Читать книгу Walking in the Yorkshire Dales: North and East. Howgills, Mallerstang, Swaledale, Wensleydale, Coverdale and Nidderdale онлайн

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While streams, even after rain, are something of a rarity on the limestone uplands of the south, dry valleys are not. Like Trow Gill and Conistone Dib, they can be stunningly spectacular – deep, narrow ravines, stepped with the walls of ancient waterfalls. Occasionally, following heavy rain, rivulets might briefly cascade through, but these bear little resemblance to the overwhelming torrents of meltwater that created them, as the last ice age came to an end. Where rivers flow uninterrupted today, they have usually worn the valley down to a bedrock of impervious stone, or else flow over deposits of clay dumped by retreating glaciers.

The extensive clint fields, or limestone pavements, also have their origins in the last ice age. Initially levelled to a bedding plane and stripped clean by glacial action, they were re-covered with clay debris when the ice finally retreated. Seeping rainwater subsequently picked out vertical lines of weakness to form the grikes, fragmenting the pavement into blocks – the clints. Eventual erosion of the thin soil cover, perhaps as a result of woodland clearance, or grazing by early man’s livestock, has once more revealed the bare pavements that are now such a striking feature. Accumulating soil in the base of the grikes holds moisture, and this shelter creates micro-habitats that are home to an astonishing variety of plantlife, rare on an otherwise quite barren landscape.