Читать книгу Short walks in the Yorkshire Dales - Collins Maps - Страница 8

Almost 300 million years ago, the rocks which are now the lowest part of the Yorkshire Dales, existed as the muddy floor of a shallow tropical sea. Those muds became slates, the bedrock of many of the Dales’ rivers. Gradually the sea filled with the teeming life of tiny crustaceans living amongst long-stemmed waterlily-like plants, called crinoids. As these plants and animals died, their bodies and shells sank slowly to the bottom of the sea, consolidating to form the colossal limestone cover which features so predominantly throughout the Dales. Much later, a huge river delta began to fill this sea, its outer deposits spreading to form shales, and on top of them came the harder gritstones, now the top-most rocks of the highest peaks.

As all this building up and smothering was taking place, land masses moved and gradually the land which was once in the tropics moved north towards its present position. During all this activity, faults occurred in the surface of the land and the rocks were pushed up and down; Malham Cove is a good example of a fault line appearing at the surface.

While the land was beginning to settle into its present shape, subterranean activity forced hot mineral solutions of ores into narrow cracks in the upper rocks. These mineral solutions were mostly lead but there were traces of silver and even gold. Chemical reactions formed calcium fluoride, which was a nuisance to later miners but is a useful raw material today. To the north, vast upsurges of volcanic dolerite created the great Whin Sill.

Around 10,000 years BC, the land, though covered by ice, was beginning to take on the outline of the Dales as we know them. As the ice melted, moraine dams created lakes in areas such as Upper Wensleydale. Mud made from ground down rocks of the high tops began to form the basis of new strata and the process of wearing down and building up began again. In geological time 10,000 years is like a few minutes to us and, once the moraine dams were breached, those muds and clays began to form the basis of today’s rich pastures of the Central Dales. The process is continuing.