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The Geology Of Quinag : The Story Recorded In The Rocks

Section B: The Story Recorded in the Rocks

The story recorded in the rocks of Quinag starts with an early Earth. The continental crust was forming when magma erupted on to the surface forming black fine grained igneous rock, similar to that in Iceland today. These were buried and then intruded by other magmas forming coarse grained granites (lighter in colour). These early-formed igneous rocks were buried even deeper (over 25 km down) and underwent major mountain building processes (about 3,000 million years ago) that turned them into a completely recrystallised banded and contorted rock called gneiss. This is the Lewisian Gneiss, which forms the foundations of Quinag, and indeed the rest of Scotland, at least as far south as the Great Glen.

This gneiss gradually moved upward in the crust, caused by erosion of the rock above. Where all the rock that was removed ended up is not known. Pulling forces then stretched the crust and allowed more magma to force its way up toward the surface along straight cracks, forming a mass of parallel black igneous intrusions called dykes. They are seen running fairly straight across the landscape today.

The Lewisian Gneiss is chemically and mineralogically diverse and the darker part in particular today releases a range of nutrients on weathering. These can form pockets of soil that support a diverse flora.

Further earth movements exposed this gneiss at the surface of a land environment about 1,200 million years ago, which had low hills and valleys. High mountains were out where the Atlantic is now, and rivers brought huge quantities of pebbles, sand and a little mud to be deposited on this land surface. There was no land vegetation, quite high temperatures and little rain, so the rivers coming from the mountains behaved quite differently to now, particularly once the low hills and valleys were covered. Rivers kept changing course, depositing and then picking up the sediment again, in a mass of temporary ever-changing river channels. The first life on land was establishing, especially in the muddy pools in deserted river channels. Cyanobacteria, which had been producing oxygen for a while in the marine environments, now started to colonise the land as well. The rocks formed are the red sandstones, conglomerates and mudstones, found in layers up to a total of 5km thick and called the Torridonian. The soils formed from the weathering of these rocks are low in nutrients and boggy if low-lying.

After some tipping-up (even gentle folding) of the Torridonian and Lewisian rocks, there followed the gradual invasion of the sea, just over 500 million years ago. This cut a flat surface and started to deposit shallow sea sediments. It was an amazing time for life in the seas of Earth, as many of our invertebrate groups evolved and started producing shells which form obvious identifiable fossils. There are intertidal and shallow-sea white sandstones (quartzites), the upper layers with burrows (the pipe rock), formed originally by a soft bodied worm. Limey muds then developed, with quite a range of invertebrates populating the shallow waters, including trilobites. Limestones with fossils then followed, before a mountain building phase lifted the sea floor up above sea level eventually forming mountains of a scale of the present-day Himalaya. The rocks of Quinag were not greatly affected by this mountain building, rather other rocks were pushed up and over them as the mountain belt was squeezed. The mountains to the east, such as Ben More Assynt, do show structures formed by low angle thrust faults at this time, bringing older rock up and over younger rock.

The limey muds and limestones provide, on weathering, some of the best growing conditions in the area, with more neutral soils and good drainage.

Nothing remains of any rocks younger than these times. If they were ever deposited, they have since been eroded. During the last few tens of millions of years, the current drainage pattern was established, with rivers particularly following narrow zones of weakness caused by the breaking and moving of rock whilst underground (faults). This was the time that defined why Quinag is where it is.

The last 2.5 million years has had a huge effect on the current shape of Quinag. Many advances and retreats of glaciers took place. At their greatest, ice sheets covered just about the whole landscape melting at the edge of the continental shelf, west of St Kilda. At that time the coast was there as well, so ice sheets started to lift off land and float there on the sea. There were periods with no ice in the area at all and the sea level was relatively higher. Vast amounts of rock have been removed and dumped at the edge of the continental shelf. The valley glaciers widened and deepened the river valleys leaving islands of rock between, one of which is Quianag, between the valley now with Loch Assynt and the sea inlet under the Kylesku bridge. In the case of Suilven the valleys which were widened were closer to each other and hence it is a narrow ridge that is left.

Figure 14: The glacial striations shown above are orientated almost north south and formed when a relatively thin glacier flowed around Quinag from the east. The striations are picked out by a dark lichen.

The quartzites and occasionally the Torridonian Sandstone are good for demonstrating the scratch marks left behind by rocks being carried in the base of the ice. The reason the quartzite is best is that once scratched, weathering has a slow effect on removing the top surface of the rock and hence keeps the scratches sharp. Other rocks would also have been scratched but have now weathered much more, up to a few centimetres in the last 10,000 years, depending on rock type. These scratches give the direction of travel of the ice at the time. Very unusually there are scratches in 2 different directions in the quartzite on the east side of Quinag. When the ice was thick (at its most recent, maximum perhaps 27,000 years ago) Quinag was less able to deflect the flow of the ice sheet, so it kept flowing roughly from east to west, whereas when the ice was less deep as it subsequently reduced from this maximum the deflection was greater around the mountain (perhaps 15,000 years ago). The most recent glaciation of all just producing a few short high up corrie glaciers.

