Collins New Naturalist Library. K. Edwards C.

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СКАЧАТЬ begin to appear in the upper levels of the shales which are referred to as the Grit Shales. Passing upwards through the rock series the grit layers become more massive. Of these there are five, of which the Kinderscout Grit may be specifically mentioned, for in the north of the district around Black Hill and Bleaklow it attains a thickness of as much as 600 feet. All these grits, however, have a lenticular form and tend to become thinner towards the south where the surface features they form are much less prominent than in the north.

      Turning now to the arrangement of the rocks it will be recalled that they have all been folded, a fact which has a marked influence upon the features of the landscape. The folds are of the two general types—upfolds or anticlines and downfolds or synclines. The sides of each fold are its limbs and these link the crest of the former with the trough of the latter. The rock layers are horizontal or nearly so at these two points but are more or less steeply dipping in the limbs.

      Erosive or denuding agencies naturally attack the upfolds to begin with and in doing so strip away the younger rocks first and the older ones in turn until in the centre of the crest the limestone is brought to view as in the Derbyshire upland. Northwards from the upland, earlier stages in this stripping process are exemplified in succession. In the moorland area much of the grit cover still remains. On the other hand, on the downfolds the youngest rocks survive longest in the centre of the trough, hence the presence of Coal Measures in the lower Goyt valley. Along the zones of country occupied by the outcropping limbs of the folds the rocks dip down from the surface and the edges of the strata produce such scarp-like features called “edges,” as Axe Edge, Froggatt Edge, Baslow Edge and Black Edge (Fig. 4, see here).

      TIME AND SCENERY

      The Peak District is but a minute portion of the earth’s crust. This latter is often spoken of as “terra firma” as though it were quite rigid. But it is rigid only in the sense that a block of wood is rigid. Anyone who has jumped off a diving-board knows, however, that when the block is so long that it becomes a plank it is springy and flexible. The rock layers which make up the Peak District are similarly flexible. Deep down in the earth beneath them lies a plastic foundation which during long periods of time has crept slowly from one region to another and the crust of rocks which rests upon it has risen and fallen accordingly. In the middle and again in late Pliocene times movements of this kind took place in the Peak District and resulted in a general uplift of the region. Each uplift probably took place in a series of stages but, for the sake of brevity with clarity, only the total results will be considered for the two occasions.

      With each total uplift of the Peak District its level above the sea was increased. Consequently all the agencies which had almost gone to sleep were aroused into activity once more; water flowed more rapidly; vertical and later on horizontal erosion were renewed. Thus it came about during the remaining 15 million years of Pliocene, Pleistocene and recent times, that rain and rivers, frost and glaciers gave to the district those magical touches of beauty which make it so attractive to its many visitors.

      The process of carving the surface was by no means haphazard. On the contrary, as the result of successive uplifts there was a majestic rhythm about its progress which inscribed its score everywhere. For those who learn to read that score the appreciation and enjoyment of the scenery are greatly enhanced. How was that score written? That is the next problem to be explored.

The key agents in carving inland scenery have always been the rivers and streams. Those of the Peak District are all tributaries of large rivers—the Mersey, Trent and Don—which in turn pour their waters into the sea. This last phrase, though trite, leads straight to the central influence that ultimately controls the whole activity of running water, for both flow and erosive activity cease at sea-level. This last term, however, lacks precision for both land and sea rise and sink independently of one another. Strictly speaking, therefore, the controlling influence is the relative level of these two and this is called the base level. Throughout its course a river flows and works only so long as its bed is above base level.

      The character of the work done by the river changes at different portions of its course. In its upper reaches the bed is steep and water flows rapidly. Like a man running swiftly, it follows almost a straight course and overcomes all obstacles that lie across its path. Downstream, as the slope of the bed becomes more gentle the rate of flow declines and the water is more easily diverted. Henceforth the river meanders from side to side.

      Upstream the more rapid flow gives to the water greater power for rolling boulders and stones along, and these by their continual passage wear grooves and channels across even the hardest rocks. This wearing of the river bed is described as vertical erosion. Downstream this power is lost. Nevertheless as the river meanders along it impinges against the outside bank of each bend. In times of flood, when the water is carrying a load of sand and gravel, it undercuts and wears the bank away. It is then said to be eroding horizontally.

      Upstream again, the valley sides come down to the margin of the water and the valley is V-shaped in cross-section. Downstream, on the other hand, as the river meanders it also erodes the lower fringes of the valley sides and thus the valley bottom is widened into a flat which increases in width as it approaches the sea. This broad flat with its cloak of gravel, silt and mud constitutes the alluvial plain.

While all these changes are going on frost and rain are busy working upon the valley side. The one pulverises the rocks and converts them into soil. The other washes away the surface of the soil and discharges it into the stream. At the same time the rain soaks into the soil and causes this to swell. In dry weather the water evaporates and the soil shrinks. This alternate wetting and drying, swelling and shrinking causes the soil to wriggle slowly downhill and ultimately to fall into the stream. This process is known as soil creep. Long after the river ceases to erode vertically these processes continue, the valley sides recede from the stream and the valley increases in width. There is, however, a limit to all these activities; they cannot be carried on below the level of the alluvial plain which serves as their local base level. As the steeper convex sides of the valley recede from the river they leave behind them a gently sloping apron of deep fertile soil which in these days is occupied by prosperous farms and villages.

      Meanwhile the plateau-like high ground between the valleys diminishes in height and extent and becomes reduced to a gentle rise of ground. In this way hilly and even mountainous country is levelled down almost to a plain, a “peneplain” (pene=almost), and is characterised by a gently undulating surface with broad open valleys and low spreading rises. This late stage in the development of landscape was attained in the Peak District in or about early Pliocene times. The level of the land relatively to the sea was about 1,200 feet lower than it is now. It remained near this level for so long a time that the work of denuding agencies was carried almost to complete fruition even in the uppermost and far inland portions of the drainage system in the region.

      The open breezy highlands of the Peak District are remnants of the peneplain then produced. The features described above are best exemplified in the limestone uplands where the rock is almost uniform in quality. An excellent viewpoint from which to see them in profile is from the summit of Thorpe Cloud. In the moorland areas in the north the alternating grits and shales have produced a rugged surface (Plate 1, see here). Upstanding peaks were absent, but even in those far-off days the plateaux and ridges rose above the general level of that ancient peneplain. Ancient indeed, for it dates back to early Pliocene times, when for several million years land and sea remained relatively stable with only small oscillations of level. The denuding processes continued their work without serious interruption until it was accomplished. At that time the Peak District did not rise above the surrounding country as it does now, for its surface was only a small part of an extensive peneplain that sloped away gently towards the far distant sea, the sea whose nearly constant level had for so long a time exerted a controlling influence that was felt along the whole length of every river and stream and across the breadth of the whole countryside.

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