Limestone 'drystone' wall at Black Keld. Except for 'Lava tubes' found in volcanic rock and caves made by faults in rocks, caves are generally found in Limestone. The limestone of the English Yorkshire Dales was formed approximately 360 million to 320 million years ago, in the geological period known as the Lower Carboniferous, or in the U.S. the Mississipian period. The Limestone was formed under the sea and is composed of the remains of crinoids (filter feeding sea 'lilies'), lime-encrusted green algae, or calcium carbonate shaped by waves. With the shifting of the Earth's plates the limestone was lifted up, far from the place of it's creation in the warm shallow seas, to form the backbone of the Pennine Hills in Northern England. It is in this rock that the caves are formed and that man has built his walls and shelters. |
