Located in a shelf on the western flank of Heron Pike, Alcock Tarn lies 1000 feet above Grasmere village. It was originally a natural tarn named Butter Crags Tarn, until the late 19th century when Mr Alcock of The Hollins in Grasmere enlarged the tarn by means of a small stone & earth dam to create a trout lake.

It is well worth the effort of climbing 1000 feet from Grasmere, being a rather attractive tarn with grassy banks and some reedbeds around the edge. The views to the south and west, with Grasmere in the valley below, are stunning.