THE

GEOLOGY

OF THE

CARBONIFEROUS LIMESTONE, YOREDALE ROCKS, AND MILLSTONE GRIT

OF

NORTH DERBYSHIRE.

CHAPTER I.

INTRODUCTION.

IN a Memoir on the Quarter-Sheets 81 N.W. and 81 S.W. of the Map of the Geological Survey of Great Britain,* an account was given of the geology of a tract of Lower Carboniferous Rocks, lying on the west of the central axis of northern England, and bounded on the north by a line through Manchester and Staleybridge, and on the south by the New Red Sandstone plain of the centre of England. In the present Memoir we propose to attempt a like task for a portion of the corresponding beds on the eastern side of the axis.

The district to be described takes in the wild moorlands and the grassy limestone slopes of the High Peak of Derbyshire, the uplands on the borders of that county and the West Riding of Yorkshire, and a long narrow strip of country running down the valley of the Derwent, through Chatsworth, Matlock, and Ambergate, towards Beiper. It lies in Sheets 88 S.E., 81 N.E., 81 S.E., 72 N.E., 82 N.W., 82 S.W., and 71 N.W. of the Geological Survey Map of England.

On the west are Glossop-a large manufacturing town, Chapel-en-leFrith, and Buxton; in the centre, the villages of Castleton, Hope, and Hathersage; on the east the chief market towns are Bakewell, Winster, Wirksworth, and Belper.

GENERAL DESCRIPTION AND PHYSICAL FEATURES OF THE COUNTRY.

The main physical features of the country are so closely connected with its geological structure that a sketch of the one cannot be given without some reference to the other, and we must therefore endeavour

* "The Geology of the Country round Stockport, Macclesfield. Congleton, and Leek." Memoirs of the Geological Survey.

U 18863.

to combine the two. Looking at the country as a whole, we may trace one great anticlinal axis, ranging a little west of north, through North Derbyshire and the West Riding of Yorkshire, the southern portion, in fact, of the great central or Penine anticlinal or "backbone" of the north of England. It throws off on the one side, with a steep westerly dip, the Coal-Measures of Lancashire and North Staffordshire, and on the other the great Coal-field of Yorkshire and Derbyshire, the dip in this direction being much more gentle. The upheaval has been greatest in the north of Derbyshire, where a large dome-shaped mass of Mountain Limestone has been brought to the surface, from which the beds slope away, gently to the north, with a very sensible dip to the east, and still more rapidly to the west; so that while in the first direction we may go far before we reach any formation higher than the lowest beds of the Millstone Grit, a short journey either to the east or west enables us to cross the whole of the Yoredale Rocks and Millstone Grit, and reach the overlying Coal-Measures. The highest points of the limestone country are about 1,500 feet above the sea, while the Millstone Grit ridges run up to 1,800, and in a few cases 2,000 feet; this is perhaps partly due to the fact that chemical and mechanical action have both had a share in the wearing away of the limestone, while the latter alone has been the main cause of the removal of the grits. The outlines of the limestone country are for the most part smooth and gently rounded, not very much unlike those of the Chalk Downs, though, owing to the greater hardness of the rock, they have not the almost voluptuous softness of the latter. Some such likeness is only to be looked for, for in each case we have a thick mass of nearly pure limestone, liable to the same atmospheric influences, chemical and mechanical. This somewhat tame and uninteresting country is channelled by deep dales or ravines, some of which are of great beauty, specially where the stream has worked its way through the thickly bedded part of the formation, and has eaten out a narrow road bounded on either side by steep and in some cases vertical cliffs, often crowned by woods, drooping shrubs, and ivy, the bright colouring and graceful outline of which contrast strongly with the cold grey tone and stern massiveness of the rock.* That these dales were originally underground watercourses, the roofs of which have been worn through by atmospheric agency, was suggested by Prof. Phillips, and the idea has been worked out by Prof. Boyd Dawkins for the valleys and caverns of the Mendip Hills.† As instances of these deep valleys we may mention Miller's Dale and Chee Tor in the valley of the Wye; the valley of the Derwent at Matlock and Cromford; and Dovedale. In the less massive parts of the formation the valleys are broader and not so rugged; Monsal Dale, in the valley of the Wye, is a good example. The features just described seem to be the result of river and atmospheric action upon a thick mass of nearly homogeneous limestone; but when we pass to the surrounding country of Millstone Grit, where hard massive sandstones are interstratified with thick beds of softer shale, we find that the same causes have produced vastly different features in the

*The impression made by these dales on our ancestors will be gathered from the following description of Dovedale written about a century ago. Proceeding, therefore, towards the edge of the plain, they came to a precipice of an astonishing height, from which was a stupendous view into a deep valley, the hills rising on the opposite side, covered with wood, nearly half a mile perpendicularly." Spiritual Quixote, Book X., Chap. VII.

