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:
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.
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 deposits Typically, 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.
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:
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.
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).