Glacial Geomorphology
Lecture 2: Glacial Landforms
GGY 166: Geomorphology of Southern Africa
Glacial Erosion
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Glaciers are capable of great erosion
High competency → material doesn’t settle out
Can transport blocks too large for other agents to move
Erode primarily in two ways:
• Plucking
• Abrasion
Plucking
• Def: Process by which pieces of bedrock are lifted out of place
by a glacier
• Occurs where meltwater penetrates the cracks and joints of
bedrock beneath a glacier and freezes
• As the water freezes it exerts tremendous leverage that pries
the rock loose
• Sediment of a variety of sizes incorporated into glacier’s load
Abrasion
• Def: The grinding and scraping of a rock surface by the friction
and impact of rock particles carried by water, wind and ice
• Acts like sandpaper to smooth and polish the land surface
• Pulverized rock termed rock flour
• When ice at the bottom of a glacier contains large rock
fragments, glacial striations can develop
• Def: Scratches/grooves in a bedrock surface caused by the
grinding action of a glacier and its load of sediment
• Used to reconstruct patterns of glacial flow
Striations
Rate of Glacial Erosion
• Controlled by 4 factors:
1.
2.
3.
4.
Rate of glacial movement
Thickness of the ice
Shape abundance and hardness of the rock fragments
contained in the ice at the base of the glacier
Erodibility of the surface beneath the glacier
Glacial Deposits
• Deposited when the ice melts
• Can play a significant role in forming the physical landscape
• General effect is to reduce local relief and thus level the
topography
• Glacial drift
• Def: All-embracing term for sediments of glacial origin no matter
how, where, or in what shape they were deposited
• Differs from other sediments in:
• Consist primarily of mechanically weathered rock
• Debris underwent little or no chemical weathering prior to
deposition
• Two types:
• Till: deposited directly by the glacier
• Stratified drift: sediments laid down by glacial meltwater
Glacial Till
(Marshak, 2008)
Landforms Caused by Glacial
Erosion
• Erosional effects of valley glaciers vs. ice sheets
• Valley glaciers:
• Tend to accentuate the irregularities of the mountain landscape by
creating steeper canyon walls and making bold peaks even more
jagged
• Ice sheets:
• Generally override the terrain and hence subdue rather than
accentuate the irregularities they encounter
• Examples of landforms associated with valley glaciers:
• Glaciated Valleys
• Arêtes and Horns
• Roches Moutonnées
Glaciated Valleys
• Glaciers take path of least resistance by following existing
stream valleys
• Prior to glaciation mountain valleys narrow and V-shaped
• During glaciation valleys widened and deepened forming Ushaped glacial troughs
• Glaciers tend to ‘straighten’ a valley by removing spurs that
extend into a valley. Leads to the formation of a truncated
spur
• Since erosion is partly due to thickness of ice, main glaciers
cut their valleys deeper than tributary glaciers leading to the
formation of hanging valleys
• Local depressions in a valley can form due to plucking and
subsequent scouring by abrasion. When they get filled with
water they are termed pater noster lakes
Glaciated Valleys
Before Glaciation
During Glaciation
After Glaciation
(Marshak, 2008)
Hanging Valley & U-shaped
Valley
Hanging Valley
U-Shaped Valley
(Marshak, 2008)
Glaciated Valleys
• Cirque
Amphitheatre-shaped basin at the head of a glacial valley
Precipitous walls on three sides but open on the downvalley side
Focal point of glacier growth since it is the zone of accumulation
Occur as irregularities in the mountainside that are subsequently
enlarged by plucking and frost wedging
• After glacier has melted away it can leave a small lake known as a
tarn
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• Fjords
• Steep-sided inlets of the sea that are present at high latitudes
where mountains are adjacent to the ocean
• Drowned glacial troughs
• Depths may exceed 1000m
• Sea-level does not act as base level for glaciers
Cirque
Fjord
(Marshak, 2008)
Horns and Arêtes
• Horns
• Def: A pyramid-like peak formed by glacial action in three or more
cirques surrounding a mountain summit
• Forms due to a group of cirques being clustered around a single
high mountain
• Arêtes
• Def: A narrow knifelike ridge separating two adjacent glaciated
valleys
• Resemble sinuous sharp edged ridges
• Develops similar to a horn but in this case the cirques exist on
opposite sides of a divide. As they grow the divide between them
is reduced to a narrow knife-like partition
Horns and Arêtes
Arête
Horn
(Marshak, 2008)
Roches Moutonées
• An asymmetrical knob of bedrock formed when glacial
abrasion smoothes the gentle slope facing the advancing ice
sheet and plucking steepens the opposite side as the ice
overrides the knob
• Owing to their mode of formation they can be used to indicate
the direction of glacial flow
(Marshak, 2008)
Landforms Caused by Glacial
Deposition
• Till deposits characteristically consist of unsorted mixtures of
many particle sizes
• Depositional Landforms:
• Glacial Erratics: An ice-transported boulder that was not derived
from the bedrock near its present site
• Moraines (till)
• Lateral moraine
• Medial moraine
• End moraine
• Drumlin (till)
• Outwash plain (stratified drift)
Moraines
• The term moraine is applied to a variety of landforms, all of which
are composed of till
• Moraines that occur exclusively in mountain valleys:
• Lateral Moraine
• Alpine glaciers moves downvalley eroding the sides
• Debris accumulates on edges of glacier as rubble falls/slides from higher
up on the valley walls
• When the ice melts this debris is dropped next to the valley walls
• Medial moraine
• Created when two alpine glaciers coalesce to form a single ice stream
• Till that was carried on the sides of each joins
• End moraine
• Deposited when a state of equilibrium is reached between
accumulation and ablation
• Although terminus is stationary, ice continues to move forward
delivering a continuous supply of sediment
• As the ice melts the till is dropped and the moraine grows
Moraines
Lateral Moraine
Medial Moraine
(Marshak, 2008)
Other Depositional Landforms
• Drumlins
• Def: A streamlined symmetrical hill composed of glacial till. The
steep side of the hill faces the direction from which the ice
advanced
• Not found as isolated landforms but occur as clusters (Drumlin
Fields)
• Outwash plains
• Def: A relatively flat gently sloping plain consisting of materials
deposited by meltwater streams in front of the margin of an ice
sheet
Drumlin
(Marshak, 2008)
Depositional Landforms
(Marshak, 2008)
References
• Marshak, S., 2008: Earth: Portrait of a Planet 3rd Edition, W.W.
Norton and Company, New York