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Glacial Deposits

Glacial Deposits. The specification states that you should be able to: Describe the deposition in glacial environments of boulder clay (till), varves , fluvio-glacial sands and gravels and explain the processes that formed them

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Glacial Deposits

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  1. Glacial Deposits The specification states that you should be able to: Describe the deposition in glacial environments of boulder clay (till), varves, fluvio-glacial sands and gravels and explain the processes that formed them Explain how to identify an ancient glacial deposit using the evidence from rocks, fossils and sedimentary structures

  2. ICE Ice erodes by: • Abrasion: ice carries embedded clasts that act like sandpaper that scrapes away at the rock below. This produces angular fragments and rock “flour” • Striations indicate the direction of ice movement • Plucking: Pieces of rock are pulled from the underlying outcrop

  3. ICE • Pluckingis most common where the underlying rock has lines of weakness eg. joints or faults, or has been weathered • Ice freezes onto the rock and as the ice moves forward it pulls the rock with it • Broken pieces of rock can fall onto the glacier due to freeze thaw on the sides of the valley

  4. ICE • Material carried by glaciers may be deposited as moraine / till • The clasts vary in size from rock flour to very large boulders (erratics) • Therefore poorly sorted and angular

  5. Boulder clay (till), • Till, or boulder clay, is material which has been deposited directly by glacier ice • Glaciers will carry within them anything from boulders the size of a car down to rock flour • Material released from the base of the ice forms deposits of till/boulder clay • As mentioned previously they are very poorly sorted

  6. fluvio-glacial sands and gravels • Sediments that were deposited by melting ice or by glacial streams are called Fluvio-glacial • The typical deposition environment is a braided stream

  7. Braided Streams • When glacial ice melts, the water moves away from the glacial in fast flowing braided streams. • The water transports vast quantities of sediment and larger debris • If the sediment load is very large in relation to the velocity of the stream, the more coarse material may start to block the stream, choking it and forcing it to constantly change it's course • The stream starts to diverge, splitting into numerous segments which split and join repeatedly • Braided streams are typically shallow and wide, surrounded by poorly sorted rock debris

  8. Outwash Plains • These are large areas of glacial sediment deposited by melt water streams furthest away from the glacier • They are formed from gravels, sands and clays, the clays being furthest away from the glacier because the smaller particles are carried furthest

  9. Climate Change • Obviously recognition of glacial deposits are an excellent palaeoenvironment indicator • Their recognition and dating has shown a succession of ice ages through time. They have also been used as evidence for continental drift • Remember the reconstruction of Gondwanaland in the Carboniferous showing the glacial deposits?

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