GEOLOGY FIELD TRIP APRIL 24TH 2019
GIGGLESWICK SCAR, CAVES AND WATER
Leader Josephine Drake
Giggleswick Scar is a fault-line scarp formed by the differential erosion of the softer Bowland Shales of the Craven Basin which lay against the harder Great Scar Limestone along the South Craven Fault. The old road from Settle going west climbs under the scar so running along the site of the South Craven Fault. Within the scar are several caves which were explored as much as is possible by non cavers not wearing proper equipment, not even hard hats but we did have torches! However, the caves are very short but do show evidence of how they were formed. Rain water falls on the tops then percolates through the limestone following lines of weakness and bedding planes. The weak carbonic acid ( Co2 in water) dissolves the limestone enlarging the routes, and where this is below the water table the cavities are filled so all the walls are eroded forming a tube – a Phreatic cave. If the route is above the water table there is only water running in the bottom so forming a deeper and narrower lower section – a Vadose cave. The caves we looked at were of the Phreatic kind, we could see scalloping on the upper walls showing the direction of flow. In one cave we could also see some slow water flow and tufa forming as the calcium carbonate settled out, sometimes known as flowstone but is crumbly, (also seen at Janet’s Foss and Gordale Scar). In the past much heavier flows of water came off impermeable rocks, which have since been eroded away from above the limestone, and flowed out from the rock at these caves – a resurgence, and is a remnant of an ancient landform as the water table was much higher then. On Giggleswick Scar there are no streams, the only source of water being rain. For the farmer and his livestock this provides a problem. We saw one or two Dew Ponds, man made hollows then lined either by mud and clay or, in this case, concrete, which collect dew, water from low cloud or rain and can be very effective. Higher up Helwith Flags had been used to form a collection point for water from a small spring then lead it into a bigger reservoir formed by concrete lined stone walls. Such is the effort required to provide water for animals here where there is no natural surface water it all having sunk into a percolation aquifer.
We had followed the walk from Exploring the Limestone Landscapes of the Three Peaks and Malham by Phillip J. Murphy, BCRA Cave Studies Series: 15
Notes from Geology trip to Giggleswick Scar
What limestone were we on?
Lower Hawes Limestone. Though this is the base layer of the Yoredale Series, in the west of the Dales it is regarded as part of the Great Scar Limestone Group as it is ‘lithologically similar to the underlying limestones’.
Tufa Travertine Stalagmites and Stalactites – what are they?
Speleothems is the name for Calcite deposits in caves. Travertine has been defined as all non-marine carbonate precipitates in or near terrestrial springs, rivers, lakes and caves. Calcareous Tufa is a form of Travertine but is less crystalline and more porous. Porosity is due to the inclusion of aquatic plants especially algae and mosses, living and dead, and clay silt and sand.
Why the layers in the ‘flows’ we saw?
Soil derived water containing increased Co2 from plants and microbes forms weak carbonic acid. This acid reacts with calcium carbonate to form a solution of calcium and bicarbonate ions. When the water emerges from the rocks at a resurgence it loses much of the soil carbon dioxide, into the atmosphere, and the calcium carbonate precipitates out. The rate at which Co2 is lost depends on the rate of flow of the water and its turbulence so variations in rainfall weather etc will alter the rate of deposition thus forming different layers.
Can the layers block the flow of water?
Gours or Rhimstone is the name for when the calcite speleothems form a dam, usually at the edge of a pool.
This is all information I have gathered this morning on further reading Caves and Karst of the Yorkshire Dales. I hope it helps. The book can be borrowed from the UWFS library.
Text by Dr. J. Drake, photographs by P. Childs.