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Glaciers. Nature’s Bulldozers CGF3M Wed. Nov. 6, 2013. Glacial Erosion . As glaciers move, they erode the land in two ways: plucking and abrasion.

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Nature’s Bulldozers


Wed. Nov. 6, 2013

glacial erosion
Glacial Erosion

As glaciers move, they erode the land in two ways: plucking and abrasion.

  • Plucking occurs when melt water penetrates the cracks and joints of bedrock beneath a glacier and then freezes. As the water freezes, it expands, prying pieces of bedrock loose. In this manner, sediment of all sizes become part of the glacier’s load.
  • Abrasion occurs when rock particles grind away the rock surface underneath, acting like a piece of sandpaper. Long scratches or grooves, called glacial striations, may be cut into the bedrock.
glacial erosion1
Glacial Erosion

Different types of glaciers erode the landscape in very different ways:

  • Alpine Glaciers carve out and accentuate the rocky surfaces of a mountain landscape
  • Continental ice sheets override the terrain and smooth it out.
factors that control the rate of erosion
Factors that control the rate of erosion:
  • How quickly the glacier moves
  • How thick the ice is
  • The shape, number, and hardness of rock fragments at the base of the glacier
  • The type of surface beneath the glacier
glacial erosion2
Glacial Erosion

The following slides outline features resulting from glacial erosion, and explains how they are formed.

U-Shaped Valley
  • Glaciers follow the path of least resistance by following the course of a pre-existing valley
  • A v-shaped valley is carved by the glacier into a U-Shaped valley.
  • The valley is now wider and deeper
Truncated Spur
  • As a glacier flows around sharp curves, it shears off pointed spurs of land that extend into the valley.
  • The result is a blunt-ended ridge of rock jutting from the side of a glacial valley.
Hanging Valley
  • A valley that drops steeply from high levels, feeding into a deeper main valley
  • Results in spectacular waterfalls
  • Created when a small tributary glacier joins a larger glacier
  • A deep inlet where mountains meet the ocean
  • Flooded valley floors, were created when glaciers melted
  • May exceed depths of 1000 metres
  • The point from which a glacier originally grew
  • Formed when the head of a glacier erodes backwards into the mountainside
  • Results in a hollowed out depression
  • Sometimes, a small lake, called a tarn, can be found at the bottom of the cirque.
  • An isolated spire of rock
  • Caused by a group of cirques around a single, high mountain
  • The Matterhorn in the Swiss Alps is the most famous example.
  • A narrow, knife-like feature
  • Caused by the growth of cirques on opposite sides of a ridge of rock
  • As the cirques grow, the ridge becomes more pronounced
glacial deposition
Glacial Deposition
  • Large volumes of sediment are eventually deposited as the glacier melts
  • All sediments deposited by a glacier are called drift.
  • Drift may include sand, clay, or boulders
  • Materials deposited directly by the glacier are called till.
  • Sediments deposited by glacial melt water are called stratified drift. Often composed of sand and gravel.
  • An elongated or oval hill formed by ice sheets
  • Usually found in clusters
  • The steep side faces the direction from which the ice advanced
  • The gentler slope points in the direction of the movement of ice
  • Common in NW Ontario!
  • A boulder found in the till or lying on the surface that differs from the underlying bedrock
  • Picked up by a glacier and may be carried far from its place of origin before the glacier deposits it
  • A long, narrow, meandering ridge of stratified drift
  • Formed when a meltwater river flows as a channel through old glacial ice and deposits sediments
Kames and Kettles
  • Kames are sediment hills- appear low and irregular
  • Kettles are depressions- formed when a block of ice becomes partially buried in drift and melts, leaving a pit in the sediment
  • Most kettles are less than 10m deep and no more than 2 km long
  • Kettles often fill with water and become ponds or small lakes
  • An accumulation of boulders, stones, or other debris carried and deposited by a glacier
  • Terminal moraines are ridges of till that mark the former position of the end of the glacier
  • Recessional moraines form during the retreat of a glacier, whenever the ice stops for a while

Alluvial fana triangle shaped deposit of gravel, sand and smaller pieces of sediment such as silt. Created by flowing water. Associated with alpine glacier

  • Outwash plain

A plain formed of glacier sediments deposited by meltwater at the terminus of the glacier. Associated with continental glacier.

in groups define the following
In groups, define the following:

Misfit stream

A stream in a valley which is larger than would be predicted based on the stream’s erosive power

Ribbon lakea long narrow lake found in a glacial trough. On area of alternating bands of hard and soft bedrock.



Loose debris or talus accumulated at the foot of a cliff comprised of angular stones and boulders

label the diagrams
Label the diagrams
  • Label the glacial features on LM 3-1a, using the terms we learned today.