The Leirinmore Fault
Major faults of the Smoo Cave area.
Being an important regional feature which is only visible in outcrop at an awkward to reach coastal locality below Leirinmore, it was worth adding some photos of the 'Leirinmore Fault'. This is one of the major normal faults of the Durness Graben which has dropped a fault block of Durness Limestone (the one which Smoo Cave is found within) against Pipe Rock quartzite of another major block.
As the fault runs inland, the quartzite becomes 'faulted out' and the Durness Limestone is placed against Lewisian Gneiss for much of it's length (before the fault continues to Gualin House and then far as the Rhiconich area). This has an indirect but important control on the volume of freshwater which Smoo Cave receives due to the vast differences in permeability between the two adjacent rock types.
The faulted boundary between the two blocks is not a clean break but a 'fault zone' of deformation which has formed a type of fault rock known as a 'cataclasite'. This shows evidence of severe crushing from repeated fault movements. Being very brittle, the Pipe Rock has been easily shattered (even several tens of metres away from the fault zone) and evidence of fault-related folding can be seen within a more resistant section which runs along the edge of the fault zone. An obvious feature of the fault zone is the reddening of the rocks, very similar to the large faults seen at Sango Bay where iron-rich minerals (e.g. hematite) formed during movement. Like at Sango Bay, the cliffs here are also called Creag Ruadh (red cliff) for the very same reason.
Numerous other (albeit minor) faults are present between the Leirinmore and Sangomore faults, one of which is particularly important to Smoo Cave as the Geodha Smoo inlet has formed along it. Evidence of vertical displacement (in the order of tens of metres) along the inlet can be seen where a distinctive metre-thick bed of white chert can be seen midway up the cliffs on its western side, but not on the eastern side (see image - facing north). Midway along the geodh, the chert bed is also cut by another fault (running near-perpendicular to the inlet) which has displaced it ~5m vertically, showing evidence of 'fault-drag' on the down-thrown side.
As the fault runs inland, the quartzite becomes 'faulted out' and the Durness Limestone is placed against Lewisian Gneiss for much of it's length (before the fault continues to Gualin House and then far as the Rhiconich area). This has an indirect but important control on the volume of freshwater which Smoo Cave receives due to the vast differences in permeability between the two adjacent rock types.
The faulted boundary between the two blocks is not a clean break but a 'fault zone' of deformation which has formed a type of fault rock known as a 'cataclasite'. This shows evidence of severe crushing from repeated fault movements. Being very brittle, the Pipe Rock has been easily shattered (even several tens of metres away from the fault zone) and evidence of fault-related folding can be seen within a more resistant section which runs along the edge of the fault zone. An obvious feature of the fault zone is the reddening of the rocks, very similar to the large faults seen at Sango Bay where iron-rich minerals (e.g. hematite) formed during movement. Like at Sango Bay, the cliffs here are also called Creag Ruadh (red cliff) for the very same reason.
Numerous other (albeit minor) faults are present between the Leirinmore and Sangomore faults, one of which is particularly important to Smoo Cave as the Geodha Smoo inlet has formed along it. Evidence of vertical displacement (in the order of tens of metres) along the inlet can be seen where a distinctive metre-thick bed of white chert can be seen midway up the cliffs on its western side, but not on the eastern side (see image - facing north). Midway along the geodh, the chert bed is also cut by another fault (running near-perpendicular to the inlet) which has displaced it ~5m vertically, showing evidence of 'fault-drag' on the down-thrown side.
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Photographs © 2006-2014 Iain Greig
Satellite image modified from Google Maps
Photographs © 2006-2014 Iain Greig
Satellite image modified from Google Maps