Knockan Crag (Scottish Gaelic: Creag a' Chnocain, "crag of the small hill") lies within the North West Highlands Geopark in the Assynt region of Scotland 21 kilometres (13 mi) north of Ullapool. During the nineteenth century Knockan Crag became the subject of much debate when geologists noted that the Moine schists at the top of the crag appeared to be older than the Cambrian and Ordovician rocks such as Durness limestone lower down. Disagreements over the processes that could have caused this to occur were referred to at the time as the "Highlands Controversy". The argument was primarily between Roderick Murchison and Archibald Geikie on the one hand and James Nicol and Charles Lapworth on the other. Murchison and Geikie believed the sequence was wrong and that the Moine schists must be the younger rocks. The controversy was finally resolved by the work of Ben Peach and John Horne whose 1907 paper on the subject remains a classic text. Peach and Horne demonstrated that the situation resulted from the action of a thrust fault - this being the first to be discovered anywhere in the world. The older rocks had been moved some 70 kilometres to the west over the top of the younger rocks due to tectonic action.
The crag is designated as a national nature reserve (NNR) due to its geological features, and is owned and managed by NatureScot. There is a car park and interpretation centre that explains the geology of the area and gives background to the Highlands Controversy, along with three waymarked trails that take visitors to points of interest across the site. The site also hosts artworks such as 'The Globe' by Joe Smith and 'Pipeworm’ by Susheila Jamieson that were commissioned to highlight the inspiration that the landscape has had on artists and poets.
