In leaky limestone country, surface water usually eventually finds its way underground. Malham Tarn is unusual because here the water doesn't sink into the ground. During the Ice Age a glacier scraped away the rock right down to the ancient slate layer underneath the limestone. This slate is impermeable and so won't let water through. The mouth of the Tarn was then sealed when the glacier melted and dropped banks of clay and gravel across it.
Open water being so rare in the area, it became a magnet for herds of wild animals during the Stone Age and hunters from far and wide gathered to take advantage. Archaeologists have found the remains of their campsites and stone tools along the shoreline.
The Tarn and adjacent raised-bog have been designated as a National Nature Reserve (NNR) and are owned and managed by the National Trust. There are two small car parks near the Tarn and a beautiful route around it together with a lovely boardwalk and a bird hide to get you up close to the wildlife. There is also a tramper - an all terrain wheelchair - available for hire from the National Trust office.