THE ANALYST.

SKETCH OF THE GEOLOGY OF DERBYSHIRE.

SECOND PART.

By J. B. JUKES, B.A. F.G.S.

HAVING, in the last number of The Analyst, given a slight sketch of each of the geological formations found within the county of Derby, I come now to the positions which they occupy, both absolutely and relatively to each other. This department of geological investigation has been correctly termed Physical Geology, and is that portion of the science which has the greatest practical importance in all mining or other operations in which a knowledge of the structure of any part of the earth's crust is desirable. Nor is its theoretical importance to the scientific geologist less than its utility to the practical man, since it is only by accurately cultivating this branch of the science that we can hope to arrive at the solution of those great dynamical problems which the disturbing forces that act upon the crust of the globe propose to our investigation.

In speaking of the geological structure of the Derbyshire district, it is necessary that we should reverse the order used in the preceding part of this paper, and begin with the lowest of the formations there mentioned, and, having thus got a base for our operations, proceed regularly in the structure of our edifice.

The reader, then, must please to imagine a substratum of mountain limestone, of the thickness and with the characters previously described, to exist over the whole of Derbyshire and the adjoining counties to the east, north, and west, for an indefinite and unknown extent, sometimes forming the surface of the country, at others buried to a great depth under other materials.* The greatest extent of

• That this is not a vain imagination will be shown presently.
VOL. IX., NO. XXV.

1.

surface formed by the mountain limestone is in the north of Derbyshire and part of the adjoining county of Stafford. If the reader will take a map in his hand, and draw an undulating line through the following places, he will get a rough notion of the extent of this district. Beginning at Burton, the boundary of the limestone runs nearly north to the small village of Dove Hole, thence, leaving Chapel-en-le-Frith about two miles to the left, proceeds just under Rushup Edge till it reaches Castleton. From Castleton it goes towards Hope, and then turns to the S. to Abney. From Abney it goes round Eyam and Stoney Middleton, and proceeds thence, by Hassop, to Ashford. From Ashford it runs close to the N. and E. of Bakewell, and proceeds by Haddon Hall to the Lathkill. Here it deflects to the W. following the course of that river till it meets the Bradford. It keeps along the eastern bank of the Bradford to its source, and then turns again to the E. by Gratton and Elton, runs just to the north of Winster and Wensley, and crosses the Derwent in Darley Dale. Hence it runs just at the back of Matlock High Torr, and recrossing the river at Cromford, continues S. to Wirksworth. From Wirksworth the boundary of the limestone turns due W. running by Carsington and Bradburn to Tissington, and thence, turning S. to Thorpe, crosses the Dove into Staffordshire. Here its boundary is irregular, but it includes the villages of Blore, Caldon, Waterfall, Grindon, Mixon, Butterton, and Ecton, and recrosses the Dove at Hartington. From Hartington, it runs up the E. bank of the Dove to Crowdygate and Glutton, then passes through Dowel to Thirkalow, and including Harper Hill and Burbage, arrives at Burton. The space enclosed in this irregular outline is occupied entirely by mountain limestone and its associated toadstone. The other places where this formation is visible at the surface would be marked on the map by an oval inclosing the village of Ashover, of about 14m. long and m. broad, having its longer axis running about 20° N. of W. and the village in the northern part of the space; by a similar oval running nearly in the same direction, but of twice the length, with Crich about in its centre; by a small circle at Birchwood Park, 5m. S. of Ashbourne; and by the larger district about Calke and Ticknal connected with the mountain limestone patches of Leicestershire, and mentioned in a former paper.

