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A wild and rugged landscape

© Patricia & Angus Macdonald /

Aerographica

Geology and landscapeTom Bradwell BGS © NERC

The Ice Age

The oldest rocks in the NW Highlands Geopark are about 3,000 million years old: the youngest are 430 million years old – but it is probably only in the last 2 million years that this spectacular landscape has taken on its now familiar appearance.

And it is only 11,000 years since the last glaciers melted in this part of Scotland – a mere blink of an eye in geological time!

The legacy of the Ice Age is everywhere, and the landscape owes much to it and the action of ice on the different rock types in reaching its present form.

How do we know? Because the ice left clues in the landscape: deep U-shaped valleys and fjords; streamlined or ice-worn bedrock, corries, arêtes, erratics, moraines – and more.

Glacial Deposition

Tom Bradwell BGS © NERC Markarfljot, SE Iceland

Transportation of glacial debris

Debris can be moved by a glacier in three ways:

Supraglacial debris – material carried on the surface of the glacier.

Englacial debris – material carried within the body of the glacier. An example is debris which has fallen into crevasses.

Subglacial debris – material carried at the base of the glacier. This type of debris is frozen into the glacier as it moves over its bed, and most glacial sediment is carried in this way. Meltwater streams beneath the ice also carry large volumes of subglacial debris.

Direct glacial depositsTypically, these consist of a variety of unsorted boulders, rocks, clays, sands and silts which have been directly deposited by ice.

Drumlins are oval hills made of till. Drumlins often appear in groups called swarms. The unsorted material is moulded by the base of the glacier to form a steep (stoss) upstream end, with a more streamlined downstream gentler lee slope.

Moraines are ridge-like and hummocky features deposited on the side or in front of the glacier. There are different types of moraines.

Erratics are boulders carried by ice, often for many kilometres, which have been deposited in areas of completely different rock type.

Till is a debris that is deposited underneath the glacier, it is a subglacial deposit. Because of the weight of the glacier, till is very compacted and can be almost as hard as concrete.

Till is a mixture of boulders with sand or clay. It is deposited under the glacier and compressed by the weight of the ice above it.

Tom Bradwell BGS © NERC

As a result it is quite compacted. The till here looks loose but is almost as hard as concrete. This till deposit is near Loch Lurgainn, south-east of Stac Pollaidh.

Till

Moraines

Lateral moraine, formed at the sides of a valley glacier.

Medial moraine, formed from lateral moraines when two valley glaciers join, and trap lateral moraines between them.

Terminal moraine, found at the maximum extent of a glacier.

Hummocky moraine, close-spaced, seemingly irregular mounds.

Depending on where they have been deposited, there are several different types of moraine:

Moraines

As glaciers melt and retreat, irregular mounds of debris containing boulders, stones and mud are dumped at the ice margins. These are known as moraines.

Some NW Scotland examples are found in the Loanan valley near Inchnadamph where hummocky, lateral and terminal moraines have been deposited.Tom Bradwell BGS © NERC Hummocky moraines

Lateral moraine — Iceland


Lateral moraine along the side of the Virkisjokull glacier.

Medial moraine — Iceland


Medial moraine on the Marsajokull glacier.

Two small glaciers coming from the ice sheet have merged to form the much larger Marsajokull, and their lateral moraines have combined to form a medial moraine.

A terminal moraine, marking the furthest extent of the glacier.

A meltwater stream can also be seen flowing from the base of the glacier and under the moraine: this will eventually form an esker once the glacier has melted.

Terminal moraine — Iceland


This is a terminal moraine being deposited at the end of a glacier in Iceland. The poorly sorted debris of boulders, stones and gravel is visible.

This is a cross-section view of moraine seen in a road cutting near Inchnadamph in NW Scotland. It is made mostly of sand, gravel and boulders which were left after the glaciers had melted.



Moraines — old and new

NW Scotland — then and now

This is the same section of moraine near Inchnadamph, and what it might look like if it was still glaciated today.


Kettle holes


A kettle hole forms by the burial, beneath moraine, of a block of ice, left behind as the glacier retreated.

