The Strac Fada Member is a distinctive layer towards the top of the Mesoproterozoic Bay of Stoer Formation, part of the Torridonian Supergroup. Evidence for a bolide impact centered on Ullapool was published by a combined team of scientists from the University of Oxford and the University of Aberdeen, in March 2008. The evidence is centred on Ullapool, a harbour town on Loch Broom in the Ross and Cromarty district of the Highland council area of northwest Scotland. This suggests it was the largest bolide impact ever to strike what are now the British Isles. The impact, which has been dated to 1177±5 million years ago, melted rock at the site and left parallel shock fractures in quartz and biotite and a tell-tale trace of iridium. Centered on the impact crater, a wide ejecta field has been traced, some 50 km across, forming the Stac Fada member within the Stoer Group of the Torridonian. The affected layer of rock, which on land stretches from Gairloch in the south to Stoer in the north is six to 22 metres thick.