, sections deep enough to show whether these beds are present or not, till we get beyond the limits of the country now under consideration. On Pendle Hill, however, near Clitheroe, sandstones which agree with them in position and character are found. In the north of the present country the black shales and limestones of the lowest division of the Yoredale Rocks are shown in Edale and the Castleton valley, but we were unable to form there any trustworthy estimate of their thickness. Between Eyam and Matlock several mines have proved them to range from 330 to 400 feet. The limestones are for the most part concretionary, or thin and found only in nodules; but here and there, especially in the south of our district, they become more regularly bedded, thicker, and so numerous and close together that it is somewhat difficult to say whether they ought to be put at the base of the Yoredale Rocks or the top of the Mountain Limestone. Millstone Grit.-The topmost bed, or Rough Rock, is, as usual, by far the most constant member of this group both in thickness and character. Its average thickness is 100 feet, and it is a massive, coarse gritstone, felspathic, and therefore crumbly. The sandstones in the upper part of the Middle Grits are for the most part represented by a single bed from 20 to 40 feet thick. But about Bradfield, Fulwood, and Longshaw there are several sandstone beds largely worked for building-stone, flags, and tilestones. The Chatsworth Grit shows a more complicated set of changes than any other bed of the group. In the centre of the district it is the coarse, massive, well-jointed gritstone, often red in colour, whose look and escarpments have been already mentioned as forming such striking objects in the landscape. This stamp it bears from Crow Chine on the north to below Chatsworth on the south. To the north of this belt the Chatsworth Grit gradually loses its coarse and massive character, and passes into a more closely grained rock. To the south of the belt of coarse rock the Chatsworth Grit puts on the form of a reddish sandstone rarely coarse, and in beds of moderate thickness. Somewhat further to the north, beyond the limits of the country we are now dealing with, it thins away. (See Geology of the Yorkshire Coalfield, p. 41.) The coal lying on the top of the Chatsworth Grit is very generally present; its average thickness is 2' 6", but at some spots it is as much as 4' 6", and at others not more than 1' 8". The beds between the Chatsworth and the Upper Kinder Scout grits show very remarkable and irregular changes in thickness. In the extreme north they are from 200 to 300 feet thick; they swell out southwards to more than 500 feet; and finally, near Crich and Belper, come down to 80 or 90 feet. Where thickest some considerable beds of sandstone are interbedded with the shales. way. The Upper Kinder Scout Grit behaves in a still more remarkable On the north it is a very coarse and massive gritstone, up to 200 feet thick. North of Hathersage it undergoes considerable change, and though still in places thickly bedded and coarse, it is in the main finely grained and mixed up with shale; it somewhat recovers its coarseness on Eyam Moors, but again about Stoney Middleton changes into finely grained sandstone; it continues to fall off in thickness and coarseness, till in Chatsworth Park it dies away, or very nearly dies away, altogether. It then recovers itself rapidly, and from Beeley down to Morley, where it is covered up by the New Red Sandstone, it is again a massive gritstone, 150 to 200 feet thick, not perhaps upon the whole quite so coarse as in the extreme north, but yet at times very rough, and even a conglomerate. Traces of coal, for the most part very thin, are found here and there on the top of this grit; about Alderwasley and Belper, however, there is a seam, 1' 6" thick, which was once worked. The case of the Lower Kinder Scout Grit is far simpler. We find it on the north a mass of coarse, very thickly bedded gritstone and conglomerate, 400 feet or more in thickness; it keeps its character, and, as far as we can judge, its thickness, till, at the southern end of Bamford Edge, it dies away very suddenly, and is not seen again. The Shale Grit can be traced from the moorlands on the north, where it is from 500 to 600 feet thick, without break down to Rowsley, where it is about 300 feet thick. Hence, southwards, the rock becomes less regular in its occurrence; no trace of it was seen between the Mountain Limestone and the Kinder Scout Grit of Robin Hood's Stride, west of Stanton, though to the east of that village a belt of it is found. It also seems to be wanting, or only represented by thin occasional bands of sandstone, along the eastern side of the Derwent valley from Rowsley down to Matlock Bank. Hereabouts, however, it again sets in as a traceable bed of sandstone, and may be followed to a point about a mile south-east of Cromford, its thickness being probably not more than 100 feet. Here it begins again to be occasionally wanting; it is not found, for instance, on the hill side below the band of Kinder Scout Grit ranging through Upper Holloway, nor between the escarpment of the same grit at the Black Rock and the Mountain Limestone. The rock sets in again to the east of Wirksworth, and forms a second escarpment below that of the Kinder Scout Grit almost as far south as the latitude of Belper, and an outlier of it is found at Kirk Ireton. It now seems to disappear for a while, but a bed of sandstone on about its horizon puts in again for a short distance at, and north of Duffield. South of this point we found no traces of the bed. As on the other side of the Penine anticlinal,* so here, traces of coal are found on the top of each of the grits, and we have moreover noticed that the surface of each is in many cases seen to have been waterworn before the overlying shales were deposited upon it; these facts tend to confirm our former notion† that an interval occurred between the deposition of each sandstone and the shale above it, during which the former became a land surface. * The Geology of the Country round Stockport, Macclesfield, Congleton, and Leek (Memoirs of the Geological Survey), p. 85. † Ibid. p. 86. |