Geological mapping in NW-Scotland: The Moine Thrust Belt
The northwestern margin of the Caledonian Orogen (430–400 Ma) in Scotland is defined by the Moine Thrust Belt, a major, gently SE-dipping décollement. The foreland (footwall) of the thrust belt consists of 2.7 to 3.1 Ga Lewisian gneisses, unconformably overlain by Torridonian sandstone, in turn covered unconformably by the Cambro-Ordovician passive continental margin sequence of Laurentia. The thrust belt is made up of shallowly (3°) dipping master thrusts, i.e., roof and sole thrusts, and numerous imbricate thrust slices between the master thrusts. The imbricate thrusts developed preferably in Cambrian-Ordovician sedimentary rocks, notably the Fucoid Beds. Total displacement on the thrust belt amounts to about 100 kilometres. Fault rocks consist of brittle faults and mylonite, which formed under greenschist-facies metamorphic conditions.
Until the beginning of the last century, the Moine Thrust Belt was subject of the famous geological controversy, the so-called “Highland Controversy”. Then it was unclear, how Moinian rocks could overly red sandstones of the Torridonian and, at the same time, form the substrate of the Old Red sandstone; both sandstone units were believed to be the same then. Similarly, in the Assynt region geologists observed that Lewisian gneisses obviously formed the substrate of the Torridonian sandstone west of what is now known as the sole thrust, whereas east of this thrust, the same gneisses cover Torridonian sandstone. It was Peach and Horne, two mapping geologists of the British Geological Survey, who solved this paradox by detailed mapping with their discovery that rocks in the Assynt region form an imbricate thrust belt (Peach at al. 1907). Recognition that rocks could be transported over large distances on shallowly dipping thrust surfaces was contemporaneous with the identification of allochthonous nappes in the European Alps. This was a major breakthrough in geological science, notably with respect to better understanding the formation of mountain belts.
The mapping course is conducted in the heart of the Moint Thrust Belt and consists of map segments of the sole thrust, Glencoul Thrust and the imbricated Cambro-Ordovician sedimentary sequences near Inchnadamph. During their field work, Peach and Horne resided in the Inchnadamph Hotel, located near the Inchnadamph Lodge, which became The Field Centre of Assynt in recent years and which hosts many university groups of geology students each year.