A BRIEF INTRODUCTION
The Assynt region has been a Mecca for geologists for over a century. There are many reasons which make it a classic area in British geology, and even in world geology. The rock formations here are so important that the area is part of the NORTH-WEST HIGHLANDS GEOPARK. Some of the oldest rocks in the world occur in Assynt; there is one of the best exposed sedimentary formations in Britain, and great fault structures can be easily observed. Rock exposure is very good largely because the landscape was stripped of soil and vegetation by the movement of ice sheets, which melted only 10,000 years ago. There is still much bare rock exposed today.
The geological history of Assynt was originally worked out by Professors Peach and Horne and their team of geologists in the early years of the last century. They saw in the rocks of Assynt structures which rivalled those to be found in the Alps, where great masses of older rock have been thrust above younger formations.
This model in the Assynt Visitor Centre depicts Assynt during and after the Ice Age, demonstrating the contribution of glacial activity to the creation of this stunning landscape
The Lewisian rocks
The oldest rocks in Assynt, which are also among the oldest in the world, belong to a group of rocks called, the Lewisian complex
. This name comes from the largest Hebridian island, Lewis
, which has extensive areas of this rock formation. The Lewisian rocks can be well seen around Lochinver
The main rock type is highly metamorphosed gneiss (pronounced ‘nice’). Metamorphism, caused by burial at great depth where pressures and temperatures are extreme, has formed the gneiss from whatever it may have been originally. It is thought that some of the gneiss was initially sedimentary rock, possibly sandstones and limestones. Some of it may have been igneous rocks, such as granite or basalt. Essentially the gneiss is a hard, greyish rock, with alternating dark and light bands. This banding is the result of segregation of low and high density minerals during metamorphism. The pale bands contain much quartz and feldspar; the darker bands are made of denser minerals including, biotite mica
. Sometimes the bands are twisted and bent, suggesting that the rock must at one time have been plastic. This Lewisian gneiss forms most of the lower ground in Assynt. Its landscape is one of hummocky low hills and water-filled hollows. It is part of the original crust of the Earth, and is similar to the oldest rocks in the great continental masses such as the Canadian shield. By studying the radio-active breakdown of minerals in the gneiss, its age has been estimated at over 2,700 million years, and that is a date for the rock being metamorphosed, so the original rocks must have been far older. Other, younger, dates have also been obtained, and these, 2,400 and 1,800 million years, are thought to represent times when the rocks were involved in periods of mountain formation. The Lewisian rocks thus belong to what geologists call the Pre-Cambrian part of geological time, which dates back from 545 million years ago to the formation of the earth 4,500 million years ago.
Many other rock types occur within the Lewisian complex. There are intrusions of basic and ultra-basic rocks, which form very dark patches and sheets running across the gneiss. Around 1,000 million years ago the Lewisian rocks seem to have been subjected to large-scale erosion. Indeed, the landscape which the gneiss forms today may be much the same as that resulting from this ancient period of erosion.
Sedimentary rocks are those formed by the deposition of particles eroded and weathered from older rocks. Around 900 million years ago, a range of mountains existed to the west, roughly where the Western Isles are today. This mountain range was actively eroded, and great river systems flowed eastwards, depositing boulders, sand and mud onto the eroded Lewisian landscape, over what is now north-western Scotland. Thousands of metres of this sediment were deposited. The sandstones tend to have a reddish-brown colour. This is partly because they contain iron oxides, and also because, in places, they are rich in pinkish feldspar, derived from the erosion of the Lewisian gneiss. Feldspar breaks down readily during weathering, and the fresh-looking feldspar in the sandstones suggests rapid deposition. In places there are layers of boulders and pebbles, possibly the result of turbulent flash-floods. On the surfaces of some of the strata are the ripples left on the original wet sand and mud by water currents, and some surfaces have patterns of cracks where the mud dried out over 800 million years ago.
is a remarkable volcanic mud flow which occurs within the reddish Torridonian strata. This mud flow contains angular rock fragments and greenish mineralised patches. Some of the earliest fossils occur in mudstones within the Torridonian strata. These are small, domed, algal structures, which probably developed in shallow lagoons. Today, the Torridonian sediments form the isolated mountains of Assynt, and the strata can be clearly seen, on sides of Suilven
, rising above the irregular gneiss platform. These sediments also form striking sea cliffs, as at Stoer Head where caves and stacks have been eroded through the strata. The surface separating the gneiss from the overlying Torridonian rocks is called an unconformity. It represents a gap in time, where millions of years are not represented by any rocks. It is remarkable that the Torridonian sedimentary rocks have existed for over 800 million years without being involved in metamorphism or folding.
On the tops of some of the Assynt mountains, such as Cul Mor
, there is a patch of pale grey rock which shimmers and sparkles in the sunlight. This rock is quartzite, and, as its name suggests it is made almost entirely of the mineral quartz. This is sand from the margins of a long lost sea that covered the area around 550 million years ago. An unconformity separates it from the underlying brownish Torridonian strata, much of which was eroded before Cambrian time. In places all the Torridonian layers were eroded, so the Cambrian rocks rest here directly on the Lewisian gneiss.The quartzite also occurs at lower levels, and is beside Loch Assynt
, not far from Skiag Bridge
Within some of the quartzite strata are the fossilised burrows of worms. The ‘pipe rock’ is a pinkish quartzite full of these vertical burrows. As the Cambrian sea spread, rusty-weathering shales and mudstones formed on the sea bed. These contain fossil trilobiteS
– a genus called Olenellus
. In Britain this trilobite occurs only in western Scotland, but it is found in Cambrian rocks in North America. This suggests that at this time Scotland was joined to North America, and a wide, deep ocean, across which Olenellus
could not migrate, separated it from the rest of Britain. Cambrian limestones were deposited above the quartzites and shales. These form massive cliffs near Inchnadamph
. The limestones produce a landscape with many classic features such as caves, potholes and limestone pavements. Here limestone soils give rise to a rare flora, visited each year by many botanists.
The Moine Rocks and the Great Thrusts
Further to the east is another group of metamorphosed Pre-Cambrian rocks, which are not as old as the Lewisian rocks, but older than the Torridonian. These are the Moine group. They are schists
and other metamorphic rocks, which originally formed deep in the Earth’s crust. These rocks are associated with a series of large thrust faults, which have carried them many kilometres from the east to ride over younger rocks in the west. The Moine Thrust is the most easterly of these thrusts, and below it are others, which bring huge slices of older rock westwards. Possibly the most spectacular and most visible of the thrusts is that at Loch Glencoul
, to the north of the Assynt region. Here, Lewisian gneiss has been thrust over younger rocks.
For more details the following may be consulted:
Geological Excursion Guide to the Assynt District of Sutherland, Johnson and Parsons, Edinburgh Geological Society.
Geologists’ Association Guide No 21: The Lewisian and Torridonian Rocks of North-West Scotland, Barber et al.
Geologists’ Association Guide The Late Pre Cambrian Geology of the Scottish Highlands and Islands, Hambrey et al.