Reading the rocks

Stoer Sandstone

The stone ripples you can see in the rocks around the north of Stoer Bay were formed some 1000 million years ago. The rock is the Stoer Group of Torridonian Sandstone, a sedimentary type of rock.

Sedimentary rock starts out as little grains carried along in rivers and streams and is built up of many layers of sediment, eventually becoming compacted under the weight of layers above until it becomes rock. Erosion along the coast has exposed these rocks and in some places you can see ripple marks made by an ancient river. (image 1)

Lewisian Gneiss

An area of NW Highlands Geopark known as ‘The Foreland’ lies to the west of the Moine Thrust Zone and includes most of the coastal townships. The Foreland is largely made up of Lewisian Gneiss, which at 3000 million years old is the oldest rock type in Britain.

Because they were once part of the same continent, the same rock type is found in North America and Greenland. The rock has been eroded and scoured by glaciers to form the many hillocks (‘cnoc’) and small lakes (‘lochans’) typical of the north west. In many areas the Lewisian Gneiss is overlaid with Torridonian Sandstone, for example at Rhu Coigach, Rhu Stoer, Oldshoremore and the mountains of Quinag, Suilven and Stac Pollaidh. (images 2, 3 & 4)

The Inselbergs

Inselberg, meaning ‘Island Mountain’ is the word used by geologists to describe these strange, isolated peaks made from Torridonian Sandstone and laid down as sand in rivers some 1,000 million years ago. Erosion during many Ice Ages has exposed and carved them to create the unforgettable and distinctive scenery we see today.

The highest Geopark peaks are capped with pale grey Cambrian Quartzite, a rock type that is harder than Torridonian Sandstone so it withstands erosion better. These Quartzite ‘caps’ have protected the sandstone below, creating the inselberg forms while the rocks all around were ground down and swept away, exposing the ancient landscapes of Lewisian Gneiss below. (images 5 & 6)

Durness Limestone

Durness Limestone was formed 480 million years ago during the Cambrian era. This rock behaves very differently from sandstone and gneiss by slowly dissolving in rain water.

Water seeps through the limestone and creates a weak acid which gradually dissolves the limestone over thousands of years. As a result, the landscape contains a number of very unusual features including large caves like Smoo and the Bone Caves, cave systems and sink holes at Traligill and the surreal ‘clints and grykes’ above Inchnadamph. In Traligill and Allt nam Uamh whole rivers disappear into sink holes and reappear further down the valley.

Another distinctive rock from this era is Pipe Rock. The little ‘pipes’ are burrows made by worms working their way through those soft sediments laid down around that ancient shoreline. Look at the beaches today when the tide is low and you will see modern worms doing the same and leaving wiggly trails of sand behind. (images 7 & 8)

Scourie Dykes

Dykes, plugs and sills are created by molten rocks – magma – intruding along fissures between other kinds of rocks. Rocks formed in this way are called igneous rocks. The Scourie Dykes are igneous intrusions forming spectacular patterns which can be seen at ‘The Multi-Coloured Rock Stop’, a car park at Laxford Bridge where the road cuttings have revealed a cross-section where three different ages of rock can be seen.

Rocks which have been heated deep in the earth until they change in form are called metamorphic rocks. Lewisian Gneiss started as an igneous rock and was later metamorphosed. Ledmore Marble (image 9) is a metamorphic rock created when local Durness Limestone was heated by the presence of magma to about 700ºC and now forms the hills north of Loch Borrolan.

Multi-coloured Rock Stop at Laxford Bridge

Ancient Lewisian Gneiss is spectacularly exposed in a road-side cutting.

Three different ages of rock can be distinguished here. Pale grey gneisses represent the original rock into which once molten sheets of dark basaltic magma were later forced. The streaks of pink granite must be the youngest of the three rock types since they can be seen cutting through both of the older types. (image 10)

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Photography courtesy of Inver Lodge Hotel, Scottish Natural Heritage, Highland Council, British Geological Survey, Sutherland Partnership, Mackay Country Archive and Iain Sarjeant.