Grey Mare’s Tail is one of the most spectacular landmarks in southern Scotland. Situated on the north side of Moffatdale's deep U-shaped valley, the falls are the product of different rates of glacial erosion during the last major glaciation around 15,000 years ago. The deep trench of the Moffat Water Valley was carved out by a huge glacier, while in the side valley a smaller glacier with far less erosive power created a much shallower valley. Once the ice melted the side valley was left elevated, or 'hanging', above the main valley.

Perched high above the Moffat Water, the Tail Burn flows through the hanging valley and then tumbles abruptly over a series of cascades and plunge pools into the valley below. With a main fall of around 60m, Grey Mare’s Tail is one of the highest waterfalls in Scotland, and the fifth highest cascade in Britain.

From the 19th century Grey Mare’s Tail became a notable tourist attraction, and was visited by a number of famous sightseers including Sir Walter Scott, who described it in his poem Marmion as the ‘roaring linn’.