Black Keld Catchment

Also within

North Yorkshire County YORKSHIRE DALES National Park

Condition (Natural England)

Habitats

Fast flowing river and stream

Geology & landscape

Geological Feature: Cave / Cavern · Rock Type: Limestone, Sandstone, Shale

Landscape processes: Karst

Description

Description The site covers an extensive area of Conistone Moor and Grassington Moor to the south westof great Whernside and approximately six kilometres north of Grassington. Two major cave systems, Langcliffe Pot and Mossdale Caverns are present and together, these caverns consist of over 20 kilometres of passages. The underground drainage system which feeds the stream resurgence at Black Keld is one of the largest and deepest in Britain, although only a small proportion of its cave passages are accessible at present. The system is unique in that the cave drainage is constrained initially within the Yoredale Limestones, but then breaks through into the underlying great Scar Limestone, resurging about 160 metres below at Black Keld close to the base of the limestone succession. These cave systems therefore breach the intervening shales and sandstones which normally act as aquicludes. These breaches are a response to geological controls and result from the penetration of stream ways through open tectonic fissures, small faults, major fractures and zones where the Middle Limestone and great Scar Limestone have been brought into lateral contact with each other through displacement along faults. Drainage within the vadose zone of the cave system is constrained by the south easterly dip of the limestones, however, the resurgence of Black Keld lies to the north west of the sinks of either cave system. The patterns of passages in both Langcliffe Pot and Mossdale Caverns are unusual for cave systems within the Yoredale Limestones and Mossdale Caverns has a divergent branching pattern which resembles the pattern of a phreatic maze in its complexity and as such, the caverns are regarded as important examples of maze caves. Other Information This is a Geological Conservation Review Site. Explanatory Geological Note The purpose of this note is to describe the nature an importance of the site, avoiding specialist terms, for the site owner and / or occupier. This note does not form part of the formal notification documents. Black Keld Catchment Area, North Yorkshire, The cave system occupies an area of approximately 1400 hectares on Conistone Moor and Grassington Moor, about six kilometres north of Grassington. Two major cave systems, Langcliffe Pot in the north and Mossdale Caverns in the south, together comprising 21 kilometres of explored passages underlie this area. Water enters Mossdale Caverns where Mossdale Beck sinks at Mossdale Scar. From there it follows a complex sequence of passageways in a general south easterly direction, eventually disappearing beyond chokes or blockages. Water enters Langcliffe Pot through several sinks and again follows a south easterly route. Again, the passageways become blocked. The placing of marker dyes in waters of these two systems has demonstrated that the water emerges at the resurgence of Black Keld in the floor of Wharfedale to the north and east of the two cave systems and in a direction almost opposite that of the stream flow within the caves. In addition, the limestone bed in which the resurgence is located in geological strata approximately 280 metres below the strata the sinks reside in. The rocks surrounding the caves consist of alternating layers of limestone, shale and sandstone. The caves passages are developed in the limestones where water passing through cracks in the limestone has dissolved the limestone and over a long period of time, opened the cracks into passages. Neither the shales or the sandstones are susceptible to dissolution by water, nor will they easily allow the passage of water through them. As the shales and sandstones intervene between the limestone bands, water should not be able to penetrate through this succession of rocks. Black Keld Catchment area clearly demonstrates the effect that geological structure can have on the development of cave systems. The rocks over much of the area dip to the south east and the cave passages have an overall trend in this direction. The cave streams have broken through the shales and sandstones along weakened zones that were fractured because the rocks had been displaced by faults. Two of these faulted zones may be seen in Langcliffe Pot where the stream has broken through the shales and sandstones and entered a lower layer of limestone. Faults have also been responsible for displacing different layers of limestone and bringing them in contact with each other, by passing the shales and sandstones in between. Black Keld Catchment Area is of importance in demonstrating the effects of geological structure on the development of a cave system.

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