Collins New Naturalist Library - K. Edwards C.

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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.

      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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       Fig. 5. Surface relief of the Peak National Park

      BEYOND THE DALES

      For many tourists the word Derbyshire spells “dales.” It is the dales they love to explore. It is up the dales they hike. When eventually they emerge into the upland, as for instance out of Lathkill Dale they lose interest for “there is nothing to see,” nothing but stone walls and tame pastures. Nevertheless, for a full appreciation of the more exciting vistas within the dales the story of these uplands must be told.

      The sequence of events detailed in the last section is known as a cycle of erosion. Whenever the level of the land rises or that of the sea sinks, the current “cycle” is interrupted or even ended. A general movement of this kind took place about the middle of the Pliocene period. The whole landscape was uplifted about 300 feet and remained at the new level for a long time. The rivers were rejuvenated and recommenced excavating their channels, first of all in their lower reaches. The point near which the steep new bed joins up with the gently sloping old one is commonly called the knick point. Below this the newly formed valley was at first a narrow gorge and ran like a trench along the floor of the broad, open, ancient valley of the former cycle. Above the knick point that valley remained unchanged. The river continued excavating its channel and the knick point receded upstream along nearly the whole length of the former valley.

      Meanwhile the sides of the gorge were worn by weathering agencies into steep and then gentle slopes. The gorge was thus slowly converted into a wide valley lying within the limits of the old one. Along its margin where the steeper side of the new valley merged into the floor of the old one there was a “break of slope,” essentially a greatly elongated extension of the knick point. Had this widening process been carried on to its utmost limit all traces of the older landscape would have been destroyed. Fortunately this cycle of erosion was interrupted in late Pliocene times and consequently relics of the earlier landscape survive in the loftiest portions of this upland.

      The late Pliocene uplift raised the general level another 200 feet and all the weathering machinery was set going once more. Through out the Pleistocene and later times, river channels were worn deeply once more and thus the dales as now seen came into being as the youngest features in the Derbyshire scenery.

      That, then, in rapidly drawn outline is the general story of this limestone scenery in the Peak District. Further details must now be considered and these vary from dale to dale mainly in association with the size of the streams.

      The Lathkill, though only a small stream, provides a pocket edition of the whole story. In the centre of its basin lies Monyash surrounded by a broad open valley formed mainly in mid-Pliocene times. The high ground enclosing this basin bears the last traces of the early Pliocene peneplanation. Downstream from Monyash the floor of the valley is gashed by the dale, the excavation of which was begun in late Pliocene times and continued until now.

      The survival of so many traces of the early phases in the development of the landscape is largely due to the fact that it lies wholly within the limestone region. Apart from the Lathkill there are no surface streams. Had such streams been present they would have inscribed an intricate pattern of new valleys and in doing so would have removed still further and larger portions of the ancient surface. The rain, however, instead of flowing off along the surface descended down cracks and joints in the rock and dissolved out underground channels along which it journeyed to the newly forming dale. Thus many of the dales are now dry at the surface, Gratton Dale being a good example (Plate VIIb, see here).

      It must not be supposed that the relics of the more ancient landscape have survived the passage of ages without undergoing change. On the contrary the whole

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