glossary
The following glossary is not exhaustive, but it contains information on some of the terms used on this website. Please refer to the glossary on the links page for a more comprehensive geological glossary.
Erratic
These are bits of rock (often large boulders) dragged along by glaciers as they move across an area. As the glaciers melt they can be left many miles from where they were picked up. They can often be left perched on top of other formations, which makes them distinctive features on the skyline.
Gneiss
This is a type of metamorphic rock. In the Knockan Crag area it is known as Lewisian gneiss after Lewis in the Outer Hebrides, as the island is almost entirely made of this old hard rock. These Lewisian gneisses are some of the oldest rocks in the World; they have been aged at between 3200 million and 2800 million years old. They were originally layers of sedimentary rocks, like sandstone and mudstone, interspersed with layers of volcanic rock, which have experienced very high temperatures and pressures deep in the Earth’s crust.
Limestone
A sedimentary rock made of calcium carbonate. In the Knockan Crag area this type of rock is known as Durness limestone after the village on the north coast. It tells us that this part of Scotland was once near the equator since limestone can only form in warm shallow water: there needs to be a certain temperature and lots of light for the chemical reaction that forms calcium carbonate to take place.
Moraine
These are piles of debris left behind by melting glaciers. Unlike an erratic, which is a single block of rock perched on top of the land, moraines are huge piles of various sizes of rocks, gravel and sand. They can form a variety of features such as long linear ‘kames’, which snake across valleys, or dumpy hillock-like ‘drumlins’ depending on how the ice melted. They are also named depending where they were on the glacier, so a terminal moraine was formed at the front of the glacier and a lateral moraine formed at the sides.
Sandstone
This is a sedimentary rock. The sandstone in the Knockan Crag area is known as Torridonian sandstones, named after the village of Torridon south of Ullapool, which is surrounded by mountains formed from this red rock. They are about 1000 million years old. Their red colour tells us that the rock was deposited on land not under the sea, as the iron minerals in the rock have ‘rusted’. They were deposited on top of the Lewisian gneiss by huge rivers which flowed across the landscape. At the time when these rocks were being deposited the land that we now call Scotland was near the South Pole.
Schist
This is a metamorphic rock. It has been affected by heat and pressure that has altered both the minerals and texture of the rock. The rocks in the Knockan Crag area are called Moine schists, named after A’Mhoine on the north coast. These rocks cover most of Sutherland and the middle of Ross-shire all the way south to the Great Glen. They were originally layers of sandstone and mudstone, which were heated up and compressed. They have been dated at about 1000 million years old.
Thrust
A thrust is a low angle fault that forms during periods of mountain building. The thrust at Knockan Crag is known as the Moine Thrust after the Moine schists, which were pushed about 70kms westwards to lie on top of the Durness limestone. This happened between 430 million and 400 million years ago when the land masses that we now call Scotland and England collided. The area over which this occurred runs from Durness on the north coast to the island of Skye, and is known as the Moine Thrust Zone.
Quartzite
This is a rock that is formed almost entirely of quartz. At Knockan Crag, the quartzite is a sedimentary rock called the Basal quartzite. It formed as a beach deposit about 590 million years ago when this part of Scotland was south of the equator and still attached to the North American continent.