Glaciation

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Glaciation
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Introduction

• The ice age was known as the Pleistocene. During
  this time, there were periods of ice advance
  glacials and periods of ice retreat
  interglacials.
• The Glacier System
• A glacier behaves like a system with inputs,
  stores, flows and outputs.
• Glacial activity is a balance between the
  accumulation of snow and ablation or loss by
  evaporation and meltwater to river systems.

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Glacial Erosion

• There are two main processes of glacial erosion
• Plucking results from glacial ice freezing onto
  solid rock. As the glacier moves away, it pulls
  with it large pieces of rock.
• Abrasion is when the material carried by a
  glacier (moraine) rubs against the rock in a
  sandpaper-like action, which erodes and smoothes
  the rock.
• NOTE for every erosional feature set of
  diagrams, you must explain the above terms.

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Features of Glacial Erosion

• Corrie
• These are armchair shaped hollows with steep
  back walls and a rock basin.
• They are also known as Cirques and Cwms.

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Formation of a Corrie

• 1) Beginning of glaciation
• 2) During glaciation -

ice
Plucking steepens the back wall
Moraine embedded in glacier
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Formation of a Corrie continued

• 3) After glaciation -

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Identifying a Corrie on an O.S. map

• Since a corrie is an armchair shaped hollow, on
  the map you are looking for a horseshoe contour
  shape.
• Tarns / lochans may also be found within the
  horseshoe.
• Names may also help, such as coire, cwm, cirque,
  cove and corrie, which may be written on the map.

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Features of Glacial Erosion

• Glacial trough / U-shaped valley
• A glacial trough is a valley with steep sides
  and a flat base, once occupied by a glacier.

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Formation of a Glacial Trough

• 1) Before glaciation
• 2) During glaciation

River meanders around interlocking spurs
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Formation of a Glacial Trough continued

• 3) After glaciation

A ribbon lake is a long lake found in the valley
floor as a result of the valley shape or moraine
damming. A misfit stream is a stream too small
to have created the valley.
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Identifying a Glacial Trough on an O.S. map

• U-shaped valleys are easy to find on an O.S. map
  (provided that you know the map extract is a
  glaciated upland).
• Look for the steep valley sides indicated by
  the closeness of the contours. The valley sides
  should be roughly straight and parallel.
• The flat valley floor is easily identified it
  usually has few contours and will look white.
  The valley floor may be occupied by a ribbon lake
  or a misfit stream.

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Identifying a Glacial Trough on an O.S. map cont
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Features of Glacial Erosion

• Arêtes and Pyramidal peaks
• When two adjacent corries erode backwards or
  sideways towards each other, the land between
  them becomes narrower, until a rocky, knife-edged
  ridge called an arête is formed. The ridge is
  sharpened by frost shattering.
• If three or more corries develop on the sides of
  a mountain, a pyramidal peak or horn may be
  developed. This has steep sides and several
  arêtes radiating from the central peak.

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Features of Glacial Erosion

• Arêtes and Pyramidal peaks

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Formation of Arêtes and Pyramidal Peaks

• 1) At start of glaciation
• 2) During glaciation

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Formation of Arêtes and Pyramidal Peaks continued

• 3) After glaciation

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Identifying an Arête on an O.S. map

• On an O.S. map, an arête is always between two
  corries, the land has contours close together and
  the highest land is in the middle. Sometimes the
  symbol for bare rock appears.

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Identifying a Pyramidal Peak on an O.S. map

• To identify a pyramidal peak, look for a spot
  height or a triangulation station, with three or
  more corries surrounding it.
• NOTE the triangulation symbol - - does NOT
  mean a pyramidal peak.

Arêtes
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Features of Glacial Erosion

• Hanging valley
• When the valleys are occupied by ice, less ice
  is found in the tributary valley. Less ice
  results in less erosion. After glaciation, the
  tributary valley is left hanging above the floor
  of the main valley. This is known as a hanging
  valley. The valley often contains a river which
  results in a waterfall into the main valley.

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Formation of a Hanging Valley

• 1) Before glaciation

Confluence of main and tributary rivers
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Formation of a Hanging Valley continued

• 2) During glaciation

Ice occupies the valleys, widening, steepening,
deepening and straightening them by plucking and
abrasion
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Formation of a Hanging Valley continued

• 3) After Glaciation

Waterfall
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Identifying a Hanging Valley on an O.S. map

• A hanging valley is the most difficult feature to
  identify on an O.S. map.
• Look for a glacial trough first, then try to look
  for a small valley hanging above the main
  valley floor.

