Dock Tarn is a small tarn located within the Lake District National Park in Cumbria, England at grid reference NY273143. It is situated on moorland at 400 metres above sea level near the summit of Great Crag, midway between Watendlath, the Stonethwaite valley and Borrowdale. It measures approximately 300 by 200 metres, and the shoreline is indented with rocky headlands and bays. There is a tiny island in the tarn with a few small Rowan trees growing on it.