without taking local topography, stratigraphy and geological history into context, and looking in detail at each rock type sample, it very difficult to say. The darker rock looks like a metamorphosed feature as it has pale quartz veins in it, which would suggest the lighter rock is quartz also, it could however be limestone or feldspar. difficult to say for certain without further detailed examination.
There could also be a multitude of geological reasons these lie against one another and their original orientation for formation (whether formed one on top of each other then geologically reoriented for example) and again, without examining the local geology, its difficult to say.
if i was there, i could say with some certainty, but a picture is little detail. not being able to look at the rock samples lustre, cleavage, grain size, density, colour, interlocking crystals or matrix supported grains (the list goes on) all make a certain explanation difficult.
Ahh, youve caught up with me , it's just that there was a rock of 2 distinct colours going across the track, if the guy with the digger would have been over a few yards more it would still be buried , I didnt notice any other bits of the dark rock on the surface, but I was walking miles to Fionn Loch .
Without knowing exactly where your picture was taken, Alexnder, I can't say for sure. But it may be that the dark rock is one of the Scourie dykes, i.e. volcanic magma, that was intruded about 2400 million years ago into older Scourian granitic gneisses, metamorphosed about 2800 million years ago. Both the Scourian gneisses and the dykes were then metamorphosed again during the Laxfordian orogeny, about 1.8 million years ago.
So the dark rock is dark because it is a meta-basalt, which has little quartz, intruded into a quartz-rich gneiss and the abrupt change is the wall of the dyke.
If you're ever on Skye or visit any of the other Tertiary volcanic centres around the Hebrides, you can see unmetamorphosed dykes intruded about 60 million years ago into older rocks. The contacts are always very sharp.
Both photos taken today round the coast from Opinan near Mellon Udrigle, the above photo shows sandstone after its been moved about a bit .
This is the same place as the above photo, it has fingers of rock going out into the sea, there are large areas of smooth rock dropping into the sea but it didnt come out well in a photo, you just have to go and stand there and soak it all in .
Thanks Jim, the twin coloured rock on the track was above Little Gruinard along the coast from me in Wester Ross. I just add rock photos in here as I know MoS passes through every now and again. Unfortunately I know very little about it, but there are loads of rocks that look as if they have been chewed by a giant and spat out
There was a clear line between the two types of rock, like two cakes cut in half and placed beside each other, they had been ground smooth by glaciers with the two faces of rock hard against/beside each other.
Your sandstones are within the Aultbea Formation of the Torridon Group. They were deposited about 800 million years ago from rivers flowing eastwards from what then must have been a large mountain range above the Outer Hebrides and the now submerged shelf to their west.
The flow directions within the sandstones suggest that 3 or 4 rivers came out of the mountains near the present-day east coast of the Outer Hebrides and spread out across a slowly-subsiding basin where the Minch and NW Highlands are now, drowning the whole area in sand.
That's very interesting Jim , thinking about it, Gruinard Hill is not made of sandstone as it's a different stone type, they widened the road 10+ years ago and the rock can be clearly seen after it was drilled and blasted.
I was thinking of walking round the coast from Opinan to Slaggan when the days are longer to see what birds I can get photos of, it will be interesting to see if its sandstone all the way round the coast.
edit, more thinking, up behind Aultbea ( about 1 mile in the hills ) where I do my peat and I get down to the bedrock it is not sandstone and the sandy grit is course stuff, nearby there are large boulders lying around ( I'll look for a photo ), so it just shows how things can change in a few miles, where I was walking round the coast this morning it appears to be all sandstone with no boulder fields.
Unfortunately, no photos of my bit of limestone pavement- will take some next time I'm there. It's approximately 30 feet to the left of Heusghar Hill summit. I was ever so pleased when I came across it. It's not exactly Malham Cove or the Burren but nevertheless!
All the north and west Coasts of Rubha Mor are Torridonian. There's a geological fault (fracture) that runs along the coast on the west side of Camas a Charraig. There are sandstones on the east side of that too, but a different kind from the Torridonian you show in your pictures. The new sandstones are much younger - Permo-Triassic, about 25 Million years old. Just east of Laide, they're overlain by some Jurassic sediment. The sandy grit you find above Aultbea could be either of these rocks. They cross the coast at Aultbea, but may not be visible under beach sands. The southernmost part of Isle of Ewe is also Permo-Triassic.
In Gruinard Bay, another fault just west of An-t Saothair brings the Torridonian sandstone back. A final fault between First and Second Coast marks the change back into Laxfordian gneiss.
I think most of it should be visible in the rocks along the coast.
That was a good find, not sure what the different colours are but its interesting to look at .
Well... I like it!
As for what the colours mean... you have to zoom in to 1:50,000, then click on the 'key' link. Each colour is a different rock type. However, this would make a lot more sense if there was also a 'stratigraphy' link showing how they're all piled on top of each other.
When I did geology as a teenager, I was frustrated by 'drift' and heartily wished that the entire country could be stripped off its its topsoil, population and wildlife, so that I could just see the rocky bits underneath. Thank goodness I never got a chance to be God for a day!
Its quiet near that in some places up here in places Paddy , I thought the different colours were different rock types but as you say it looks good anyway .
When I did geology as a teenager, I was frustrated by 'drift' and heartily wished that the entire country could be stripped off its its topsoil, population and wildlife, so that I could just see the rocky bits underneath. Thank goodness I never got a chance to be God for a day!
Damned right, Paddy, what about my archaeology in the upper layers?!! My wish was that all the occupation layers post-Roman would disappear
When I did geology as a teenager, I was frustrated by 'drift' and heartily wished that the entire country could be stripped off its its topsoil, population and wildlife, so that I could just see the rocky bits underneath. Thank goodness I never got a chance to be God for a day!
Suggest a trip to East Greenland, then, Paddy. Beautiful 3-D exposure of the rocks on the surface and along the 2 km high fjord sides, with little or no cover except in the bottom of the glacial valleys.
, it's just that there was a rock of 2 distinct colours going across the track, if the guy with the digger would have been over a few yards more it would still be buried 
, I didnt notice any other bits of the dark rock on the surface, but I was walking miles to Fionn Loch 
. 
