LITMIR.BIZ

CHAPTER 2

       THE ROCKS AND THEIR HISTORY

       The hills are shadows, and they flow

       From form to form, and nothing stands;

       They melt like mist, the solid lands,

       Like clouds they shape themselves and go.

      ALFRED TENNYSON: In Memoriam

      A WANDERER returning to his native village in the Peak District finds his favourite haunts unchanged. The dales and streams, cliffs, hills and moors are all just the same as they were in his youth. If, however, he has studied the rocks they tell him that this peaceful, enchanting scene is but an episode in a long and eventful story which moves so slowly that for the brief period of his lifetime it seems to have been at a standstill. His going and coming have been no more than the flicks of a fly’s wings. That story is recorded for him in the rocks of the district; in the limestone of the uplands on the south; in the grits capping the moorlands of the north and forming the ridges which girt the uplands on either side; and in the shales which underlie the fertile vales that lie between the areas occupied by these two types of rock (Fig. 2, see here).

      THE FOUNDATION ROCK

The limestone teems with fossils which may be seen and collected wherever the surface of the rock has been washed by the rain for a long time or etched by weak acids seeping down from the covering of soil. These fossils are the remains of creatures that lived in an ancient sea which 280 millions of years ago occupied the whole district. How different was the outlook then! Blue sea extended to the horizon in all directions except the south where, in the offing, stood the miniature mountainous island of Charnwood which lay across the area now known as the Midlands. Its rivers were small and carried very little sand and silt into the sea, the waters of which were in consequence clean and clear. The sea was of no great depth and the scenery of its sunlit floor varied from place to place. Here and there were forests of stone lilies, animals that were allied to the starfishes. Each one grew upon a tall stalk built up of rings of lime piled one upon another to a height of eight or ten feet. The main body of the creature was at the top and carried five branching arms spread out like the fronds of a palm tree, to catch both the sunshine and the small organisms upon which it fed. When the stone lily died its flesh decayed and the fairy bead-like rings of the stalk and the limy framework of the arms and body fell to the floor of the forest and in the course of many generations built up deep deposits of calcareous debris.

      Out in the open, beyond the bounds of the stone lily forests, lay coral reefs. These were produced by the combined activities of myriads of polyps. Superficially they resembled modern reefs but the structural details of the individual corals were strikingly different. Surrounding the forests and reefs were spacious wastes of mud formed from the shells and bodies of minute organisms which fell in a perpetual drizzle from the waters overhead. Burrowing in the mud or crawling over its surface were many worms and other creatures that fed upon the mud or caught the drizzle as it fell. These latter included lamp-shells, a type of animal that is scarce today but was then varied and numerous and played a much more important part in the economy of the sea floor. Like cockles and mussels their bodies were also enclosed in shells, often prettily shaped and ornamented. They were usually small shells but some were giants a foot or more in diameter.

      Of special interest were certain curious molluscs belonging to the far-off ancestral stock of the Pearly Nautilus which lives today in the waters of the Pacific Ocean. Like this, they had shells which were divided into a succession of chambers separated by thin partitions. One large type is known as Orthoceras because its shell was straight and not closely coiled like that of the nautilus. Provided with a battery of tentacles round its mouth and with an apparatus for jet propulsion, it preyed upon fishes and other more peaceful creatures. The Goniatites were much smaller 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.

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