Deposition in Glacial Landscapes



  • Glaciers deposit their load when their capacity to transport material is reduced. This usually occurs as a direct result of ablation during seasonal periods of retreat or during de-glaciation. However, material can also be deposited during advance or when the glacier becomes overloaded with debris.
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  • All material deposited during glaciation is known as drift. This can be subdivided into tilt, which is material deposited directly by the ice and outwash, which is material deposited by meltwater. The latter is also known as glacio-fluvial material.
  • It is estimated that glacial deposits currently cover about 8% of the earth's surface. In Europe they cover almost 30% and are mainly material left by earlier ice masses that have since retreated. East Anglia has deposits of up to 143m thick. However, in the gulf of Alaska they are, in places, 5000m thick. In active glacial areas, it is possible that rates of deposition are in the order of 6m every 100 years.
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Till Characteristics

  • Angular or subangular in shape- as it has been embedded in the ice and has not been subjected to further erosion processes, particularly by meltwater which would make it smooth and rounded, although it ma have been altered in an earlier erosion period by meltwater before being entrained, transported and deposited by glaciers
  • Unsorted- when glaciers deposit material and sizes are deposited en masse, together. When water deposits material, it loses energy progressively and deposits material in a size based sequence
  • Unstratified- glacial till is dropped in mounds and ridges rather than in layers, which is typical of water borne deposits.
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Two types of till

Two Types of Till:

  • Lodgement Till- this is material deposited by advancing ice. Due to the downward pressure exerted by thick ice, subglacial debris may be pressed and pushed into existing valley floor material and left behind as the ice moves forward. This may be enhanced by localised pressure melting around individual particles that are under significant weight and pressure. Drumlins are the main example of landforms of this type.
  • Ablation Till- This is material deposited by melting ice from glaciers that are stagnant or in retreat, either temporarily during a warm period or at the end of the glacial event. Most glaical  depositional landforms are of this type.
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