Blackford Hill is a prominent Edinburgh landmark, one of the seven hills of Edinburgh. It is composed of a single rock unit, a layer of purple-coloured andesite lava formed about 410 million years ago in the Devonian Period. This is part of the Pentland – Fairmilehead – Braid – Blackford volcanic succession (Pentland Hills Volcanic Formation), and the andesite lava flow at Blackford Hill is one of the youngest lavas in this succession. The lava is underlain by a thin, poorly exposed volcanic ash layer, called a tuff, and a lava flow that forms Braid Hills to the south. The Hermitage of Braid lies between Blackford and the Braid Hills, and is a deep wooded gorge containing the Braid Burn.
Agassiz Rock, located to the south of Blackford Hill near Blackford Quarry, has an important place in the history of geology: at this site in 1840, the visiting Swiss geologist Louis Agassiz proclaimed scratches and rock moulding as “the work of an ice sheet”. This was the first recognition of the role of ice erosion in shaping Scotland’s landscape and was reported in the Scotsman Newspaper by its Editor, Charles MacLaren.
Ice had an important role in shaping the wider landscape, with ice movement from west to east. This gives Blackford Hill, the smaller Corbie’s Crag near the summit, and several of the other hills of Edinburgh a common ‘crag and tail’ shape, with a steep westerly side and more gentle slope to the east.