The condition of the first and principal district above mentioned will be best understood by imagining the rocks to have been originally horizontal at a lower level, and then to have been elevated by an expansive force acting from below, until they were swollen

up, as

it were, into an irregular, low-curved, dome-shaped mass, with many great cracks and fissures intersecting each other at right angles; then a partial settling to have taken place, and some of the pieces divided by those fissures to have slipped below, while others were elevated above their previously common level. This will give a rough notion of the present broken and disjointed state of the mass, where, however, the fractures are clearly traceable to a common cause, and where the general inclination of the beds slopes on every side from the central position. That this is the general position of the beds may be seen by actual inspection. Along the western boundary some miles N. and S. of Buxton, the limestone dips everywhere to the W. passing regularly under the superincumbent strata. tracing it towards Castleton, it first dips N.W. and about that place due north. Between Castleton and Bakewell the limestone has an easterly inclination; and the same general dip may be observed throughout the eastern boundary all the way to Wirksworth. The whole of this eastern portion, however, is very remarkable for the great folds or flexures exhibited by the limestone strata. Scarcely any portion is a perfect plane, but it is all bent into regular and alternate elevations and depressions, like mighty ridges and furrows, the sides of which dip respectively to the N. and S. while the whole, taken as a mass, dips invariably and sometimes rapidly to the east. Of these great corrugations, Matlock High Torr and the adjacent cliffs down to Cromford, expose very beautiful examples, the strata of each cliff, while viewed in front, dipping evidently, on either hand, from the summit of the hill to the bed of the river, or N. and S. respectively, while a cross section would show them to be, at the same time, dipping into the hill, or towards the E. at a considerable angle.* The remarkable curve of the boundary of the limestone between Winster and Youlgreave, is owing to this peculiar structure, increased perhaps by a positive dislocation. It is caused by a depression or downward curve of the limestone between these two places, the hollow being filled up by the superior rocks; the bottom of this hollow dipping, doubtless, to the E. while its sides incline to the N. and S. respectively.†

The dip of the beds along the southern boundary is not so regular

The quarries at Matlock Bridge show this very clearly. The whole hill of Masson Lowe, of which these beautiful cliffs are but small portions cut off by the gorge of the river, would exhibit the same structure on a larger scale could its stratification be exposed.

+ See section No. 2.

as that on the W. N. and E. as, for some miles west from Wirksworth, the limestone is cut off by a great fault, which causes a sudden downcast (instead of a regular dip) to the S. All the way to the Dove the limestone is much broken and disturbed, but whenever a dip is seen along the whole line between Wirksworth and Thorpe, it is, I believe, invariably towards the south. Having established the fact of the limestone dipping, at each part of its boundary, from the central portion of the district, and passing, for the most part, regularly under the superior strata, the next fact that would strike our attention is, that we cannot proceed far from the boundary towards the central position without coming abruptly to the edge of a precipice or steep slope, or, geologically speaking, an escarpment. This is a necessary consequence of the position of the rocks, since, as the beds rise towards the central portion wherever they are cut through by a fault, and the inside piece depressed, or a valley in any way formed, the broken edges of the beds must be exposed to view, and a steep cliff or escarpment formed. From the inclination of the beds, too, the escarpments on the western side must face the E. while those on the eastern side look towards the W.* This position of the escarpments and inclination of the strata, however, must be understood very generally, and does not hold good on advancing far into the limestone country, there being nothing like any well-defined central ridge or axis of the district from which the beds dip equally on either hand, all the central portion being broken through by faults, and everything like regularity of position destroyed. On looking at a map on which the outline pointed out before has been traced, it will be seen that the limestone district is narrowest in the middle, or between the Dove and the Bradford, while N. and S. of that tract it expands to a considerable extent. This middle tract is comparatively undisturbed, and I am not at present aware of any place between Hartington and Middleton, by Youlgreave, where the toadstone exists at the surface. In all the limestone district, however, north of this line, we continually find the toadstone either at the tops of the hills or the sides of the

It would have been next to impossible, for instance, for a steep cliff to have been formed, like the Matlock High Torr, on the opposite side of the river, or with its face to the E. because, as soon as the ravine was formed, the beds which dip towards it on that side, being cut through, would be de prived of their support, and slide one over another into the valley: this, in fact, has, at some time, taken place, and the side of the hill opposite t High Torr is covered with broken ruins. The beds of the Torr itsel the contrary, dipping from the ravine and inst

can scarcely by any possibility fall into th