Coire Glas: A window into the Great Glen Fault

Overview of the Great Glen from the Coire Glas site (BGS © UKRI 2026).

To help de-risk renewable energy infrastructure and potential sites of energy storage, characterise underground nuclear storage facilities and proposed nuclear power stations and assess seismic hazards, it is essential to understand crustal-scale fault zones. Yet, finding rocks associated with the core of major crustal fault zones can be extremely difficult because faults typically comprise very weak, deformed and altered rocks that are highly susceptible to erosion. 

As part of the ground investigations at the Coire Glas project, a proposed pumped storage hydroelectric power station in the Scottish Highlands (coireglas.com), Scottish Southern Energy (SSE) and its contractors drilled hundreds of metres of core, reaching up to 500 m below ground level, and a 1,200-m-long exploratory adit from Loch Lochy towards the planned undergound powerhouse cavern. A combination of horizontal and vertical cores were drilled from the end of the adit across the base of the mountain. The drill cores and the adit penetrated the fault’s core, then went through the fault damage zone and beyond into less damaged rock, providing a unique view of the Great Glen’s fault rocks. 

Drilling geological core is incredibly expensive and is normally only justifiable at such extensive depths as part of major energy or infrastructure projects like Coire Glas. An added challenge with the Great Glen Fault is that, beyond its location in the remote Highlands, fault rock can be highly altered and weak, thus presenting a technical challenge to successful drilling. 

Underground fault ‘observatories’ have been developed for other major structures such as the San Andreas Fault, USA, and the Alpine Fault Zone, New Zealand, but these only provide limited access to the associated fault rocks and adjacent damage zones. Coire Glas provides unprecedented insight into the inner workings and behaviour of crustal-scale faults, and scientists from the British Geological Survey (BGS) and Durham University were given access to the site to review new drill core and sections. 

Schematic model showing how key renewable energy infrastructure interacts with the geology of crustal-scale fault zones (BGS © UKRI 2026).

The Great Glen Fault 

The Great Glen Fault is the largest geological fault in the UK and is one of the most significant manifestations of late Caledonian orogenesis. It was also re-activated during the later Paleozoic, Mesozoic and Cenozoic, although there is no unequivocal evidence that it is still active today.  

The fault stretches from Ireland all the way through Scotland and continues offshore, as part of the Walls Boundary Fault, to Shetland—a total along-strike distance of roughly 1,000 km. It has a profound influence on the landscape within northern Scotland and forms the major Great Glen valley, which is characterised by the deep lochs of Loch Ness and Loch Lochy.  

The Caledonian Orogeny represents the final closure of the Iapetus Ocean during the collision of Laurentia (North America and Scotland), Baltica (Scandinavia) and Avalonia (England and Europe) in late Ordovician to early Devonian times. This orogeny is associated with a major phase of thrusting and metamorphism, followed by the development of a series of north-east trending, sinistral (left lateral), strike-slip faults including the Great Glen Fault. The strain field associated with these faults also profoundly influences the development of a whole series of early to middle Devonian basins stretching from Ireland to Norway, which formed as the Caledonian Mountains began to collapse.  

Today, the fault underlies the Great Glen, having been scoured out by glaciers during the Pleistocene glaciations. As a result, the Great Glen fault rocks remain mostly hidden to the human eye by the waters of Loch Ness, Loch Oich and Loch Lochy. Quaternary glacial deposits are found along the valley floor, and slope deposits on the valley sides. Occasional exposures of mylonites are found scattered along the Great Glen, from Torcastle (near Fort William), along the shores of Loch Ness to Rosemarkie on the Moray Firth coast (near Inverness). These rocks represent exhumed slivers of deeper crustal, amphibolite grade, ductile deformation associated with sinistral shear during the late Caledonian at depths greater than 15 km. Later brittle features that formed at shallower depths are observed within these exposures, but they are relatively weakly developed.  

3D LiDAR of the exploratory adit projected within the BGS Geovisionary software with selected adit face photographs (BGS © UKRI 2026).

The geology of Coire Glas 

The Coire Glas project site is located on the northern side of Loch Lochy in the Great Glen. The proposed pumped storage hydroelectric scheme, with a potential generating capacity of up to 1,300 MW and a storage capacity of 30 GWh, is being developed by SSE Renewables. It is the first large-scale pumped storage hydroelectric project to be developed in the UK for more than 40 years and will more than double Great Britain’s existing electricity storage capacity. It will comprise an upper reservoir in Coire Glas itself, a series of tunnels and shafts, and a large cavern that will house the power station, with Loch Lochy functioning as the lower reservoir. Much of the underground workings occur within rocks that are part of the core or damage zone of the Great Glen Fault, with potentially profound geotechnical and engineering implications as to rock quality. 

The bedrock of the Coire Glas site includes fault rock associated with the Great Glen Fault and a sequence of metasediments of the Loch Ness (formerly Moine) Supergroup. The grey metasedimentary rocks, which are mainly psammite with subsidiary pelite and semipelite, have undergone amphibolite-grade metamorphism as part of the Caledonian and older orogenies, and form the main protolith rock for this part of the Great Glen Fault Zone. The site also includes numerous igneous intrusive rock bodies, ranging from metamorphosed amphibolites and diorites within the Loch Ness Supergroup to larger granites and pegmatites intruded along Caledonian structures, and a set of relatively undeformed dolerites. 

The Coire Glas project is an example of how in-depth ground investigation can provide significant benefits for the long-term success of major renewable energy projects, as well as leading to exciting new science 

The fault zone comprises heavily fractured, metasedimentary and igneous rocks with subsidiary fault zones that range from a granular scale to tens of metres wide. The fault fills range from breccia to cataclasite to clay gouge. The entire fault zone shows pervasive iron oxide alteration, giving a pinkish-red colour.  

A key feature of the Great Glen Fault Zone is its often-pervasive network of veins that demonstrate a complex fluid-flow history intimately associated with the brittle deformation. Within the main damage zone, multiple types of vein fill include hematite, quartz and several types of carbonate. Vein fills are particularly focused along brittle faults. Zones of east-to-west trending dolerite dykes and dolomite-filled fractures crosscut the Caledonian structural fabric and likely formed during later (possibly late Carboniferous to early Permian) phases of brittle deformation. The dolerite dykes range from relatively fresh to heavily altered by the incoming, carbonate-rich fluids. 

The northern boundary of the fault zone within the Coire Glas site is marked by a major, low-angle fault intersected by the exploratory adit. This fault comprises a 30-m-wide zone of fault breccia with localised zones of cataclasite and clay gouge. The entire zone has experienced extensive hydrothermal alteration with quartz and dolomite veining. Beyond this feature, the rocks become more massive with a low fracture density, a reduction in fault density and fewer veins. 

Drill core from the Coire Glas site showing key fault textures, vein fi ll and hydrothermal alteration (BGS © UKRI). 2026

The fault core 

Close to the shore of Loch Lochy, five boreholes were drilled in what is essentially the fault core of the Great Glen Fault. The LCW03 core intersected a 10-m-thick section of hard, pale-green fault rock with a well-developed foliation quite distinct from those seen elsewhere in the fault zone. The fabric indicates a thick fault unit that has experienced high levels of shearing, where existing pink granites and pegmatites are preserved as visually striking, lower-strain augen strongly wrapped by the fault rock fabric.  

The fabric of the LCW03 fault in some ways resembles the deeper mylonitic fault rocks seen in slivers elsewhere along the Great Glen. However, the fault lacks any wide-spread grain-scale crystal plasticity. Instead, it shows evidence of distributed, brittle, grain-scale deformation and is dominated by cataclasis and solution-precipitation creep textures. These deformation processes are typical of major intermediate depth (5 to 10 km) fault zones elsewhere in the world. 

Photograph of the green, foliated fault rock with augen of granite and psammite in LCW03 (BGS © UKRI 2026).

Although the core was not orientated, it preserves a reversal in the direction of shear. A large part of the analysed sample underwent later hydrothermal alteration during this reversal, characterised by dolomite, iron dolomite and calcite mineralisation. The core is interpreted as representing a unit of crushed fault rock formed in the core of the Great Glen Fault that has been hardened by some form of carbonate metasomatism during the subsequent reversal in shear sense, possibly in late Carboniferous to early Permian times.  

The LCW03 core and other drill cores are currently stored at the BGS National Geological Repository, the UK’s foremost collection of geological samples, where it will be made available for future research purposes. This will enable the long-term preservation of the Coire Glas core, allowing scientists to study and attempt to unlock secrets of the Great Glen Fault long into the future.  

