Collins New Naturalist Library. K. Edwards C.

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СКАЧАТЬ and their shells were closely coiled. In them the partitions between the chambers were folded, sometimes in sharp angles (gonia=angle) which suggested the name. These beautiful little creatures may be pictured as spending their days flitting or crawling over the coral reefs and browsing upon the coral polyps.

      Fig. 2. Geology of the Peak District. A–B is the line of the section shown in Fig. 4 (see here)

      The shells of all these animals added their quota to the deposits that were being laid down upon the sea floor. Nevertheless, though the sea was shallow, it did not become filled, for its foundations were subsiding at about the same rate as the deposits were accumulating. Thus it came about that they ultimately attained a thickness of nearly 2,000 feet.

      A temporary but fascinating feature in this submarine scenery was the occasional presence of small volcanoes. The ashes which they shot forth into the waters above settled down and became mixed with the mud beneath. Sheets of lava, full of steam bubbles, were poured out and flowed far and wide over the sea floor (Fig. 3, see here). The dark-coloured rock into which these lavas solidified is known as Toadstone and has a striking appearance due to the fact that the bubbles have been filled with a white mineral.

      All the deposits described above consolidated and became the limestone which forms the Derbyshire upland. It is sometimes called the Mountain Limestone but to geologists it is known as the Carboniferous Limestone.

      THE LATER ROCKS

      The northern shores of the Carboniferous Limestone sea lay 200 miles away and stretched across the centre of the Scottish region. Scotland, at that time, was part of a great North Atlantic continent drained by large rivers flowing southwards. The general geographical picture thus presented was not unlike that of the United States with the Mississippi flowing into the Gulf of Mexico. These rivers carried the debris formed by the destruction of the uplands and by the rain washing the plains into the sea. Deltas of grit and sand were formed and banks built out along the shoreline. The fine muds were, however, carried farther afield and eventually reached the Peak District. Then for some time the sea-water was alternately clear and turbid, but eventually the latter condition prevailed. The mud accumulated and in course of time became those rocks known as the Edale shales which underlie the peaceful meadows of Edale and Darley Dale.

      The fauna in the waters underwent a corresponding change. The stone lilies, corals and many of the brachiopods and molluscs departed from the area. A few of the last remained and were joined by other kinds of goniatites and bivalves. Meanwhile the deltas and sandbanks extended and began to invade the district from time to time. The quality of the water also changed from being saline through brackish to fresh. Marine animals disappeared and were succeeded by a less varied and sparse population. At first this included Lingula, a curious tongue-shaped brachiopod which had already existed for 250 millions of years from Cambrian times onwards, and was destined to continue in the world for a similar stretch of time until the present day. As the waters freshened still more, Lingula and its associates migrated elsewhere and were replaced by crowds of bivalves such as Carbonicola which resembled the mussel of the present-day rivers and canals. Towards the close of this phase in the story of the Peak District the occasional influxes of coarse sediments became more copious. Banks of grit and gravel were formed and ultimately became that massive hard rock known as Millstone Grit.

       Fig. 3. Volcanic rocks occurring in part of the limestone of the Peak District. (Based on H. H. Bemrose)

      Owing to slight oscillatory movements in the level of the region these deposits were sometimes raised above water level and produced a low-lying landscape of sandbank and water channels. Spores wafted by the breezes, seeds carried by the streams from the continent enabled plants of the northern continent to settle on this new land surface. In the warm moist atmosphere they quickly germinated and produced a jungle growth of fern-like plants and strange-looking trees. Of the latter the smaller ones resembled the Tree Ferns which now grow in the tropical forests of the East Indies. Others, known as Calamites, were closely related to the horse-tails, those tall weeds which look like miniature Christmas trees and today grow profusely in wet waste places. Some of the trees towered to a height of 60 or 80 feet and had trunks as much as five feet thick. The bark was often decorated with scale-like markings which suggested the name Lepidodendron (lepido=scale) for these trees. Their branches and twigs had a furry covering of small lancet-shaped leaves. The modern relatives of these trees are not to be sought for in luxuriant forests but on bleak moorlands where the Stags Horn Moss (Lycopodium) is to be found straggling through the grass. These plants do not grow from seeds but, like ferns, they reproduce by means of minute pollen-like spores.

      Among the undergrowth of ferns and in the pools and sluggish streams were lowly types of four-legged animals, represented today by newts or salamanders, creatures which resemble fishes in that they lay their eggs in water and their young must spend at least the early part of life breathing by means of gills.

      These conditions, so different from the present, lasted long enough for deep deposits of vegetable debris to accumulate and become peat. A gentle down-sinking of the whole region then ensued. The sandbanks with their cloak of peat were submerged and were gradually buried under thick beds of mud and clay. The peat, squeezed by the pressure of the increasing load and changed by complex chemical reactions ultimately turned into coal. The mud also was compacted into shale.

      The sequence of events last portrayed was repeated a number of times and thus was built up a series of massive grit layers interspersed with shales and occasional seams of coal. These rocks were all destined, in the fullness of time, to play a large part in the scenery and other amenities of the Peak District. The layers of grit and shale, which had attained a thickness of a thousand feet, extended far beyond the bounds of the district and covered a vast area including the north of England and southern Scotland.

      The transport of so much sediment from the North Atlantic continent implies a corresponding destruction of the rocks in its uplands and a general wearing down of its whole surface to lower levels. As one outcome of all this the formation of coarse grit ceased and henceforth only fine sand, silt and mud were transported into the southern waters. Deposition and sinking went on in unison continuously, except for an occasional upward oscillation which converted much of the region, including our district, into an extensive fenland of mud-flats, upon which forests grew once more and thick peats accumulated. When the sinking movement was renewed, sea-water sometimes flowed in and spread everywhere, bringing with it marine animals, especially goniatites. Such marine conditions lasted only for a short time and gave place to a long period of fresh-water conditions when mud accumulated and buried the peat, which was in turn converted into coal.

      In this way 4,000 feet of clays, shales and fine sandstone with occasional coal-seams and marine bands accumulated. These are spoken of collectively as the Coal Measures. The economic importance of the coal suggested the word Carboniferous as the most suitable name for the whole sequence of rocks hitherto dealt with, including Mountain Limestone, Edale Shales, Millstone Grits and Coal Measures; and for the long period of time, amounting to 60 millions of years, which they represent.

      The countryside now occupied by these Coal Measures is comparatively low-lying. Apart from tip heaps it has a scenic beauty of its own but makes no contribution to the amenities of the Peak District. It does however provide the basis for the livelihood of dense populations. From its cities at week-ends and holiday СКАЧАТЬ