No. I. Section given by Midland Railway. p. 18. Scale, 200 feet to 1 inch. No. IV. Millclose Mine, p. 146. VI. Section at Wirksworth, p 24. No. VII. Section at Ashover, p. 78. VIII. Section at Crich, p. 82. The numbers in each section refer to the same beds as on the pages quoted. We may now turn to the boundary of the Mountain Limestone, which winds through the country with many a sharp turn and re-entering angle, owing sometimes to faults, sometimes to sharp foldings of the beds. In spite of these irregularities the line is not very difficult to trace, for even in the absence of sections there is a marked difference between the short sweet turf of the limestone, and the sour rushy growth on the overlying shales: sometimes, too, the junction is marked by a line of swallow-holes, into which the streams which drain the surrounding country sink on reaching the limestone, and continue their course underground. We will start at the Odin Mine near Castleton,* the extreme northern point of the limestone tract, and work our way round along its western, southern, and eastern edges. Between our starting point and Buxton the boundary is at some spots formed by faults which bring the lower, thickly-bedded, white limestones against the Yoredale shales; while at others these latter are regularly overlaid by the upper, thinly-bedded, cherty limestones : along part of the line there is not evidence to determine whether the boundary is a fault or not. The hills, Treak Cliff,* Windy Knoll,* and Middle Hill,* are all formed of massive white limestone, and we nowhere find any trace of the upper cherty beds, and the limestone must, therefore, here be bounded by a succession of faults. From Middle Hill to Perryfoot,* the shape of the ground points to a natural junction, and the line is well marked by a line of swallow-holes; it is rather doubtful, however, from the character of the rock, whether it belongs to the topmost beds or not, and therefore the question whether the boundary is a fault or not must be left open. Between Perryfoot and Sparrow Pit the boundary seems faulted, but on turning down the road from the latter point to Chapel-en-le-Frith, we find the upper cherty beds dipping below Yoredale shales at angles of 15° to 30°. We shall show below, however, that a fault must run between the limestone and the Shale Grit of Bolt Edge.* Turning down the valley to Buxton,† we find many sections showing the upper cherty beds of the limestone passing regularly below the black shales, and the boundary though broken by cross-faults is a natural one. Between the top of the limestone, however, and the base of the Shale Grit to the west, there is room for only about 500 feet of measures (see Horizontal Section of the Geological Survey, Sheet 70), and as the combined thickness of the two groups of the Yoredale Rocks is most likely far greater than this, a fault must lie between the two. In the tunnel of the Midland Railway at Doveholes, a fault was crossed a little to the west of the junction of the limestone and shale, and we have already seen that in the railway cutting east of Buxton the limestone is faulted against shales (p. 21, fig. 2); between these two points, then, we have drawn the fault, keeping it a little to the west of the limestone boundary. We have mentioned in the Memoir on Sheets 81 N.W. and S.W. (p. 49) that a fault most likely runs between the Mountain Limestone and Shale Grit at Buxton, but we have no further evidence to offer. From the house called the Pecks,‡ a mile and a half west of Buxton, a line of faulted boundary reaches to beyond Hartington. The existence of this fault was pointed out by Farey, and though his generalisations were somewhat too sweeping, and many of his statements have to be modified when we come to details, yet the keen-eyed old geologist was here at least right in the main. Farey's views will be found in a paper * In Map 81 N.E. see p. 41. † In Map 81 S.E. In Map 81 S.W. in the Philosophical Transactions for 1811, entitied "An account of the Great Derbyshire Denudation, by Mr. J. Farey, senior. In a letter to the Right Hon. Sir Joseph Banks." A capital summary is given in Conybeare and Phillips' "Outlines of the Geology of England and Wales," p. 450. We also gave our reasons for suspecting some such fault in the Memoir on the adjoining Quarter-sheet 81 S.W. (pp. 48, 49), and a detailed examination has confirmed our first impressions. The following considerations lead one to suspect the boundary to be a fault. The limestone mostly rises from the plain or valley to the west in so steep a cliff that nothing short of a dip nearly vertical could carry it below the Yoredale shales which occupy that valley. The junction of the shale and limestone can be traced with great accuracy; and, when laid down on a map, we see at a glance, from its notched outline, that, if not a fault itself, it is broken by many cross-faults. Specially we notice the way in which the