Glacial Deposits

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Glacial Deposits

• The specification states that you should be able
  to
• Describe the characteristic products of
  sedimentation in glacial environments
• boulder clay (till),
• fluvio-glacial sands and gravels
• and explain the processes which formed them.
• Explain how glacial deposits can be used to
  interpret climatic change.

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Erosion by 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.

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Glacial Moraines
Till deposited as moraine, when the glacier melts
is characterised by poor sorting, angular and
randomly orientated clasts.
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Glacial deposits

• 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 tend to be angular.

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• Erratics found in glacial tills are rocks which
  do not outcrop in the area where they have been
  found.
• They can be useful in determining the direction
  of ice movement.

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Boulder clay (till)

• TILL or Boulder Clay, is material deposited
  directly from melting ice.
• Glaciers and ice sheets 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.
• they are very poorly sorted.

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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.

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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.

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Outwash Plains

• These are large areas of glacial sediment
  deposited by melt water streams furthest away
  from the ice sheet.
• They are formed from gravels, sands and clays,
  the clays being furthest away from the ice sheet
  because the smaller particles are carried
  furthest.
• Meltwater deposits tend to be better sorted than
  glacial deposits and may be cross bedded like
  some river sands.
• Fine sediment such as clays can be deposited in
  glacial lakes as varves

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Till
Fluvial glacial

• Poorly sorted
• Angular
• Clay rich
• No bedding

• Well sorted
• Sub rounded
• Sand composition
• bedding

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Glacial deposits and Climate Change

• recognition of glacial deposits is an excellent
  palaeoenvironment indicator of arctic conditions.
• used as evidence for continental drift and the
  reconstruction of Gondwanaland in the
  Carboniferous.