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Features of Glacial Erosion

• Roche Moutonée
• This is a smaller erosional feature, but it
  helps to indicate the direction of ice movement.
  These are not indicated on O.S. maps.

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Formation of a Roche Moutonée

• (only 1 diagram)

On lee side re-freezing and plucking of
loosened material.
Piece of hard, more resistant rock on valley floor
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Glacial Deposition

• Transportation by ice
• Rock debris (moraine) may be carried in three
  ways
• Supraglacially on the surface of the glacier.
• Englacially moraine is carried within the
  glacier.
• Subglacially under the glacier by ice as ground
  moraine or by meltwater streams as a result of
  melting.

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Glacial Deposition

• Deposition by ice
• The collective name for all of the boulders,
  gravel, sand and clay deposited under glacial
  conditions is drift. Drift can be divided into
  till / boulder clay all material deposited by
  the ice (unsorted) and fluvioglacial material
  deposited by meltwater streams (sorted).

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Glacial Deposition

• Deposition by ice
• Till Deposits
• Till is an unsorted mixture of rocks, clays and
  sands. There are two types of till
• Lodgement till (ground moraine) - which is
  smeared on the valley floor when the weight is
  too great for the glacier to move it.
• Ablation till a combination of englacial and
  supraglacial moraine which is dumped in situ when
  the glacier begins to melt.

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Features of Glacial Deposition

• Erratics
• These are boulders picked up and carried by ice
  and deposited in areas of a totally different
  rock type. By determining where boulders came
  from, it is possible to track ice movements.

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Features of Glacial Deposition

• Moraine
• There are six types of moraine
• Lateral from frost shattering of valley sides
  and carried along the edges of glaciers. When
  the glacier melts, it leaves embankments of this
  material along the valley side.
• Medial found in the centre of a valley as a
  result of the merging of two lateral moraines at
  the confluence of two glaciers.

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Features of Glacial Deposition

• Moraine continued
• Ground featureless till deposited on the
  valley floor.
• Terminal also known as end moraine a high
  mound of material found right across the valley
  at right angles to and marking the maximum extent
  of the glacier.
• Recessional marks a retreat of the ice when
  the glacier stayed stationary to allow the build
  up of material. Usually parallel to the terminal
  moraine.

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Features of Glacial Deposition

• Moraine continued
• Push develop if it becomes cold again and the
  ice may temporarily advance.

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Features of Glacial Deposition

• Drumlin
• Drumlins are large, smooth, elongated mounds of
  glacial till. They were thought to have been
  formed when the glacier became overloaded with
  material and had to deposit some. They were
  later streamlined by further ice movement. They
  are almost symmetrical across their long axis,
  which is parallel to the direction from which the
  ice came. The highest point is nearest to the
  stoss side, which faces the direction from which
  the ice came. Drumlins usually occur in swarms.

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Drumlin

• NOTE this is the most likely depositional
  feature you will be asked about. You should be
  able to draw the diagram and write the text on
  the previous slide in your answer.

Overhead view
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Features of fluvioglacial deposition

• These features were moulded by meltwater
• Outwash Plains Made up of gravels, sands and
  furthest from the snout, clays. They are
  deposited by the meltwater streams from the
  glacier. The material may have been deposited by
  ice and later picked up and deposited by
  meltwater streams beyond the maximum extent of
  ice sheets. It may also be deposited on top of
  the till as the glacier retreats.

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Features of fluvioglacial deposition continued

• Kames Kame Terraces Kames are undulating
  mounds of sand and gravel deposited unevenly by
  meltwater like a series of deltas along the front
  of a stationary or slowly melting ice sheet.
  Kame Terraces are ridges of sand and gravel found
  along the sides of valleys. They are deposited
  by the meltwater streams flowing between the
  glacier and the valley wall (these are sorted
  whereas lateral moraines are not).
• Eskers Long, narrow, sinuous ridges of sorted
  sands and gravels. They were deposited by
  subglacial streams during ice retreat.

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Features of fluvioglacial deposition continued

• Kettles These form when blocks of ice are left
  when the glacier retreats. Then they are buried
  by fluvioglacial material. When the ice blocks
  melt, they leave hollows which often fill with
  water to form kettle holes and kame and kettle
  topography.
• Braided streams channels of meltwater become
  overloaded with coarse material and the streams
  divide.