Lewisian Gneiss Complex:
Building the continental crust (2900 to 1750
million years ago)
The crust of NW Scotland, together with parts of
Greenland and North America that make up the ancient
continent of Laurentia, was built up mainly from
igneous rocks that crystallized around 2900 to 2700
million years ago. At that time, the rocks we now see
were deep in the Earth's crust. They were deformed
and metamorphosed at very high temperatures,
producing gneisses with a folded layering. Actually,
there were two periods of deformation and
metamorphism (named the Scourian and
Laxfordian events), separated by a stable
period when the crust fractured and allowed in basic
magma that crystallized as a set of dykes.
To see the rocks, visit: Scourie; Achmelvich; Laxford
Folded gneisses at Achmelvich, formed deep in the
continental crust
Torridonian red beds:
Rivers and lakes (1200 and 1000 million years
ago)
By 1200 million years the old continent had been
eroded down to a landscape of low rocky hills. A
great thickness of red sandstone, brought in by
rivers, buried the old hills. Lakes formed, and then
dried out. This cycle of events happened twice: the
older red sandstones (the Stoer Group) were tilted,
eroded and overlain about 1000 million years ago by
an even greater thickness of river-deposited red
sandstone, the Torridon Group. At the present day the
sandstones are being eroded away, so that in places
the ancient landscape can be seen once more.
To see the rocks, visit: Clachtoll; Stoer; Loch Assynt
Dried-out lake sediments: polygonal cracks in
mudstone filled with light-coloured sand
Cambrian sediments:
Shallow sea (540 to about 490 million years
ago)
After another long interval, this part of Laurentia
found itself next to the ocean. The deep ocean in the
Cambrian period (about 540 million years ago) was
away to the south-east, but here in NW Scotland the
water was shallow and relatively quiet. A rather thin
sequence of sandstones, siltstones and limestones
formed in coastal sand-bars, tidal flats and in the
shallow sea. Now there were animals active on the sea
floor, burrowing and grazing: we can see the imprints
they left. Also there were tiny shelled creatures,
unlike anything known today. Limestone deposition
continued until about 490 million years, into the
Ordovician period.
To see the rocks, visit: Skiag Bridge; Glencoul
Hard white quartz sandstones laid down in the
Cambrian sea
Igneous rocks of the Assynt area:
Intrusions of unusual magma (430 million years
ago)
At about 430 million years ago this part of NW
Scotland was intruded by magmas, which crystallized
in the form of sills (sheets parallel to the bedding
of the enclosing sediments), dykes (steep,
cross-cutting sheets) and plutons (larger, rounded or
irregular masses). These are unusual igneous rocks of
alkaline composition (rich in potassium and
sodium). The typical rock is a syenite, rich
in potassium feldspar, rather than a granite or
gabbro. The heat from the intrusions metamorphosed
nearby Cambro-Ordovician limestones, converting them
into marble.
White-spotted borolanite, a variety of
syenite
Moine Thrust zone:
Tectonics: collision of continents, and overthrust
faulting
Meanwhile, a range of mountains, the Caledonian
mountain belt, was forming to the south-east, as
other continents began to collide with Laurentia
during the Ordovician period. At about 430 to 420
million years ago a vast slab of metamorphic rocks,
the Moine schists, was pushed out of the mountain
belt, over the top of the rocks of NW Scotland, much
as the Himalayas are today being pushed out over
India. Along the base of the Moine rocks, the intense
shearing deformation produced a fine-grained platy
rock called mylonite. Here in NW Scotland about 100
years ago, pioneering geologists discovered one of
the world's most impressive zones of overthrusting:
they recognized the significance of the mylonite, and
observed that older metamorphic rocks lay on top of
younger sedimentary rocks.
Platy mylonites, very strongly sheared rocks of
the Moine Thrust zone
