Collins New Naturalist Library. K. Edwards C.

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CHAPTER 3

       THE MAKING OF THE SCENERY

       Approach we then this classic ground:

       More gentle name was never found

       By chance, nor more of picturing sound

       To tell the spirit of the scene.

      EDMUND BLUNDEN: Dovedale

      THE SCENERY of the Peak District is a treasury of features, some new, some old. Relics of the far-distant past are closely linked with others of more recent origin; some beautiful, others rugged, weird, mystifying. It is the outcome of a long process of sculpturing which was produced by slow incessant change. Throughout all these ages, however, the rocks have remained unchanged in quality and arrangement and have exerted a constant though passive influence upon the landscape. A brief consideration of the nature of that influence will provide a useful background against which to watch the development of the scenery as it exists today.

      There are three main types of rocks in the district—limestone, grit and shale. Volcanic tuffs and intrusive dolerite play a minor but interesting part.

      ROCKS AND SCENERY

      Limestone is a hard, almost impervious stone. Consequently very little rain-water penetrates into its substance. This fact shields it from the shattering action of frost. On the other hand, it is slightly soluble in natural water. Just as a cube of sugar becomes rounded as it dissolves in tea, so the contours of the limestone surface tend to develop smooth curving outlines.

      Even the purest limestone contains small quantities of dust and other earthy materials. This is left behind on the surface of the rock and accumulates to form soil. On slopes the soil is usually too shallow to give firm foothold for trees, except for the ash, but it is covered by a carpet of grass and flowers which provides valuable pasturage. The more level ground on the platforms and along valley bottoms has a covering of much deeper soil. Here trees and bushes may flourish except where the altitude is such as to expose them to the influences of strong winds.

      The debris shed by the vegetation becomes mixed with the soil. There it rots and produces weak humic acids which are taken into the water percolating down through the soil and thus intensify its solvent action upon the limestone.

      Limestone, like other rocks, is arranged in layers or strata, each of which is broken into more or less cubical blocks by the presence of two sets of cracks known as joints and bedding planes. Descending water finds its way down even the finest of the joints and along the closest bedding planes, and by dissolving the stone on either side widens these into open fissures. Eventually these become so spacious that most of the rain-water abandons the streams upon the surface and flows away along these newly formed underground channels. Below a certain depth all cracks and fissures are permanently filled with water. That depth is spoken of as the water-table and thus becomes the surface along which these underground streams flow.

      Sometimes a surface valley is deep enough for its floor to lie along or even slightly below the water-table. In these circumstances a surface stream or river is maintained which is remarkably constant in its flow and rarely swells into flood in wet seasons or shrinks and disappears in times of drought. The Dove is a good example of this type of river.

      Grit, on the other hand, is practically insoluble in water. It is, however, so porous that rain-water is quickly absorbed and, soaking inwards away from the surface, leaves this dry. The water, however, fills all the pores and cracks in the deeper layer, which then become saturated. The dry superficial parts of the grit are consequently less liable to destruction by frost action and therefore retain their angular forms and sharp edges. Some grits contain grains of felspar, a mineral which is gradually rotted by natural water. In these cases the rock disintegrates more rapidly.

      Shale is more finely porous than grit. Water therefore percolates into it more slowly and is retained in its surface films. There its prolonged presence softens the rock and favours the pulverising action of frost. In areas where shale is the dominant rock, vertical erosion by streams and valley-widening proceed more rapidly. In these areas landslips occur on the steeper slopes and under the overwhelming pressure of superincumbent rocks the shaley sides of the valleys may even begin to bulge. Examples of such features may be seen around Edale and other vales.

      The limestone, which is the oldest rock in this district, underlies all the others and occurs as a great mass at least 1,500 feet thick. In its uppermost portions thin layers of shale appear. Passing upwards these thicken and the intervening bands of limestone become thinner and eventually disappear. The succeeding 1,000 feet of shales are called the Edale Shales.

      Thin СКАЧАТЬ