boundary winds back up many of the valleys which cut across it. As the fall of the valleys is in the same direction as the dip of the beds, a tongue of limestone, longer as the dip was smaller, ought to run down each valley, while in fact we constantly find a wedge of shale running up the valley into the body of the limestone, and abutting on either side against cliffs of that rock in which the beds are seen to lie nearly flat. This, of course, can only be accounted for by faults bounding the valley on either side, and meeting at the point of the wedge. The lower part of Glutton Dale is a capital instance. On examining the line more closely we find the upper cherty beds everywhere wanting, the rock immediately in contact with the shales being a massive white limestone, such as is only found some way down in the formation; also the limestone is sometimes seen close to the boundary to be rising to the west; and, lastly, we shall be able to point to a few sections where the faulted boundary is actually seen. In spite of these explanations we are not without fear that the extremely complicated array of faults laid down on the map will be looked upon with suspicion, and treated as a bit of fancy mapping.* We have however some confidence in the accuracy of the lines and the sufficiency of the evidence, and we think it not unlikely that a large fault, if it could be traced with the same minuteness as in the present case, would often be found to have a like broken and jagged line, and that the lines we are usually obliged to be content with as showing the course of faults are in many cases only very rough approximations, giving their general direction, but by no means all their windings and sharp bends. Mostly this series of faults brings black Yoredale shales directly against massive white limestone. Here and there, however, when the throw has not been so great, or the denudation has reached lower down, small patches of the upper cherty beds of the limestone are found on its down throw side. * In some instances the complication has probably been increased through the liability of limestone to what may be termed "underground denudation." Caverns are formed in this rock, particularly along lines of fault, and by the eventual collapse of their roofs become converted into wall-sided ravines. It is not uncommon to find masses of the rock, which formerly lay above the limestone, dropped in a more or less broken condition into such hollows. It sometimes happens also that whole sheets of limestone are dissolved away along the outcrop, leaving a mere parting between the over- and under-lying beds.-A.S. The following points are worth special notice: In the long tongue of Yoredale shales which runs up between the farm houses Stoop* and Green Sides,* the eastern boundary fault was marked by a line of swallow-holes, in some of which black shales were seen abutting against massive white limestone. The fissure here and elsewhere was filled with crumbly oxide of iron. The sharp peaks of Park* and Chrome hills,* which we had often seen from a distance, are so widely different from the usual rounded outlines of the limestone country, that they had long been a puzzle to us. On the spot however the explanation is easy enough. They are masses of limestone bounded by faults on all sides; and while the softer shale, which once reached to their tops, has been carried away by rain and atmospheric wear, the hard limestone has held out against these attacks, and stands up like the conical mounds of soil often left in a railway-cutting by which the contractor measures how much has been dug away. Behind these hills is a broad flat-bottomed amphitheatre, at the head of which stands the farmhouse Dowel,* shut in on three sides by steep limestone hills, and cut off from the shale flat to the west by a narrow wall of limestone rock. A stream crosses this, running above the farmhouse in a deep, narrow, steep-sided limestone gorge, then winding across the flat, and escaping through another narrow cleft in the western limestone wall into the Dove. And here the same explanation applies. The amphitheatre is occupied by a mass of shale which has been let down by four faults into the limestone, and which once filled the hollow up to the top of the bounding hills. The stream has cut a channel across; and those parts of its valley which are in limestone are still narrow, because that rock does not easily break up under the action of the weather; the shale, on the other hand, crumbles more quickly and is washed by rain and rivulets into the brook and so carried away, and therefore in it the valley has been widened out, up to the edges of the harder limestone. At Crowdecote,* we found the section given in fig. 4. It shows how complicated is the faulting along this line, but the details could not be put on the small one-inch map. Fig. 4. Section and Plan to show the faulted nature of the Limestone Boundary at Crowdecote. |