Geology as inspiration for landscape photography - the other side of the terrane boundary

In a previous post I described an image made of an intrusive granite sheet cutting through Lewisian Gneiss on the north side of Loch Laxford, on one side of the Laxford shear zone a candidate terrane boundary that happened in Deep Time. This post is about a trip a few days later to make a landscape image of the terrane boundary from on the other side.

View along the strike of gneiss foliation. The pink rock is a foliated granitic intrusion into the steeply dipping gneiss.

There are a number of known terrane boundaries in the UK, many are covered by younger rocks, submerged under water or overlain by superficial deposits, whilst others require a walk. The NW Highlands of Scotland provides an opportunity to drive across one and on the way view the rocks. Below is a Google street view of a road cutting south of Scourie that shows the character of the Lewisian gneiss complex in the northern part of the central district .or the Assynt terrane. At a first glance it looks like sedimentary rock dipping gently across the road.



Zooming in on the outcrops and the nature of the rock becomes more apparent, thanks to the attention of geology hammers aka hammer rash, with fresh (hammered) surfaces reveal alternating bands of dark and pale minerals or gneiss foliation.



Heading north from Scourie, towards Loch Laxford the nature of the gneiss changes, the dips get steeper, the strike of the gneiss has changed direction and the gneiss has been subject to intense strain. The Laxford Shear Zone has been entered.


Now a google street view from the north side of Loch Laxford  and view south. The rock has been drilled and blasted providing fresh exposures of the Lewisian Gneiss Complex and in the layby is an interpretation panel, the layby is one of the stops on the Rock route in the NW Highlands Geopark. The rock in this view has an apparent structural dip to the west of south.



A closer look at the exposure. There are 3 rocks colours - Black, Pink and Grey. The Grey rock is gneiss, the dark rocks are intrusive dykes that have cut across the gneiss foliation and the pink rock is intrusive granite, that has cut across the gneiss and dyke and tectonic forces have deformed everything in view.



It is reasonable to suggest that the outcrop of the Lewisian gneiss complex, does appear different from the Google Street view south of Scourie. Journeying on towards Laxford Bridge shows how the rocks change in dip and are subject to more intense tectonic forces, until south of Loch Laxford in the road cuttings alongside Loch na Claise Fearna the postulated terrane boundary is crossed. The road cuttings between Riconnich and Scourie represent a journey through over a Billion years of continental crust evolution from the Archean to the Proterozoic.

There are a couple of papers available online that provide an overview of past and recent developments concerning the Lewisian Gneiss Complex and the geology of the Laxford Shear Zone.

2010 The Laxford Shear Zone: an end-Archaean terrane boundary? *K. M. Goodenough, R. G. Park, M. Krabbendam, J. S. Myers, J. Wheeler, S. C. Loughlin, Q. G. Crowley, C. R. L. Friend, A. Beach, P. D. Kinny & R. H. Graham,  In: LAW, R. D., BUTLER, R. W. H., HOLDSWORTH, R. E., KRABBENDAM, M. & STRACHAN, R. A. (eds) Continental Tectonics and Mountain Building: The Legacy of Peach and Horne. Geological Society, London, Special Publications, 335, 103–120 

2013 New U-Pb age constraints for the Laxford Shear Zone, NW Scotland: Evidence for tectono-magmatic processes associated with the formation of a Paleoproterozoic supercontinent
K.M. Goodenough, Q.G. Crowley, M. Krabbendam, S.F. Parryd

Fair sized garnets in mafic gneiss (metagabbro) with felsic rims 

The area I had in mind was one I had visited briefly in October 2011, before it was cut short by really poor weather, proper drookit. Although I had managed to locate an outcrop of metagabbro with some impressively large garnets. This has lingered fairly high on my list of places to revisit as the views from a couple of elevated areas held some photographic potential. Despite its modest height on an ordnance survey map, glacial ice has exploited weaknesses in the geological grain and it is definitely terrain for putting on the Big Boots.

Degrading blanket peat

A landscape photograph isn't going to convey zircon ages or metamorphic facies, but tectonothermal events leave an imprint on rock and intrusive rocks that show field relationships, so there's some hope. Especially when the area has been aerially scoured by glacial ice.

View along the strike of gneiss foliation. The pink rock is a foliated granitic intrusion cutting across steeply dipping gneiss.

A view of Loch Laxford and geology of the Laxford shear zone

Despite the less than promising weather forecast, the weather delivered what the NW Highlands is renowned for in late autumn, bands of rain/sleet/hail, with broken cloud following on behind, a hooley blowing with gusts of some intensity and some photogenic fleeting December light around too. I was fortunate.

NW Highlands atmospheric lighting