times streams of hikers and pleasure-seekers pour forth to find renewal of energy and refreshment of mind and spirit in the dales and on the moors.
THE ARRANGEMENT OF THE ROCKS
The process of forming those rocks which make up the Peak District was completed millions of years ago. Two sets of agencies then came into action and recorded their activities in quite different characters. One, which was concerned with rearranging the rocks, completed its work in a relatively short period of time. The other, concerned with destroying and carving the rock of the district, experienced a long interruption of its activities during the Mesozoic Era.
The hiker on Kinderscout or Bleaklow rejoices in the fresh breezes and in the fact that he is 2,000 feet or more above the sea. Though he takes that fact for granted it is nevertheless full of significance. Kinderscout is a little plateau with a relatively flat surface defined along its margins by rough, often precipitous slopes of Millstone Grit, for that is the rock which immediately underlies the plateau. When it was formed, that slab of grit was part of an extensive sandbank lying close to sea-level. As already seen, it was subsequently buried under Coal Measures down to a depth of some 5,000 feet, that is to say 7,000 feet below its present level. How then did it come to be at its present height? This was brought about by the joint action of two types of movement of the earth’s crust, of which the first must now be discussed.
Just as the process of burial described above was being completed, great revolutionary events began to take place across the south of the British area and the north of France. Mighty pressures in the earth’s crust acting from north and south crumpled the rocks along a belt of country about 200 miles wide and heaved them up into a mountain range of Alpine and even of Himalayan proportions. A small backwash of these great events was felt in the region of the Peak District. That threw the Carboniferous rocks into a number of folds which have exerted an important influence upon the physical features as seen today. As one result, the western margin of the district was crumpled into a series of narrow folds which merged into the main axis of the Pennines. Chief among these was a broad fold, elongated from south to north, which is known as the Derbyshire Dome. In cross-section it is asymmetrical and has the form of a wave that is just about to break; that is to say its eastern side or limb rises slowly to a crest from which the western limb drops down more rapidly and is broken by a series of rock fractures or lines of faulting. The slowly rising eastern limb is itself crossed by a succession of minor folds which run at right angles to the crest and which curve southwards beyond the east margin of the dome.
It is, of course, impossible to see a complete section across this and the other folds at any one point, but the visitor wandering from place to place, along footpath, road or rail, will see the rock layers exposed to view in valley sides, in railway cuttings and quarries, or in cliff faces. Sometimes they are seen to lie flat and horizontal. At other times they dip gently or even steeply. Geologists have carefully measured these angles and recorded them upon maps from which they have been able to piece together the structure of the region as a whole (Fig. 4, see here).
The contemplation of such folded rocks creates the impression of a time of turmoil and grievous upset, a time when nature took the rocks in her hands and squeezed them as a child squeezes its plasticine. That impression is, however, quite false. The moulding of the rocks into these varied and complex forms was spread over a period of several millions of years and took place so slowly that except for an occasional earthquake the placidity of the scene was never disturbed.
It must not be supposed that any part of the district ever attained such lofty heights as 7,000 feet or more, for as the crests of the folds rose above the general level of the surrounding country, they became the targets for the destructive action of such agencies as frost, rain and running water. In this way the Coal Measures, while they were being slowly uplifted, were simultaneously skimmed off the upfolds and the debris was transported to the lowlands which during the succeeding or Permian period, were to some extent covered by sea. At the base of the Permian rocks today there is a layer of this debris, a rock called Breccia made up of angular fragments of Carboniferous rocks. This lies unconformably upon the upturned and bevelled edges of Coal Measure shales, sandstones and coal-seams. This unconformity is a silent witness to the fact that when this Breccia was being laid down the Coal Measures covering the upfolds in the Peak District had been removed. The Millstone Grits thus exposed were then attacked by the same destructive agencies. Being more resistant they were not so completely destroyed except over the south of the dome where the limestone core was exposed to view. Along its margins the limestone layers tilted downwards and disappeared under the surrounding shales and grits.
In this way the general surface plan of the district was established no less than 220 million years ago. In the southern half it exhibits a large roughly oval area of limestone encircled by Millstone Grit and shales with grit layers, which are arranged in a pericline; that is to say they dip outwards from the centre of the limestone dome. In the northern half of the area the grit covering remains. There the rocks are horizontal or nearly so along a north-south axis, but on either side of this they dip outwards beneath the east and west flanks of the dome.
Meanwhile, the mighty folding movements along the axes of the Armorican mountains had ceased but the destructive agencies went on working. As long as any part of this intensely folded zone stood up above the level of the lowlands the rivers flowed down the slopes and spread their burden of gravel, sand and mud across the plains. Thus as the mountains were being laid low the plains were being levelled up. The Peak District with its folds thus became only a small feature in the vast expanse and was therefore gradually covered up and buried under the waste from the distant mountains and remained out of sight throughout the whole of the Mesozoic Era.
A PROLONGED BURIAL
Subsequent to this burial, a cavalcade of great events swept across the Peak District without leaving any trace of its passage upon the present landscape. Nevertheless the story would be incomplete without some reference to those events.
The uprising of the Armorican mountains athwart the path of the dominant rain-bearing winds greatly influenced the climate of the British area. The warm moist climate which encouraged the growth of the Coal Measure forests and swamps passed away, a dry arid climate set in and barren deserts replaced the luxuriant forests and tangled swamps. The Peak District area lay hidden away under the centre of a vast plain swept by hot winds and dust-storms.
The section is drawn along the line A-B as shown on the geological map in Fig. 2 (see here). The individual grits of the Millstone Grit series, being lenticular in torm cannot be traced continuously across the area. Those outcropping in the east are indicated by inital letters as follows: K-Kinderscout Grit, C-Chatsworth Grit, RR-Rough Rock. On the west side Black Edge is formed by the Chatsworth Grit. (Prepared by Prof. W. B. R. King, Sc.D., F.R.S., based by permission on maps of the Geological Survey and other sources)
Fig. 4. Geological section across the central Peak District
Far away on the distant mountains, during rainy seasons, the rivers became swollen into flood. Their waters spread on the plains as shallow transitory lakes and deposited their sand and mud in thin sheets everywhere. Sometimes the flood-waters found their way into and replenished the more permanent lakes. When the dry season set in and the floods ceased the temporary lakes were dried up and the permanent were reduced by intense evaporation. In both cases the salts that were in solution were left behind. The lakes consequently became more and more salty until such minerals as calcium sulphate and sodium chloride were precipitated. In modern times these chemical deposits have been exploited for gypsum and alabaster in south Derbyshire and for salt in Cheshire.
