Great Urswick Tarn

Urswick Tarn, a rare marl tarn at Great Urswick in South Cumbria.
A marl tarn or lake was created by glacial activity carving out a hollow in the limestone bedrock. A normal lake has a mud floor, whereas a marl lake is surrounded by limestone which gives the water a high calcium carbonate content and often a turquoise tint. Thanks to a band of impermeable shale in the limestone at the base of Great Urswick Tarn, the water has been retained.