Shake Holes near Malham Tarn EarthCache
Shake Holes near Malham Tarn
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Ths cache has been placed with the kind permission of the National Trust Estates Office; Yorkshire Dales.
Malhamdale. Bill Bryson once described the area as possibly heaven on Earth, is spoilt for choice in dramatic scenery and features.
Visitors to the area will gather for the delights of Malham Cove, Gordale Scar, Malham Tarn and Watlowes valley to name but a few.
However to the more observant, especially those on foot, there are distinct pock marks and holes in the ground. These litter the area and are especially numerous in the area between Malham's Cove and Tarn.
Limestone is an organic, sedimentary rock. This means it was formed from the remains of tiny shells and micro-skeletons deposited on the sea bed. They were compressed to form solid rock. Limestone is made up of calcium carbonate and reacts with diluted hydrochloric acid. Limestone is formed in layers - called bedding planes. These bedding planes contain vertical cracks called joints. Joints and bedding planes make the rock permeable as the acidic rainwater expolits and erodes the cracks.
Sinkholes are common where the rock below the land surface is limestone. As the rock dissolves, spaces and caverns develop underground. If there is not enough support for the land above the spaces then a sudden collapse of the land surface can occur. In areas of soil or boulder clay coverage the material is washed into the cracks or fissures below causing the land to slump.
While not as dramatic as swallow holes where surface water disappears deep underground, they are, in their own way an important geographcal feature of the landscape. This water will percolate down and reappear at Airehead Springs, a couple of miles south of the cove.
In the case here between Abbotts Hills and Torlery Edge, they look like shallow depressions formed by mining activity or military shell craters.
To claim this cache please:
1) Go into the large shake hole at the GZ. Estimate the maximum length, width and depth of the formation
2) Estimate the height difference between the shake hole and resurgent stream at the base of Malham Cove
3) Suggest what the landscape may look like in a wetter climate. Why will there be so much surface water? What might cause this to happen
Would be very grateful for any photos you would like to include.
Additional Hints
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