Dock Tarn is a natural tarn standing on moorland close to the summit of Great Crag in the Central Fells area of the Lake District. Its surface elevation is 400 m (1,312 ft) and it covers an area of around 300 m by 200 m (984 ft by 656 ft), with a tiny island which carries a few small rowan trees. The tarn is a peaceful haven for picnicking and viewing wildlife, particularly dragonflies which thrive in the reeds growing in the water in spring and summer. Red deer may be seen in the surrounding moorland. The tarn's outflow is Willygrass Gill; and it can be approached either from Watendlath via a 2.5 km / 1.5 mi walk along path that skirts the summit of Great Crag, or from Stonethwaite, a far more strenuous walk.