Lewisian, Torridonian and Moine Rocks of Scotland
(2009)
GCR Volume No. 34
Mendum, J.R., Barber, A.J., Butler, R.W.H., Flinn, D., Goodenough, K.M., Krabbendam, M., Park, R.G. and Stewart, A.D.
This volume describes the outcrops of Precambrian rocks that lie north-west of the Great Glen Fault in the North-west Highlands of Scotland and in the Outer Hebrides and parts of Shetland. The rocks and their convoluted tectonic and metamorphic histories chart the first 2700 million years of the geological development of Great Britain.

Summary

 

The Lewisian, Torridonian and Moine rocks of Scotland were formed when continents were in far different positions to those seen today and provide tantalizing fragmental evidence of multiple ocean–continent and continent–continent collisions, lost volcanic arcs, and long-disappeared oceans.  The geological provenance of Highland Scotland is bound up with that of Greenland and North America to which it was attached until some 55 million years ago.

 

The main component networks are: the Lewisian Gneiss Complex, whose igneous protoliths and resultant gneisses date back to over 3,000 million years; the Torridonian rocks, mainly sandstones deposited 1,200 to 1,000 million years ago; and the Moine Supergroup, sandstones, siltstones and mudstones, similarly deposited at around 1,000 to 950 million years ago.  The Moine rocks have been deformed and metamorphosed during several separate orogenic events, an early Neoproterozoic Knoydartian Event, and later Caledonian events that occurred in two main phases, an early Ordovician Grampian Event and a later mid-Silurian Scandian Event.  The Grampian Event reflects the collision of an oceanic arc with the former Laurentian continent whereas the later Scandian Event reflects the collision of Baltica with Laurentia.  This last Caledonian orogenic event resulted in the thrusting of the deformed and metamorphosed Moine Supergroup and its underlying Lewisianoid basement, some 100 km towards the WNW onto the Lewisian Gneiss Complex and Torridonian and Cambrian sedimentary cover of the Foreland.  These distinctive foreland units are incorporated in the Moine Thrust Belt where they form complex nappes and thrust systems.  This was one of the places where large-scale thrusting was first recognized back in the 1880s.  It now provides a natural laboratory where the complex mechanisms and timing of such tectonic processes have been studied in detail and models for thrust formation have been rigorously tested.

 

Igneous intrusions within the Lewisian and Moine rocks both pre-date and post-date the main deformation events and many have been used in pioneering geochronological studies.

 

The GCR sites encompass a very wide range of rock types of different ages, affected by tectonic processes that have occurred over many millions of years.  Indeed, only a few sites focus on a single geological feature.  The mountain scenery of the North-west Highlands in places strongly reflects these varied rock-types.  This is nowhere better illustrated than in the Assynt district of Sutherland, where the Torridonian basal unconformity has been exhumed and sandstone mountains protrude upwards from the peneplained surface of the Lewisian gneisses.  However, elsewhere it is Cainozoic peneplained surfaces and more-recent glacial processes that have controlled the current topography, effectively masking the complex lithological and structural template beneath.

 
 
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722 pages, illustrations, A4 hardback
ISBN 978 1 86107 483 6
 
Please cite as: Mendum, J.R., Barber, A.J., Butler, R.W.H., Flinn, D., Goodenough, K.M., Krabbendam, M., Park, R.G. and Stewart, A.D. (2009) Lewisian, Torridonian and Moine Rocks of Scotland, Geological Conservation Review Series, No. 34, Joint Nature Conservation Committee, Peterborough, 722 pp.