Malham Tarn is an upland lake. A tarn forming in an area of limestone is very unusual. The reason why the water has accumulated is because the bedrock beneath the tarn is impervious Silurian slate. The outflow stream from Malham Tarn disappears into the ground through sink holes (Water Sinks) in the limestone, and reappears at Airehead Springs. The tarn was formed from glacial melt water after the last ice age 18000 years ago.
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