Geosites Project

Superb exposures of the Moine Thrust, associated deformed "Lewisianoid" rocks carried by it and some of the best exposures in the UK of normal faults and their fault rocks in carbonates, associated with basin development offshore.

The Sango Bay outcrops are critical for establishing minimum displacements on the base of allochthonous, mylonite-bearing thrust sheets in northern Scotland. This was first recognized by Peach & Horne in the late 19th century and hence the GCR site is of considerable historical importance. The outcrops are clean and readily accessible, making them admirably suited for study, unlike some other parts of the Moine Thrust Belt. Further, there is an excellent array of outcrops that show the progressive retrogression and shearing of Lewisianoid gneisses related to the thrusting. The kinematic indicators in the high-strain -Oystershell Rock- are particularly clear and imply a consistent WNW-directed emplacement direction.
The site also contains exceptional exposures of post-Caledonian (probably Devonian) normal faults with spectacular carbonate-rich fault rocks. These show complex cyclic histories of brecciation and cementation that are characteristic of seismogenic faults. This is also one of the very few places where Caledonian thrusts are clearly cut by post-Caledonian normal faults with the overprinting of the earlier mylonites by later cataclasites clearly demonstrated. Although some workers have inferred that Caledonian thrusts have been preferentially re-activated during post-Caledonian normal faulting, in the Sango Bay outcrops this type of behaviour is not seen in the exposed, shallow crustal level faulting.