A steep little gorge in NE Dumfries and Galloway, Hartfell Score lies on the southwestern flank of Arthur's Seat, below Hart Fell. The Spa Well Burn descends through it. The gorge is of geological interest comprising blocks of black Moffat Shale which were thrust upwards with the closure of the Iapetus Ocean around 425 million years ago. These Ordovician sediments contain graptolite fossils, which were used by Charles Lapworth in 1878 to decode the complex geological structure of the area. It was also described in 1899 by Ben Peach (1842 - 1926) and John Horne (1848 - 1928). The gorge has also been explored for copper and the remains of a trial adit can be observed. Further upstream is a Hartfell Spa, a chalybeate spring.
In 2004, the rocks at Hartfell Score were proposed as a Global Boundary Stratotype Section and Point (GSSP), the internationally-agreed reference section, for the base of the middle stage of the Upper Ordovician on the geological time scale.
