Definition of Lewisian in English:



  • 1Relating to or denoting the earlier stage of the Proterozoic aeon in NW Scotland, from about 2,500 to 1,100 million years ago, when the oldest rocks in Britain were deposited.

    • ‘This displacement has been achieved with virtually no internal deformation within the Lewisian rocks of the thrust sheet, save a few spaced fractures, now marked by epidotechlorite seams.’
    • ‘The Assynt region, NW Scotland, is a world-famous site to view the Moine Thrust Zone and its foreland geology of metamorphic Lewisian basement overlain by red-bed deposits of the Torridonian and the Cambro-Ordovician shelf sequence.’
    • ‘The Archaean to Proterozoic Lewisian complex, exposed along and in the Moine Thrust Zone, represents the last stages of the growth and reworking of the Hebridean craton.’
    • ‘Therefore, the Lewisian rocks from the Assynt terrane of the Caledonian foreland are an unlikely source for any of the sediment.’
    • ‘In contrast, the Lewisian gneiss and basal Torridonian within the Main Ring Fault are not in situ, and the depth at which they were metamorphosed may have varied during the development of the igneous complex.’
    1. 1.1as noun the Lewisian The Lewisian stage or the system of rocks deposited during it.


Mid 19th century: from the name of the island of Lewis and Harris in the Outer Hebrides (where the chief outcrops of these rocks are found) + -ian.