LITMIR.BIZ

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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.

      At last this arid period known as the Triassic drew to a close. Its plains began to subside, were carried down below sea-level so that these too became submerged. This event ushered in the second division of the Mesozoic Era, known as the Jurassic Period. At first this new sea tended to be muddy but later its waters were often clear during long stretches of time and then limy deposits were laid down. The latter contained numerous small round pellets resembling the roe of fishes. For this reason the rocks formed from these deposits are known as Oolitic Limestones (oos=egg).

      The animals that came along with these marine waters were many of them similar to but not identical with those that lived in the Carboniferous sea. There were some near relatives of the Pearly Nautilus but they were few in number and all closely coiled. Descendants of the goniatites abounded but their shells were more prettily ornamented and the partitions between their chambers were complexly folded and even frilled. They are therefore distinguished under the name Ammonites. When the waters were clean and clear, stone lilies and corals became common but differed from their Carboniferous allies in many details. Sea-urchins, sea-snails and bivalves also contributed their quota to the building up of the Jurassic deposits and rocks.

      Shoals of fishes swarmed in the open waters. Their bodies were clothed in an armour-like mosaic of thick bony scales covered with shiny enamel. They were preyed upon by a new type of animal unknown in Carboniferous waters. These were large reptiles shaped like fishes and almost as perfectly adapted for a life spent wholly in the sea. Conspicuous among these was Ichthyosaurus, a fearsome creature with a large mouth as well-equipped with sharp teeth as that of the crocodile today.

      During Jurassic times the floor of the sea was subject to occasional undulatory movements. The crests of some of these undulations rose above sea-level and formed long stretches of land on which grew strange-looking trees allied to the Monkey Puzzle (Araucaria) and to tree ferns. As yet there were no herbs with coloured flowers, no grasses, no grassy swards. The landscape must have been a drab expanse, varied here and there only by the dark green foliage of the trees. The scene was, however, enlivened by the presence of many kinds of reptiles of all sizes, crawling, running on all fours or leaping like the kangaroo. Some small forms fed on insects. The larger ones fed on foliage or preyed upon their fellows. There were also some with such large bulky bodies that only by wading up to their necks in water could they support their great weight. Other lightly-built reptiles had large flimsy wings like those of the bats. These pursued and fed upon dragonflies and other flying insects which were at home in the air. As yet there were no birds. From all this it is evident that this period was one in which reptiles dominated air, land and sea. There were a few furred animals or mammals, but they were no larger than rats and mice and survived largely because they led a furtive existence in the crevices of rocks or hiding in the foliage of the trees.

      The Jurassic Period was brought to a close by a slow, nearly uniform, lowering of the earth’s crust in the British area. The sea flowed

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