The larger part of the island is formed from olivine-phyric basalt flows erupted during the Palaeocene epoch. Together with flows of hawaiite and mugearite, these form the Eigg Lava Formation. The Sgurr is formed from porphyritic rhyolitic pitchstone erupted into a valley during the Eocene epoch. It displays columnar jointing formed as the lava cooled. Outcrops of an underlying conglomerate containing clasts of Torridonian sandstone and basalt are visible in places. These igneous and sedimentary rocks are collected together as the Sgurr of Eigg Pitchstone Formation.