When the ice slowly thaws and the covering material collapses, a depression forms.

Depending on the size of the ice block, kettle holes can be anything from 1 metre to ½ km in diameter, and if they contain water they may form kettle lochs or lochans.

Recently-formed kettlehole, Virkisjökull, SE Iceland

Kettle holes - NW Scotand


There are relatively few kettle holes in NW Scotland – this kettle lochan is one of a small group about 5 km east of Strath Kanaird.

NC 224956

An erratic is a rock which does not belong in its local area which has been transported as if on a conveyor belt of glacial ice before being dumped there.

They are commonly found in areas of completely different geology.

This large erratic of Lewisian gneiss was carried for 10 km and eventually dropped at the coast south of Stoer onto Torridonian sandstone when the ice melted. The difference in rock types is easy to see.

Erratics

Kathryn Goodenough BGS © NERC

This huge boulder in the Swiss Alps will become an erratic when the ice supporting it melts.

Why hasn’t the ice melted yet?

© www.glaciers-online.net

An erratic in the making

Fluvioglacial deposits

As opposed to direct glacial deposits, fluvioglacial deposits have been transported by over long distances by water and are mostly made up of

sorted and layered sand & gravel.

Outwash plains (sandur) and fluvioglacial fans are wide areas of sorted sands & gravels which are deposited at the mouths of glacial meltwater rivers. These rivers are often braided.

Eskers are long, winding ridges of layered sand & gravel similar to railway embankments. They are formed in narrow tunnels beneath the ice in which meltwater streams flows during deglaciation.

Kames are small hillocks of layered sand & gravel: kame terraces are found at the sides of glacial valleys and are composed of layered sand & gravel. They are formed when material is deposited by meltwater streams between the edge of the glacier and valley side.

Several large fluvioglacial fans lie just to the south of the Geopark along the northern side of Loch Broom, and the town of Ullapool is built on one.

The fan comprises a 20 metre thick wedge of sand and gravel, laid down by glacial meltwater flowing SW out of Glen Achall at the end of the last glaciation.

Fluvioglacial fans

© Graham Scott View over Ullapool (NH 1394)

Eskers

Andrew Christie BGS © NERC

Although not common in NW Scotland, eskers provide valuable glacial clues. Subglacial streams normally flow towards the glacier’s end – so as the ice retreated the eskers which were deposited tell us about the shape and extent of former glaciers.

This large Lanarkshire esker shows the glacier’s original direction and cross-section: compare its size against the houses on the left.

Kames and kame terraces

Kames and kame terraces are rare in NW Scotland – this kame terrace is near Kingussie. It has a characteristic flat top and steep edge, and can be seen from the roadside.

© Ailith Stewart NH 735998

Acknowledgements

Authors

Rod Owen (British Geological Survey)Tom Bradwell (British Geological Survey)Jackie Yuill (Perth Academy)Murdo MacPherson (Kinlochbervie High School)

Special thanks to

Paul Ewing (Arbroath High School)Maarten Krabbendam (British Geological Survey) Isobel MacPhail (North West Highlands Geopark Development Officer)

This CD has been produced with co-funding from the Royal Scottish Geographical Society (RSGS).

CD cover artwork by Miranda MacDonald (Kinlochbervie High School).

Rod Owen and Tom Bradwell publish with the permission of the Executive Director of the British Geological Survey (NERC).

Acknowledgements (cont.)

all maps and photographs in this resource are British Geological Survey © NERC, with the exception of the following:

• P.&A. Macdonald/Aerographica (slide 2)

• www.glaciers.online.net (slide 19)

• I. MacPhail, NW Highlands Geopark Development Officer (NW Highlands Geopark logo, Global Geoparks logo, European Geoparks logo)

Copyright for all images on the slides listed below belongs to the named author, and reproduction of the images from www.geograph.org.uk is licensed for reuse made under a Creative Commons License.

• A. Stewart (slide 23)

• G. Scott (slide 21)

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Unless otherwise stated the copyright of materials derived from the British Geological Survey's work is vested in the Natural Environment Research Council