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A natural upland lake brought to the surface by the north craven fault, malham tarn lies on a floor of older (400 million years old) impervious silurian slate covered with thick glacial drift and marl deposits. it has a surface area of around 150 acres, average depth is approximately 2.4 metres and the maximum depth is around 4.4 metres.
The tarn basis was dammed by a moraine from the retreat of the devensian ice-sheet from the last ice age and prior to silting and mire development the tarn was originally twice it's present size.
The inflow waters have passed through lime-rich soil, and the one outflow stream flows out for a short distance before sinking into the carboniferous limestone when it reaches the line of the north craven fault and re-emerges further down at airehead springs.
In 1791 the level of the tarn was raised by approximately four feet (1.2m) with the construction of the dam and weir at tarn foot by lord ribblesdale. thus the previous shore-line and beach is now a drowned landscape and has also resulted in the erosion of tarn moss, a raised bog to the west of the tarn which has grown out at the mouth of the tributary streams coming off fountains fell.
The tarn was granted to fountains abbey and its medieval monks by william de percy in the 12th century, with all its fishing rights - the tarn has a reputation for its excellent trout. it also harbours a varied population of water birds - curlews, mallards, and greater crested grebe among them - protected in a sanctuary on the western shore.
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