10 Truncated spur
Bonus question: what scale category would these landforms be in?
Lesson title: Erosional
2A.7 Glacial erosion creates distinctive landforms and
contributes to glaciated landscapes
N.B. Start of Enquiry Question 3
• Erosional processes: abrasion, plucking (aka quarrying)
• Dilation and fracture and traction (caused by crushing)
• Meltwater flow (caused by basal melting)
• Weathering: freeze-thaw aka frost shattering
• Mass movement: Solifluction, creep
• Cirques/corries, arêtes, pyramidal peaks, glacial troughs (U-
shaped valleys), truncated spurs, hanging valleys and ribbon lakes
--- all macro/meso/ micro features?
These are all created by the processes previously covered in
cirque/ valley glacier locations (temperate glaciers)- weathering
(freeze-thaw), plucking, abrasion.
Next we need to look at landforms formed by ice sheet scouring:
1. Knock and lochan (macro-scale)
2. Roches moutonées (meso)
3. Crag and tail (meso)
Term Scale Appearance Description
Erosional Landforms Cirque/ corrie
U shaped valley (trough)
Crag and tail
Knock and lochan
For scale specify if it is macro, meso or micro scale.
Focus on these
Complete others as
revision task for
• A ribbon lake is a long and narrow, finger-shaped lake, usually found in a glacial trough. They are macro-scale formations
• Its formation begins when a glacier moves over an area containing alternate bands of hard and soft bedrock. the rate of
erosion at the base will vary largely due to the underlying bedrock.
• If the glacier moves over a softer rock it will erode more easily than a harder one. This creates a ‘stepped’ long profile (not
completely smooth), with hollows called rock basins.
• On either side of the rock basin, the more resistant rock is eroded less and these outcrops of harder rock are known as
rock bars, which act as dams between which rainwater may accumulate after the retreat of the ice age, filling up the rock
basin and creating a ribbon lake.
• Another way they can form is by terminal moraines blocking the meltwaters route downstream and acting as a dam thus
forming a lake behind it.
• Sometimes these deep glacial troughs fill with seawater as the sea level rises as a result of melting ice increasing sea
levels. These are known as fjords (flooded glacial valleys)- common in Norway (aka sea lochs in Scotland).
Scouring (erosional process) & knock and lochan
Scouring occurs when ice sheets and glaciers expand
out beyond constrained mountain valleys and erode large
areas of lower relief land.
This type of erosion is typical of warm-based ice
(temperate glaciers) which are moving relatively slowly
and eroding the bedrock beneath (subglacial).
The landforms are largely dependent on the bedrock
(hard/soft) leading to different rates in erosion.
In Scotland landscapes to experience scouring are
known as knock and lochan (macro-scale) as the higher
areas of resistant rock (knocks) are interspersed with
small lakes (lochans).
Definition of knock and lochan: a glacially-scoured
lowland area which displays alternating roches
moutonnées (cnoc: a small rock hill in Gaelic) and eroded
hollows often containing small lakes (lochans).
Fracturing often determines the locations of linear
valleys and rock basins.
Linton(1963) coined this term to describe the highly
irregular topography on the Lewisian gneiss of the NW
Highlands and Outer Hebrides.
Roches moutonées (meso-scale erosional
These form as a result of glaciers eroding
an outcrop of hard rock. They have a clear
stoss (upside) and lee (downside) side. On
the stoss side abrasion smooths the rock
surface. On the leeward side (at the end of
the outcrop) the pressure levels suddenly
drop and a gap between the glacier and the
bed may be present. Subgalcial meltwater
fills this gap, refreezes onto the glacier
base and plucks the underside creating a
rough side (contrasting the smoothness of
the top of the rock)
Striations are common as subglacial
material scratches the rock as the glacier
passes over the top of it.
e.g. Cairngorms, Scotland (300m long and
30m in height)
Crag and tail (meso-scale landforms)
• The most famous crag and tail is the site of Edinburgh Castle. The castle sits on an igneous
intrusion which is incredibly resistant to erosion.
• This hard rock provides a ‘pressure shadow’ on its lee side resulting in the soft rock behind it
being eroded less than that around it.
• Crag and tail landforms are normally larger than roche moutonées.
• These can also be classed as depositional as sediment is sometimes deposited in the shadow zone
creating a tail of till.
Irregular chips and fractures in the rock . Chatter
marks are commonly 1–5 centimetres (1/2–2 inches)
but may be submicroscopic or as much as 50 cm in
length. They occur mainly on hard, brittle rocks
such as granite and are formed under a glacier by
the pressure and impact of boulders moved along
by irregular rolling or sliding. The resulting pattern
of impacts has been likened to the “chatter” of a
carpenter’s chisel slipping along the surface of a
piece of wood. Chatter marks are commonly
arranged in nested series, with the orientation of
the fractures at right angles to the direction of
Crescentic gouges are similar though more regular
in appearance and are formed by the juddering
effect of moving ice. The crescents are usually
concave up-glacier (this image therefore indicates
the glacier moved from top to bottom).
Crescentic gouges on
bedrock of gabbro, Loch
Coruisk, Isle of Skye,
Scotland. Photo M. J.
Linear fine scratches formed by
the abrasive effect of debris-rich
ice sliding over bedrock.
Intersecting sets of striae show
differences in flow direction.
Exceptionally well-preserved striated pavement of Permo-
Carboniferous age (c. 290 m.y. old) near Douglas in the Great
Karoo region of South Africa. Photo M. J. Hambrey.
Quick Quiz- erosional landforms
1. Give an example of knock and lochan.
2. Name three macro scale landforms.
3. What processes create roche moutonées?
4. What process creates chattermarks?
5. Give an example of an arête
6. Give an example of a pyramid peak.
7. How are striations formed?
8. What are the two main causes of ribbon lakes?
9. Give an example of a ribbon lake.
10. Draw a corrie.