LITMIR.BIZ

      The dramatic rock wall of Malham Cove (Walk 20)

      Neatly laid down in horizontal bands, the whole area, known as the Askrigg Block, was subsequently uplifted by earth movement, putting the Carboniferous strata of the block far above the younger rocks that lie to the south. The block tilts gently backwards, and whereas weathering has exposed the older limestones in the south west of the area, the more recent Yoredale rocks remain on top to the north east. The lines of fracture are dramatically evident in the three main Craven Faults, which cut across the southern part of the National Park. Giggleswick Scar – the line of towering cliffs overlooking the B6480 west of Settle – is part of the South Craven Fault, which continues its line south east towards Skipton. The Mid-Craven Fault is marked by a long line of cliffs of which Malham Cove and Gordale Scar are a part, while the North Craven Fault runs parallel to it at the southern lip of Malham Tarn. In a few places, the limestone of the block has been worn away to expose rocks from an even earlier era, known as the Ordovician, which, unlike the even, Carboniferous formations, are extravagantly crumpled and consist of slates, grits and mudstones. They can be seen in the quarries of Ribblesdale and around Ingleton, and are also exposed as an impervious basement layer in the southern valleys, perhaps most vividly in Thornton Force and along the Ingleton Falls.

      The character of the Howgills clearly sets them apart from the rest of the Dales and, with the Middleton Fells, they are separated from the Askrigg Block by another fracture line – the Dent Fault. This runs in a rough north–south line east of Sedbergh, but here the displacement has been in the opposite direction, elevating the older rocks that lie to the west. Geologically these hills are part of the Lake District and are composed of much-folded metamorphosed slates and grits from the Silurian period, about a hundred million years older than the Carboniferous rocks making up the Askrigg Block. Their grassy flanks sweep steeply upwards from deep ravines to broad rounded tops, whose long interconnecting ridges, once attained, offer immensely satisfying walking.

      Some of the most spectacular scenery of the park is to be found in the areas dominated by the Great Scar Limestone – Malham Cove, Gordale, Kingsdale, Twisleton, Lower Ribblesdale and the middle reaches of Wharfedale. Towering lines of white cliffs and scars, shake holes, sinks, potholes, caves, disappearing and resurgent streams and rivers, dry valleys and waterfalls, clints and grikes are all features of this remarkable karst landscape. Overlooking the fault lines, the cliffs result from the upward movement of the Askrigg Block, but the terraces along the valley side are due to the relative resistance of different layers to erosive weathering. A similar picture is seen in the Yoredale Series, where the successive bands of limestone are comparatively harder than the intervening strata of sandstones, producing a stepped profile such as that vividly presented above Buckden. It is this same process that gives rise to the many spectacular waterfalls of the region, the water cascading over a lip of hard limestone, but undercutting into the softer rock which lies below.

      The most intriguing features of karst landscapes are those that result from the solubility of the bedrock in rain water. Its slight acidity dissolves the stone, exploiting crevices and vertical stress fractures and ultimately creating the awe-inspiring potholes and caves for which the area is famous. Whole rivers are swallowed into the ground, either in abruptly sensational falls such as Gaping Gill or merely ‘evaporating’ before your eyes as does the stream emanating from Malham Tarn or the River Skirfare in its passage through Littondale. Just as magical is their reappearance further down the valley, the river having coursed between the two points deep underground in dark and constricted passages and fissures that are the province of intrepid potholers and cave divers. In places, some dramatic passages are sufficiently accessible to have been opened as show caves, allowing visitors to marvel at the fantastic stalactites, stalagmites and other formations created as incessant drips of the lime-rich water have evaporated over millennia leaving the lime behind. Occasionally, similar deposits are also seen on the surface in the form of tufa, where calcite is precipitated from the cascading water.

      While streams, even after rain, are something of a rarity on the limestone uplands, 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 rain water 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 their shelter creates micro-habitats that are home to an astonishing variety of plant life, rare on an otherwise quite barren landscape.