- The

On the Caverns of Burrington Combe. By Messrs. W. A. Sanford and W. B. Dawkins.

scenery. Here the outcrop of each grit-bed forms a long, unbroken ridge, sloping gently in the direction of the dip, and often at nearly the same angle, and ending on the other side in a sharp cliff or "edge," below which is a broad valley hollowed out in the underlying beds of softer shale. The stream at the bottom of this valley will often be found to run along the top of the grit-bed next below, which slopes up and forms the other side of the valley, ending in its turn in a cliff on the next hill-top, beyond which there will be another steep-sided valley. The section below, which is drawn from nature, on a true scale, will explain this.

Fig. 1. Section showing the general Outline of the Grit Country.

B

A

B

A

The sandstone beds are shown by dotting, and each forms a little cliff or bank at the top of each ridge, as at A; the steep hill-side below is formed of shale, and at the bottom of the valley we have a brook at B running on the top of the grit next below. To this general type all the main ridges and the valleys between them conform more or less closely, though it would not be easy to find many sections which satisfy it as fully as the one just given.

We may here mention that these wellmarked features are a great help in the geological mapping of the country. The baselines of the several gritstone beds are so clearly given by the sharp edges just mentioned, that they may often from some commanding height be sketched in for miles round. And it has been our usual custom to trace them in first of all in this way, and then walk along each line separately, correcting any errors in our first rough determination by the help of what evidence there may be on the spot. The small brooks, which run down the flanks of the escarpment into the main longitudinal valleys just described, are very useful in this way. At the base of each gritstone there is usually a cliff over which the water falls in a little cascade into a deep pool below. The moisture and spray are ever wearing away the shale at the base of the cliff, and masses of sandstone from time to time lose their support and tumble over, and the valley is thus gradually worked farther and farther back into the hill, and a clean section always shown of the junction of the grit and shale. The upper boundaries of the gritstones are by no means so clearly marked. Here, however, the shape of the ground is often a good guide, for the upper surface of the sandstone often forms a long gentle slope, over which the shales come on in low hummocks rising after a while into a steeper bank. When these natural features are indistinct we look to the changes in the nature of the ground, from the dry sandstone soil to the cold wet land of the shales; or to the herbage, rushes being

found in the shale, and heath and furze growing more plentifully on the sandstone; and here we may specially notice the little Viola lutea, a crowded belt of which, perhaps not many yards broad, is often seen along the outcrop of a sandstone bed, while not a single plant will be found on the shales which come out on either side. Viola palustris in the same way sticks to the shales, but it is too rare to be of any use as a geological guide. If all these helps fail us we recollect that each upper boundary runs between two consecutive base lines; and as the latter can be mostly fixed with great accuracy, the amount of error in the determination of the former can never be very large.

Drainage. Nearly the whole of our district lies on the east side of the great watershed of central England, the line of which ranges over Combs Moss, along Rushup Edge and Cowbarn, and a little to the east of Kinder Scout, the westerly flank of the Peak; thence it runs northwards till it meets the ridge formed by the northern outcrop of the Millstone Grit, which it follows for about four miles, when it strikes away northwards into country with which at present we have no

concern.

The brooks to the west of this line flow into the Goyt or Etherow, and so find their way into the Mersey.

The streams rising on the east of the central watershed flow either into the Derwent, the Don, or the Dove, and so in the end their waters are carried into the Humber.

The Derwent rises in the moorlands on the borders of Derbyshire and Yorkshire and forms for some distance the boundary between these countries; its course is a little east of south, and it falls into the Trent below Derby.

The chief feeders of the Derwent in the northern part of its course are the small rivers Alport, Ashop, and Noe. The first two join at Alport Bridge and fall into the Derwent at Ashopton. The last flows along the vale of Edale and joins the main stream at Mytham Bridge. We might perhaps say with more truth that here the waters of four streams are gathered into one channel and form the Derwent, for above this point the river which bears that name has little claim to be regarded before the others as the main stream. Lower down, the Derwent receives the Wye, which rises near Buxton, at Rowsley; and the Amber at Ambergate.

The Dove rises on Axe Edge and flows past Ashbourne into the Trent near Burton.