In recent years huge grooves (mega-grooves) have been observed from satellite imagery cut especially into the very hard quartzite (and further east the Moine Schists). These were not just big scratch marks but were probably formed by the action of high-pressure turbid flow of water carrying sediment, which flowed under the ice sheets. This has yet to be fully understood. Some of these mega-grooves can be seen cut from east to west on the west of the road just north of Skiag Bridge road junction. In the photograph below the mega-grooves are running up hill with quartzite exposed between them.

Figure 15: The parallel quartzite ridges run up the hillside. The tops are exposed and the grooves between are filled by peat and vegetation.
Figure 16: Right beside the road between Skiag and Unapool and below the hairpin bends there is a canyon cut by glacial meltwaters, in a fault zone in the quartzites.
Figure 17: This also has an old channel running out from the canyon across the landscape of Torridonian Sandstone.

As well as erosional features, there are some depositional materials formed as the last ice retreated. These loose deposits on the surface are advantageous to plant life, as they often provide rock materials that form soils more readily than solid rock, and also sometimes provide better drainage.

Moraines of several types are to be found on the surface, especially in hollows that existed in the land surface. The rock types carried depend on the geology of the source areas of the ice, which were further east and hence can be useful sometimes to indicate where the ice has come from. There is a wide range of sizes of fragments in these deposits, with erratics being the largest size, which can be left standing on the land surface after all the finer materials are washed away. Torrents of water, produced during the melting of the ice, transported and sorted the mixed moraine, and deposited it into hollows or swept it away out of the area.

Peat started to accumulate on the land’s surface about 5,000 years ago as the climate became wetter and accumulated bog prevented complete plant decay. This is formed mostly from sphagnum moss, which is compressed and partly decomposed, but also preserves other vegetation from these times. Tree stumps, often from the base of the peat, can be seen being re-exhumed as the peat erodes in some places at the present day.

The modern day burns also move sediment around and can form pockets of better drained ground which may also release some nutrients. The Unapool Burn is an example. Frost shattering on exposed rock faces, where the layers of Torridonian Sandstone forms high cliffs, particularly on the west face of Quinag, has led to extensive development of scree slopes, with scree skirts and boulder trains developing where scree slopes become unstable. These may have started to form whilst the lower ground still had lower-lying glaciers moving through.

Figure 18: An erratic on the skyline.

Plant and animal life advanced into the area from the south in the interglacials, and then retreated again in glacial periods to find conditions best suited for their establishment. Right now (the current interglacial!), the species that prefer colder conditions have retreated higher up the mountains, and as the average temperature increases, they occupy isolated islands that will eventually be able to retreat upward no further. This is a process that will have happened many times before. There is little evidence of the fauna that lived here during the interglacials unless, at the time, their bones were washed into cave systems in the limestone. The land surface is swept clean by each glacial period, destroying any evidence that was there, so it is only the cave systems that retain it. Animals such as reindeer, brown bear, polar bear, lynx, lemming and arctic fox were present, and their remains have been found.

It may be that humans only advanced this far north during the last interglacial. Human activity is a further overprint on the landscape as we see it today. Humans have occupied the landscape, built on it, farmed it and adapted its vegetation to best suit their own needs. This is recorded to a certain extent in the current plant distribution, the pollen in the sediments on the floor of the lochans, and peat and soil records.

The Study Of The Geology

Humans have become increasingly inquisitive about the story that has shaped our landscape and contributed to the plant and animal distribution patterns that we see today. A fuller account of how the ideas of the geology of the area developed are to be found on the North West Highlands UNESCO Global Geopark website. As will be seen from this account, the area just to the east of Quinag was at the very forefront of the development of mountain building theories.  Read more about the Highlands Controversy

Figure 19: Sir Roderick Murchison

Sir Roderick Murchison was one of the earliest geologists to try and establish the geology of the area. He was so enthusiastic about Quinag that he used it as the frontispiece of his major book on the rocks of this age, called The Silurian System (1839). He didn’t quite get the ages of the rocks right at that stage. His Laurentian is what we now call Lewisian, his Cambrian is the Torridonian, and his Lower Silurian our Cambrian.

Figure 20: Murchison’s frontispiece to The Silurian System, Quinag from Inchnadmaph

The small team of geologists who really mapped out the complex geology of the area in the late 1880s and 1890s were led by Ben Peach and John Horne. They were responsible for developing ideas on the processes that took place during mountain building. The original ideas having been established on Loch Eriboll in the early 1880s by Charles Lapworth.

Ben Peach enjoyed drawing and painting the mountains of the area, which included Quinag shown below.

Figure 21. Assynt remains a wonderful outdoor laboratory, which is used by university groups and researchers to further the understanding of our planet. Many undergraduate groups visit during a year, as part of their field training. They are often to be seen along road cuttings beside Loch Assynt and over to Loch Glencoul, with their high visibility jackets and hard hats.


This article was written by Pete Harrison, Geologist for the North West Highlands Geopark.

British Geological Survey Data UKRI (2021). Base mapping is provided by ESRI.

The 2D and 3D visualisations come from the British Geological Survey. Such images can be constructed for any part of the UK, using the link to the BGS viewer below.  

Geology Of Britain Viewer

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