The mountains and wild uplands of north west Scotland are determined largely by their geology and the geological processes that have sculpted them. Geology influences relief, soils and in doing so determines the plants and thus the highland economy and culture of this sparsely populated area.
Geology is the bedrock and foundation of the Highlands.
Beinn Eighe National Nature Reserve has a wonderful waymarked Mountain Trail. There are a number of interesting geological features pointed out a way points including Trumpet Rock and Pipe Rocks (quartizite rocks bearing the signs of burrowing by wormlike creatures in the Cambrian period 542 - 488 Ma ago.) The walk also allows you to see the Kinlochewe Thrust and faulting.
Pipe Rock
Further afield there is a Geodiversity Trail running between Arbroath and Authmithie and the Geopark at Knockan Crag just north of Ullapool.
The mountains act as a record of the immense geological processes that have formed them.
Three rock types are seen within a very short walk of Coulags. First the Lewisian gneisses (basement rocks), Torridonian sandstones, quartzites and then the superficial covering of glacial and post-glacial deposition (screes, peat and alluvium).
The oldest rocks are Pre Cambrian Lewisian gneiss 1200-1600 Ma, these are intensely metamorphosed are are banded in alternating light and dark bands, sometimes these show signs of flowing and melting a bit like toffee. These are exposed on Cnoc nan Each the small hill behind the house, but also at Torr na Iolaire.
Then there are the Torridonian sandstones formed 800-1000 Ma. These may be the characteristically red rocks exposed in great thicknesses in Torridon, but some of the sandstones are almost purple in colour and contain small pebbles. Fuar Tholl and much of the ridge towards Sgorr Ruadh are Torridonian sandstones.
Cambrian quartzites, the youngest rocks, formed 600 Ma ago, are also exposed in the area. The quarzites are white crystalline rocks which give rise to a lunar landscape they can be seen clearly on the path around Maol Chean-dhearg in Loch Coire Ruadh Stac, but also on the western side of the Fionn abhainn and on Carn Eididh on the flanks of Fuar Tholl. The paths show up quite white with the crunchy angular weathered material.
Trumpet rock - quartzite
The reason all three rocks are exposed in such close proximity is due to the presence of the Moine Thrust. When the Iapetus Ocean closed during the Silurian period 417-443 Ma ago huge slices of rock were pushed piggy back fashion over each other, leading to a repetition of rock sequences but also a great thickening of the crust. A terrific view of the Moine Thrust can be seen from the south side of Loch Carron; the geological structure being clearly seen from the shape of the mountains and the "dipping lines" running across the hills.
Covering all these rocks in the valleys are more recent sediments from the Quaternary Period 2.4.miillion years to 5000 years ago when there were a succession of ice advances interspersed by interglacials. Various types of moraine can be recognised - there is a terminal moraine west of Coulags in Glen Carron, a distinctive lateral moraine on the western mountain side as you follow the valley of Fionn abhainn, but mainly it is a muddled mix of hummocky moraines such as those by the cottage or fluvio-glacial and glacial materials partially reworked by fluvial action. Three clearly distinguishable river terraces are seen along Glen Carron and the changes wrought by isostatic recovery can also be seen in the waterfalls which are so characteristic of the area. Along the coastline the impact of the higher sealevels in the past followed by isostatic recovery can also be seen in the frequency of raised beaches, coastal depositional features and the salt marshes found at the end of Loch Carron.
