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Arnaboll

Lairg, Highlands, Scotland

Arnaboll

'The Arnaboll Thrust at its type locality - also the type locality for mylonites'

From Rob Butler

Ben Arnaboll, on the eastern shores of Loch Eriboll, is one of the most famous sites in world structural geology. It was here, in the mid 1880s that the “Highland Controversy” was settled – by Charles Lapworth releasing that the rock sequence here was created not simply by the layer-upon-layer superposition of sedimentary rocks but had been stacked up by vast tectonic faults.

Archibald Geikie subsequently coined the term “thrust” for these structures – with the Arnaboll Thrust being the inspiration for this. In Geikie’s words:

"When a geologist finds…gneiss overlying gently inclined sheets of fossiliferous quartzite, shale and limestone, he may be excused if he begins to wonder whether he himself is not really standing on his head.”

Arnaboll

The Arnaboll Thrust is doubly important for this is where Lapworth went on to describe the process of dynamic metamorphism, where the original character of the gneisses carried by the thrust (and of the quartzites below) has been recrystallized by deformation. He coined these new rocks “mylonites”, one of the beginnings of the science of rock deformation and microstructural geology. 

Nominated by: Rob Butler

100 Great Geosites

Related Links

Images (top to bottom):

  • Arnaboll Thrust © Rob Butler
  • Fold-thrust complexes at Heilam © Sylvia Duckworth (Source Geograph.org.uk) Creative Commons Attribution-ShareAlike 2.0 license.
Earth Science Week 2017

Earth Science Week
October 7-15
Theme: 'Our Restless Earth'