NW Scotland contains some of the best exposed geology in the British Isles, a unique region of unrivalled diversity. It records such diverse events as the early growth of continental crust more than two billion years ago, landscapes and sediments almost one billion years old, several periods of ancient mountain building, basin development during the past two hundred million years with analogues for processes responsible for the UK's offshore oil and gas resources, a short but dramatic period of volcanic activity some 55 million years ago and the sculpting of the modern landscape through glacial action in recent times. Given the relatively easy access to Victorian seats of learning it is not surprising that NW Scotland inspired seminal studies in many earth science disciplines. Amongst the most impressive of these was the discovery and description of the Moine Thrust Belt in the 1880s. Since then this belt has played a major role in the development of concepts and methods in structural geology together with broader issues of mountain building. It has also been important for understanding the particular evolution of the ancient Caledonian mountain belt to which it defines the western margin in Europe.
The importance of the structural geology of the Moine Thrust Belt for the heritage of UK geoscience led, in the 1970s and early 80s, to the designation of a collection of Sites of Special Scientific Interest. These and other prospective SSSIs formed the core of the network of sites collated under the UK-wide Geological Conservation Review. The GCR was initiated by the old Nature Conservancy Council and is now promoted by the Joint Nature Conservation Committee. The over-arching principle is to document and justify for conservation, the truly key field locations of the UK's geological heritage. The site networks are themed to a particular topic. Examples include the Quaternary of Scotland, Caledonian Structures in Britain (south of the Midland Valley of Scotland) and the British Tertiary Volcanic Province. The aim of a network of sites is to record a representative range of geology within a specific topic, not only the best examples of specific features but also those representative of diversity and of historical importance. In the context of the Moine Thrust Belt, the existing array of GCR sites aims to show the range of structural styles along the belt and reflect the historical development of tectonic research in NW Scotland. However, since 1980 and the initial designation of GCR sites in NW Scotland, there has been a burst in academic research in the Moine Thrust Belt. This has including extensive remapping of ground not seriously studied since the original work by the Geological Survey in the 1880s. Further, the tendency to pigeonhole geological topics under distinct networks, a policy that runs counter to the integrative, systems approach of much current geoscience research, has left some key aspects of Moine Thrust Belt geology unrepresented in the GCR. For these reasons it is proposed that the existing Moine Thrust Belt network be enlarged to increase its relevance to the geoscience community in particular and to Scotland's geological heritage in general.
The network of GCR sites should satisfy a number of different demands.
The existing GCR sites are fairly good at achieving the first two ambitions. It is in the third category that the current network falls short.
The following ten representative transects are proposed here, together with the constituent GCR sites (new and proposed):
Eriboll (existing GCR site), Faraidh Head (existing GCR site), Sango Bay (Durness - NEW)
This transect shows extensive development of mylonites in the hanging-wall to the Moine Thrust, basement thrust sheet (Arnabol), imbrication of Cambrian strata, the scale of horizontal displacements on the Moine Thrust, strain and deformation mechanisms in thrust sheets, disruption of the thrust belt by post-Caledonian faulting. The existing Eriboll site gave the world two key structural terms - "thrust" (Geikie 1884) and "mylonite" (Lapworth 1884) and, with the Glencoul site, was key in establishing thrust tectonics in NW Scotland (Lapworth 1884).
The Sango Bay site was key in providing a clue to the scale of horizontal displacement on the Moine Thrust for 19th century researchers. It remains one of the most accessible outcrops of the Moine thrust surface and its associated mylonites in NW Scotland. It also includes amongst the finest exposures of post Caledonian faults in the British Isles.
NEW
This transect shows the most extensive development of imbricate thrust systems within a single rock unit (Pipe Rock Formation). The transect includes amongst the world's finest examples of detailed thrust structures exposed in 3D and is one of the type-examples of duplex structures. It also shows critical evidence for foreland-directed thrust belt migration (piggy-back thrusting).
Note: although Foinaven is not currently part of the Moine Thrust Belt network, it is an existing SSSI for
(existing GCR site).
This transect includes two of the most famous sites in thrust tectonics - the Glencoul Thrust (one of the first such structures recognised, Callaway 1884) and the Stack of Glencoul (for its spectacular exposures of mylonites and the ductile Moine Thrust). It is the finest transect involving the thin (600m) basement thrust sheets which remains one of the outstanding feature of the thrust belt. The mylonites and related structures show evidence for extensive extensional flow.
Central Assynt (composite existing site), Inchnadamph (composite existing site), Cam Loch (existing site).