Mineral map image generated using SEM of Great Glen Fault rock. The pale, purple-pink areas are interconnected networks of weak clay minerals formed due to alteration of the host rock minerals (mainly feldspar), into which shearing has localised. Host rock quartz (yellow) is highly fractured and veined. Pale and
dark blue areas are regions of later carbonate veins and cement. Image is 5 mm wide (BGS © UKRI 2026).

Once in a lifetime 

The Coire Glas site presents a once-in-a-lifetime opportunity to study a range of different geological processes that operated during the formation of the biggest and most famous crustal-scale fault structure in the UK. The work done by the geologists from BGS and Durham University, engineering geologists from SSE Renewables, COWI and Stantec, as well as drillers from Strabag and Fugro, has enabled a new understanding of brittle and hydrothermal fluid history at intermediate to shallow crustal depths within the fault zone. New drill core provides evidence for brittle fault structures on a range of scales that are acting as pathways for carbonate-rich fluids and magmas, potentially sourced from deep within the crust.  

The continuous, 800-m-long, underground exposure through the Great Glen Fault also helps us to understand the 3D relationship between key features within the fault zone, including fault structures, igneous intrusions, fracture networks and hydrothermal alteration. These processes have important implications for the engineering properties of rocks in complex fault zones, such as local to regional hydrothermal and deformational processes that alter the short- and long-term geotechnical properties of a rock mass.  

Coire Glas presents a once-in-a-lifetime opportunity to study the biggest and most famous crustal-scale fault structure in the UK 

The Coire Glas project is an example of how in-depth ground investigation can provide significant benefits for the long-term success of major renewable energy projects, as well as leading to exciting new science. This success was due to effective collaboration between geologists and engineering geologists to accurately characterise the geotechnical properties of complex, deformed, and altered rocks associated with the Great Glen Fault. This included providing consistent and accurate fault-rock descriptions in geotechnical baseline reports; creating a 3D geological model to accurately project major geological structures across the site; and understanding the strength properties of hydrothermally altered fault rock.  

The work has helped reduce uncertainty and risk for the main pumped storage development but has so far only scratched the surface of what can be achieved. With the core now stored at BGS, there is a major opportunity for many years of exciting research. 

Anyone interested in further studies on these unique cores is invited to contact the authors. 

Authors

Romesh Palamakumbura Senior geologist at the British Geological Survey, UK romesh@bgs.ac.uk 

Maarten Krabbendam Chief geologist (Scotland) at the British Geological Survey, UK 

Prof Bob Holdsworth Durham University, UK 

Further reading

• Holdsworth, R E, Stewart, M, Imber, J, and Strachan, R A. 2001. The structure and rheological evolution of reactivated continental fault zones: a review and case study.  115–137 In Continental Reactivation and Reworking. Miller, J A, Holdsworth, R E, Buick, I S, and Hand, M (editors). Geological Society, London, Special Publications, Vol. 184. doi.org/10.1144/GSL.SP.2001.184.01.07 

  Law, R D, Strachan, R, Thirlwall, M, and Thigpen, J R. 2024. The Caledonian Orogeny: Late Ordovician–Early Devonian tectonic and magmatic events associated with closure of the Iapetus Ocean. 205–257 in The Geology of Scotland. Smith, M, and Strachan, R (editors). Geological Society, London, Geology Of Series. (London, UK: Geological Society of London.). doi.org/10.1144/GOS5-2022-71 

  Law, R D, Thigpen, J R, Mako, C A, Kylander-Clark, A, Caddick, M J, Moore, L R, Becker, C, Holdsworth, R E, Strachan, R Am and Leslie, A G. 2025. The timing and significance of mid-crustal shearing and exhumation of amphibolite-facies rocks along the Great Glen Fault Zone, Scotland. Journal of the Geological Society, Vol.182(4), jgs2024-264. doi.org/10.1144/jgs2024-264 

  Smith, M, and Strachan, R (editors). 2024. The Geology of Scotland. Fifth edition. Geological Society, London, Geology Of Series. (London, UK: Geological Society of London). doi.org/10.1144/GOS5 

  Stewart, M, Strachan, R A, and Holdsworth, R E. 1999. Structure and early kinematic history of the Great Glen Fault Zone, Scotland. Tectonics, Vol. 18(2), 326–342. doi.org/10.1029/1998TC900033

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