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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.

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Presentation on theme: "Unless otherwise stated the copyright of materials derived from the British Geological Survey's work is vested in the Natural Environment Research Council."— Presentation transcript:

1 Unless otherwise stated the copyright of materials derived from the British Geological Survey's work is vested in the Natural Environment Research Council (NERC), and is acknowledged as ‘British Geological Survey © NERC, 2007. All Rights Reserved’. The user of this CD-Rom is granted permission to access the publication and reproduce free of charge in hardcopy (analogue/paper) format for the purpose of teaching/educational/instructional/informative purposes, either as teaching aids for classes and lecturers, projects or exams set for students. The source of the BGS materials is to be acknowledged. Any permission to reproduce BGS/NERC material does not extend to the reproduction of third party's copyright protected material. The reproduction of BGS material, or that of other copyright owners, in any work intended for commercial usage and sale or distribution to the public or any outside organization is prohibited. This includes any courses that may be prepared/presented by individuals from the educational establishment (teaching/academic staff, research associates, or other employees) for the gaining of any commercial advantage, or for the purposes of supplying outside concerns who may have commercial interests. © NERC, 2007. All Rights Reserved.

2 A wild and rugged landscape © Patricia & Angus Macdonald / Aerographica

3 Geology and landscape Tom Bradwell BGS © NERC

4 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 ice-worn bedrock; corries; arêtes, erratics, moraines and more.

5 Glacial Erosion Tom Bradwell BGS © NERC

6 Processes of glacial erosion Abrasion: Fragments carried in the base of glaciers grind in to the bedrock just like sandpaper on wood. This grinding leaves long grooves called striations if the debris is coarse or smooth polished rock surfaces if the debris is fine. Freeze-thaw: Bedrock beneath glaciers form cracks along planes of weakness. Water enters these cracks and freezes to form ice, which takes up more room. This forces the cracks wider. The water thaws and refreezes, again and again, further widening the cracks and eventually loosening fragments of rock. Plucking: As a result of freeze-thaw, the loose fragments are picked up and pulled away by ice and embedded into the base of the glacier as it moves downhill. The three steps of glacial erosion are:

7 Glacial processes: freeze-thaw Here, water entered what were originally small cracks in this rock. When water freezes it expands, so the ice forced the cracks to widen. The ice thawed, and when more water entered the cracks and froze again, they were forced even wider. This cycle is called freeze- thaw or frost shattering and was repeated, again and again, until the rock was split into several pieces. Maarten Krabbendam BGS © NERCFreeze-thaw on Conival (NC 305195)

8 Glacial processes: plucking & abrasion Glacial ice freezes on to bedrock, fractures it and then pulls it into the base and side of the glacier. The process is made easier where frost action has already loosened the material and the rock is well-jointed. Surfaces affected by plucking tend to be rough and jagged. Here, shallow striations seen on the smooth bedrock surface are evidence of abrasion: they also indicate the direction of ice flow. Tom Bradwell BGS © NERC

9 Arêtes Corries Pyramidal peaks Hanging valleys U-shaped valleys Fjords Cnoc and lochan landscapes Large-scale glacial erosion features in NW Scotland

10 Arêtes Tom Bradwell BGS © NERC Sharp, narrow, dividing ridges of rock separating glacial troughs and corries are called arêtes. This one forms a knife-edge ridge running south from Ben More Assynt (NC 319202). The deep glacial trough of Glen Oykel is on the right- hand side and Glen Cassley is on the left.

11 Corries are large bowl- shaped hollows carved by ice. They normally have a steep backwall (often with an arête at the top), a saucer-shaped corrie floor, and a rock lip at the corrie entrance. In some cases a small rounded lochan occurs in a depression in the corrie floor. In this picture a stream flows to the loch below. Tom Bradwell BGS © NERC Corries Garbh Coire, Conival (NC 305198)

12 Pyramidal peaks (horns) A good example of a pyramidal peak in NW Scotland is Sgùrr an Fhidhleir, approximately 1km north of Ben Mór Coigach. It is 700 metres high. Tom Bradwell BGS © NERC Isolated mountains with 3 or 4 faces formed by the backcutting of glacial corries are called pyramidal peaks, or horns. NC 094055

13 Hanging valleys Valleys left perched above the main valley floor – caused by glacial erosion happening at two separate times are known as hanging valleys. The picture shows a hanging valley on the north side of Cranstackie, 6 km NW of Foinaven. Maarten Krabbendam BGS © NERC(NC 34 56)

14 U-shaped valleys U-shaped valleys (glacial troughs) are formed where pre-glacial valleys are overdeepened and widened by large glaciers during an ice age. They have steep sides and a wide floor; commonly rivers or streams flowing through them are small in relation to the trough. They are called misfit rivers as they lack the energy needed to erode such large troughs. Kathryn Goodenough BGS © NERCStrath More from Ben Hope (NC 478502)

15 Fjords Deep U-shaped valleys or glens are carved by ice, and where they cut down below sea level they form fjords. The deeply indented coastline of NW Scotland is typical of a highly glaciated landscape. This picture is of Loch Glendhu viewed from Kylesku – a classic fjord carved by ice over several glaciations. Tom Bradwell BGS © NERCNC 25 33

16 Cnoc and lochan landscape The NW Highlands are characterised by large areas of rugged cnoc and lochan scenery. This is a lowland of irregular relief usually consisting of rough bare Lewisian gneiss where intense glacial erosion has scoured the surfaces leaving rocky knolls (cnocs) and small basins (lochans). © Rod OwenView west from Cnoc á Chàise (NC 175431)

17 Small scale glacial erosion features in NW Scotland Roches moutonnées Striations Glacial meltwater channels

18 Roches moutonnées Roches moutonn ées are resistant bedrock features commonly found in glacially sculpted terrain. As a glacier flows over bedrock it smooths the upstream (stoss) side by abrasion, and fractures the steeper, craggy downstream (lee) side by plucking. They can be up to 10 metres high and 100 metres long. Nick Gollege BGS © NERC ice flow

19 Roches moutonnées — NW Scotland This roche moutonnée in upper Glen Coul is about 3 metres high. In this valley the last ice sheet flowed NW (left to right) towards the sea, as shown by the smoothed top (stoss) and the rough plucked face (lee). Tom Bradwell BGS © NERCNC 295285

20 Striations Tom Bradwell BGS © NERC A product of abrasion, striations are small straight scratches on rocks formed as the glacier, armed with hard rock fragments, moves over the bed. They are normally long, parallel, and a few millimetres deep and can be used to tell which way the ice was flowing. The striations in this photo, some over 1 metre long, are cut into sandstone next to the football pitch at Ullapool and show that during the last glaciation an ice sheet flowed NW across this ground towards the sea.

21 Glacial meltwater channels Tom Bradwell BGS © NERC Glaciers release large amounts of water as they melt. This water flows as turbulent rivers that carry large amounts of debris. This meltwater channel from the Steinholtjokull glacier in Iceland is carving its way through bouldery debris.

22 Glacial meltwater channels – NW Scotland This meltwater channel is in Glen Oykel. Meltwater channels are often very large relative to the streams that now occupy the valleys – the streams are too small to have cut the valley and are called misfit streams. Tom Bradwell BGS © NERC

23 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).

24 Acknowledgements (cont.) all maps and photographs in this resource are British Geological Survey © NERC, with the exception of the following: P.&A. Macdonald/Aerographica (slides 2) I. MacPhail, NW Highlands Geopark Development Officer (NW Highlands Geopark logo) R. Owen (slide 16)

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