The Assynt culmination is historically the most studied part of the thrust belt. This transect shows the involvement of basement sheets and, through the preservation of klippen (apparently of the Ben More sheet), shows the scale of displacements in the footwall to the Moine Thrust. The area is also key for showing the relationship between Caledonian structures and syn-tectonic igneous intrusions which constrain timing of orogenic events in NW Scotland. The igneous sites are well represented in the GCR network covering Caledonian igneous rocks.
(existing GCR site)
This famous section in southern Assynt shows how the Moine sheet has moved late, truncating structures in its footwall so that the Moine now lies directly against the foreland. Note that strictly the fault at the base of the Moine sheet is not the Moine Thrust but a later structure. This transect is important for demonstrating this deviation from the otherwise commonplace foreland-directed evolution of thrusting. It may indeed represent the localised equivalent to the ductile extensional flow that modified the higher part of the Glencoul transect.
NEW
About a quarter of the length of the Moine Thrust Belt is characterised by the Moine Thrust lying directly against Cambrian strata of the Caledonian foreland, or separated from it by only a few metres of "far-travelled" slices of Torridonian and Lewisian rocks. The best-exposed transect through this geology, from Torridonian up through a tectonically decapitated Cambrian succession, lies east of An Teallach. The Moine Thrust carries a few metres of highly deformed Torridonian. This site is not currently part of the Moine thrust Belt network but is part of the existing GCR network on Quaternary geology.
Meall a'Ghuibhais (existing GCR site), Slioch-Glen Bruachaig (NEW)
The thrust belt in the Torridon area is represented by a wide imbricate system. The northern termination of this structure is at Loch Maree. This Caledonian structure is apparently nucleated upon a Precambrian fault (the Loch Maree Fault) that in turn controls the thickness of Torridonian strata. The Slioch transect includes arguably the most spectacular incised unconformity surface in the British Isles, a key section into a basement thrust sheet (the Kinlochewe sheet) and a window into imbricated Cambrian strata (one of the very few such features in NW Scotland). These relationships complement those on the south side of Loch Maree (existing Meall a'Ghuibhais site) which include Torridonian-Cambrian imbricate systems and a klippe of the Kinlochewe sheet. The outcrops of the Kinlochewe sheet in Glen Bruachaig are of historical importance in developing the early thrust interpretations in NW Scotland and for the application of petrographic studies in structural geology.
NEW
South of Loch Maree the Moine Thrust Belt is represented not only by a high-level basement sheet (the Kinlochewe/Kishorn sheet) but also by an imbricate stack of Torridonian and Cambrian strata. The imbricate slices are, in contrast to those made up of just the Cambrian quartzites, much thicker and the corresponding thrust belt structure is much more simple. Consequently the detailed 3D geometry of individual thrust slices may be traced for many kilometres giving unrivalled insight into the architecture of thrust ramps and related folds. The best exposed and most continuous transects crop out on the ridges that include the tops Ruadh Stac Mor, Beinn Liath Mor and Sgorr Ruadh.
(existing GCR site)
The lateral termination of thrust culiminations can rarely be defined although there is a general notion that it can occur by the systematic lateral climb of imbricate thrusts to higher stratigraphic levels. In this site these geometries are clearly evident as imbricates of Torridonian and Cambrian quartzites (see Sgorr Ruadh transect) cut upsect to the SW into the younger Durness carbonates. On a large scale this results in a culmination in the overlying Kishorn thrust sheet. This sheet is important because it marks the northern limit of major folding, a style of deformation that dominates the thrust belt further south. The transect also contains a zone of Mesozoic faulting that runs offshore into the Hebrides basin system.
(existing GCR sites Ord, Tarskavaig, Slumbay Island)
The southernmost part of the Moine Thrust Belt is radically different to the rest. It is dominated by a major west-facing inclined syncline (the Lochalsh syncline) with an overturned limb of over 8 km width. Other key features include the enigmatic Ord Window, probably one of the least understood structures in the entire thrust belt. It is a tectonic inlier of faulted and folded Cambrian strata within the Kishorn sheet.
The existing GCR sites reflect both the history and the range of structures, both in style and scale, that can be found in the Moine thrust belt. However, they currently do not represent the ways in which individual structures interact to form a thrust belt. The Moine Thrust Belt displays a rich diversity in transect style. Several of the transects are of international importance - particularly for demonstrating the sequence and relative timing of thrust development , the ways in which basement can be involved in upper crustal thrusting. Others are important for understanding the evolution of the Scottish Caledonides through testifying to the scale of displacements. Consequently a selection of FIVE new GCR sites are proposed which will greatly enrich the existing network for the Moine Thrust Belt. The new proposed GCR sites are: