Deep beneath the Highlands — Eilrig Shear Zone

The smooth-bored TBM tunnel at Glendoe is 8 km long by 5 m in diameter and will channel water from a new reservoir into an underground hydroelectric power station and then out into Loch Ness.

Its construction delivered an unrivalled research opportunity for our geologists to examine architecture of the Caledonian Orogen in the Grampian Highlands of Scotland.

Folding and shearing

In 2008, we recorded extraordinary examples of folding and shearing throughout these smooth-bored tunnels, including a complete transect through the 1.5 km thick Eilrig Shear Zone (ESZ).  The ESZ is unique in the Grampian Terrane and may ultimately prove to be linked with the Moine Thrust Belt and thus represent a preserved fragment of the Caledonian orogenic front south-east of the Great Glen Fault.

Grampian Terrane structural baseline

The Glendoe tunnels have now delivered a unique structural baseline and provide an unrivalled opportunity for rigorous study of the geological evolution of this part of the Grampian Terrane. We have also have made observations, measurements and photographs recorded on site throughout the construction process from the dam foundations to the TBM and aqueduct tunnels.

Men in Glendoe tunnel

Digging at Glendoe

Geological mapping

The surface mapping in the region is relatively modern and detailed (BGS, 1993). Whilst building a new well-constrained 3-D model for the geology surrounding the ESZ which integrates all available data, we can attempt to accurately characterize:

  • the structural history of the shear zone and compare this with events in the hanging wall and footwall rocks;
  • the age of deformational events using Ar/Ar and Rb-Sr geochronology applied to syn-tectonic micas in ESZ mylonites;
  • the age and provenance of the footwall succession using U-Pb geochronology applied to detrital zircons.

If any robust linkage is possible across the Great Glen Fault, we can then seek to quantify net Caledonian displacement on this system and hence input to North Atlantic pre-Permian reconstructions.

This work will improve the understanding of the history and architecture of a major terrane boundary representing one of the most important tectonic structures in the UK; enhancing expertise and skills in the area of ductile to brittle-ductile thrust tectonics, characterization and dating of the processes involved

Further information

Geology in Glendoe tunnel

Thanks to Scottish and Southern Energy, their consultants Jacobs, and contractors Hochtief Glendoe JV for assistance and access to the tunnel.

More information about Scottish and Southern Energy's Glendoe Hydro Scheme