Yorkshire Dales

The rocks which dominate this landscape were laid down as marine sediments during Carboniferous times. This geological period began about 350 million years ago and lasted for some 80 million years. Older rocks survive in the Howgill Fells where erosion has produced a distinct and dramatic landscape of smooth, rounded hills with long shoulders, dissected by deep, steep-sided valleys. Rocks similar to those found in the Howgill Fells underlie the Carboniferous rocks and are visible at the surface in small though economically important inliers in mid-Ribblesdale. Changes in sea level were mainly responsible for the different types of rock formed during the Carboniferous period. The Great Scar Limestone, the youngest of these rocks, was formed by the slow deposition of shell debris and chemical precipitates upon the floor of a shallow tropical sea. Minor breaks in this accumulation are indicated by bedding planes and some thin bands of shale. Over millions of years the sediments were compressed and then recrystallised or metamorphosed into a very strong rock which is resistant to most forms of erosion. Like other limestones this is scarcely affected by pure water but when rainwater absorbs carbon dioxide it becomes slightly acidic and then the rock is slowly dissolved and removed in solution. Carbon dioxide is given off by plants and animals: the richer the surface vegetation the greater the amount of available gas and thus the greater the acidity of both rainwater and surface water and the faster the limestone is dissolved. The principal outcrops of Great Scar Limestone and its distinctive landforms, technically known as ‘karst’, lie in the south-west part of the Dales, between Kingsdale and Wharfedale, an area historically known as Craven. Here it forms a wide, sweeping, now largely treeless landscape with thin calcareous soils and short green-turfed pastures subdivided by white dry-stone walls, and with dramatic outcrops of bare rock, either as steep cliffs known as ‘scars’ or as seemingly barren, but lichen-covered, limestone pavements. The distinctive profiles of the Three Peaks of Ingleborough, Whernside and Pen-y-Ghent rise above this karst landscape. Streams flowing off the hills disappear through sinkholes into a complex network of underground passages, some to reappear as springs. The limestone has been quarried for centuries. Its purity means that it is very important for the chemical industry but today the main product is aggregate for the construction industry. Limestone is also a main constituent of the Yoredale Series rocks which overlie the Great Scar Limestone. Deltas emerging from rivers eroding the landmass to the north dropped muds and sands into the seas in which these rocks were formed. Periodic earth movements influenced both sea depth and delta activity, resulting in the rhythmic succession of deposits of limestone, shale and sandstone, the latter sometimes overlain by a thin coal seam, followed by limestone, shale and sandstone….which together form the Yoredale Series. These are nearly horizontally bedded, dipping only slightly north-east. Erosion has created distinctive step-like terraces along the valley sides. This is clearly seen in Wensleydale and Pen-y-Ghent Gill where the hard limestone scars stand out like the risers of a giant staircase with the easily weathered shales the treads. The scars, and the steep scree slopes immediately beneath them, are sometimes clothed with long narrow woods of oak and ash. Numerous small streams cascade down these terraces, the lips of the limestone outcrops creating the waterfalls which are such a feature of the northern dales. Some, like Hardraw Force and Whitfield Gill Force, are well known, others not even named. Other waterfalls and rapids, such as Wainwath Force and Kisdon gorge in Swaledale, formed where the rivers themselves cut across the different rocks. The sandstone outcrops were quarried and mined to produce roofing slates and building stone. The coal seams were a valuable fuel supply, especially for the numerous small lime kilns built along the limestone outcrops to burn lime to sweeten the pastures and for building purposes. Shales and sandstones (some of the coarser sandstones are called gritstones) of the Millstone Grit Series dominate Nidderdale and the watersheds in the northern part of the Dales. On the high ground they create a much harsher landscape than the limestone country; a landscape of acid grass or heather moors with extensive cotton grass peat bogs where drainage is impeded. The sandstones and gritstones were also quarried as building stone and, as the name suggests, some beds were used for millstones. The Series includes thin coal beds, the best known being worked at Tan Hill at the head of Arkengarthdale. The metalliferous minerals which were so important to the economy of the area were formed during the major mountain building period which marked the end of the Carboniferous Period. Hot saline liquids forced their way up through numerous faults and fissures in the Carboniferous rocks forming veins as they cooled and crystallised. The mineral deposits vary considerably in extent which made lead mining a highly speculative venture. The Carboniferous rocks were subsequently covered by younger rocks and uplifted into a mountain chain. This has since eroded to such an extent that younger rocks no longer exist in the Dales.

The effects of the Ice Age

Major ice sheets have advanced and retreated across Britain at least three times in the last half million years bringing with them distinct forms of erosion and deposition. The effects of the latest, or Devensian, glaciation which began some 80,000 years ago, are the most apparent in the Yorkshire Dales. This glaciation was not constant – the generally bitter cold being occasionally broken by milder episodes called interstadials. We are living in an interstadial now, known as the Flandrian warm stage. All of the Yorkshire Dales, with the exception of some of the higher peaks, were covered with ice during the last glaciation. Existing river valleys provided easy routes for massive ice sheets which spread from the north and west. With each advance of this ice, glaciers scoured and deepened the principal valleys into a characteristic U-shaped form, straightening out previously formed kinks and spurs. This is best seen at Kilnsey Crag, close to where a glacier moving down Littondale met the Wharfedale ice flow. Some high ground deflected the flows of ice. Buckden Pike diverted ice from Langstrothdale southwards down Wharfedale and north along Bishopdale, protecting in its lee the narrow, still V-shaped, valley of Walden Beck. This increased ice flow over-deepened the valley of Bishopdale. Then, as the ice sheets slowly retreated, the valley floor silted up as a glacial lake partly dammed by ice and debris from another glacier which, fed by snow which had accumulated on the high ground around Baugh and Great Shunner Fells, continued to flow down Wensleydale. Lower Bishopdale is still very prone to flooding. The ice sheets which covered the Yorkshire Dales carried with them vast quantities of rocks, sand and clay. As the ice retreated this mixture was dropped to form the boulder clay or drift deposits which blanket the floors and sides of many of the dales. Sometimes this boulder clay was partly overridden by the ice and moulded into drumlins. These small smooth egg-shaped hillocks are best seen in Upper Ribblesdale where they are aligned north-west – south-east in the direction of the main ice flow. Terminal moraines, the deposits dropped at the front of a glacier, mark stages in the retreat of the ice sheets. The moraines sometimes blocked river valleys, as at Grinton and Gunnerside in Swaledale, causing lakes to form behind them. Some tributary streams entering the moraine lakes created deltas which, as at Buckden and Kettlewell in Upper Wharfedale, now provide drier settlement sites in otherwise flat and marshy valley bottoms.

Karst

Ice action, though powerful, is only part of the erosion history of the Dales. Water has been equally important, both as a physical erosive force and through chemical action, notably on the limestones which are soluble in acidic water. This can be clearly seen where boulders, known as erratics, carried from nearby Crummackdale during the last glaciation and deposited on top of limestone, have since protected the underlying rock from attack by rainwater. Some boulders at Norber are now perched on limestone blocks as much as 30cm (1ft) above the surrounding surface. The most distinctive surface features of the karst are the areas of limestone pavement. Any overburden of soil, debris or weaker rock was scoured away by glacial activity leaving a smooth rock surface. Rainwater has seeped into the joints and other fractures of the limestone, slowly enlarging them by dissolving the rock walls to form gaping fissures, known as grykes. The blocks of bedrock between the grykes are called clints. Little soil has developed on the clints as limestone is almost totally eroded by solution but small pockets exist in some of the grykes. These, protected from sun and wind and from grazing animals, support plant communities of great botanical interest which belie the barren impression given by distant views. In some areas overlying walls indicate that man’s agricultural practices have also played a part in the creation of limestone pavement by continuing or accelerating the process of soil erosion. Scattered throughout the karst landscape are numerous other depressions where water has seeped into fissures in the limestone. Shakeholes, most of which are only a few metres wide and deep, are the hollows created where the soil and soft rocks overlying the limestone have been washed down the fissures. Cracks and tears in the vegetation and soil cover on the sides of shakeholes show that they are still actively developing. Shakeholes are most numerous where the overlying drift is two to three metres thickalong buried shale-limestone boundaries. They should not be confused with the circular depressions, generally surrounded by small spoil heaps, left by shallow shaft mining for coal and lead. Water from surface streams sometimes disappears into sinkholes, also known as swallow holes. The best-known example is Gaping Ghyll, south of Ingleborough, where Fell Beck plunges down a deep, open shaft into a massive underground chamber. Fifteen kilometres (9 miles) of caves and passages linked to this cavern have been mapped by geologists and cavers. The cave systems were initially formed by rain and ground water slowly percolating down joints and cracks and along bedding planes until it found an exit or resurgence. Once a drainage route was established, dissolved material was carried away and a labyrinth of interlacing passages and caves slowly created. Underground, water continues to erode and seek the lowest level and so sometimes cave passages are abandoned. The shapes of these fossil caves provide evidence of the levels of ancient water tables and valley floors and are important clues to the evolution of the landscape. Frozen ground, a lack of flowing water and absence of plant life to increase the acidity of water meant that cave development virtually ceased during the ice advances. As the ice retreated the blocking of cave passages by ice and other debris sometimes forced meltwater to gouge out new courses and abandon old cave drainage systems. This process was accentuated by the deepening of valleys which meant that many caves such as Victoria Cave were left isolated on hill sides. Information taken from outofoblivion.org.uk
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