in Shetland the Wester Keolka Shear was proposed to represent the Moine Thrust, although more recent radiometric dating results suggest that this is unlikely to be the case. The link between the structures imaged offshore and those onshore remains unclear. Southwestward continuation Southwest of Skye the course of the Moine Thrust Belt becomes uncertain. It is assumed that it must pass to the southeast of the mainly Lewisian islands of Coll and Tiree. It is often shown passing through the Sound of Iona west of Mull, although no thrusts are actually exposed. It is presumed to be truncated by the Great Glen Fault zone somewhere southwest of Mull. Timing Movement on the Moine Thrust Belt has been dated partly on its relationship with a series of igneous intrusions in the Assynt Window and partly from the results of radiometric dating of deformed rocks from the footwall or hanging-wall of the thrusts. The intrusions near Loch Borralan and Loch Ailsh have similar chemistry, both being forms of syenite, and have been dated at 430±4 Ma and 430.6±0.3 Ma respectively suggesting that they were intruded at about the same time. The Loch Ailsh intrusion outcrops within the Ben More thrust sheet and predates movement on that structure. The Loch Borralan intrusion lies beneath the Ben More Thrust. Locally the intrusion has been said to cut across the Ben More Thrust, but a lack of the metamorphic effects found elsewhere along the margin of the intrusion suggest that this contact is tectonic rather than intrusive. In the upper part of the Borralan intrusion, there is evidence of deformation of the syenite while it was still hot, suggesting that thrusting started soon after it was intruded. IUGS geological heritage site In respect of it being 'the classic orogenic front of significant importance in both modern and historical tectonics research', the International Union of Geological Sciences (IUGS) included the 'Moine Thrust Zone' in its assemblage of 100 'geological heritage sites' around the world in a listing published in October 2022. The organisation defines an IUGS Geological Heritage Site as 'a key place with geological elements and/or processes of international scientific relevance, used as a reference, and/or with a substantial contribution to the development of geological sciences through history.' See also Knockan Crag Inchnadamph Geology of Scotland References Further reading Plate tectonics Structural geology Geology of Scotland First 100 IUGS Geological Heritage Sites