Week 7: Landforms and landscapes from glacial erosion Flashcards
Classification of erosional landscapes
SMALL = mm to a few m
INTERMEDIATE = m to a few km
LARGE = km to 10s km
LANDSCAPES = 100s km
Controls on erosion and form reference
Sugen and John 1976
Controls on erosion and form
Glaciological
Substratum
Topography
Time
Controls on erosion and form: glaciological
PRIMARY CONTROL
Stress (basal shear/normal)
Pw and routing
Ice flow direction/velocity
Thermal regime
Controls on erosion and form: substratum
Structure
Lithology
Degree of pre-glacial weathering
Permeability
e.g. before start of quaternary = warm and tropical = lots of cover by soft, friable, easily eroded material = thick, weathering mantle b/c deep weathering of Earth’s surface
Controls on erosion and form: topography
Morphology of glacier bed
Controls on erosion and form: time
Duration of glaciation
Small landforms (+reference)
Benn and Evans 1998
Striae
Rat tails
Gouges/fractures/chattermarks
P-forms
Small: striae =
Small scratches/thin grooves on bedrock/clast surface due to glacial abrasion
N.B. clast does more work than substrate “glacial polish”
- not continuous
- fine grained, hard rocks (quartzite)
- cross-cutting
Striae: nailheads
No. of brittle failures that end abruptly (not continuous)
Striae: ice direction
Guided by topography
Thickens = more independent/diffluent flow
Thins during deglaciation = trapped in topography
Small: rat tails =
Small version of ‘crag and tails’
Down-ice from resistant nodules
Differential erosion around
Tail on lee side
UNAMBIGUOUS ice flow direction
Small: gouges/fractures/chattermarks =
Crescentric/lunate cracks due to fracture plane dip (=ice direction?)
Few cm wide
Often associated with grooves
Stick-slip behaviour (bed mosaic)
Crescentric =
Concave down ice
Lunate =
Concave up ice
Small: p-forms =
“Plastically moulded forms”
Smoothed depression eroded into bedrock
Vary in size
Transverse/longitudinal to ice flow or NO directional trend
P-forms possible mechanisms:
- Debris-rich basal ice
- Saturated till
- Subglacial meltwater
- Ice-water mixtures
P-forms, references:
SHAW 1988:
Similar to water-eroded scour marks
GOLDTHWAITE 1979:
Striae on surface = glacial
SHAW 1988:
Striae created after, tortuous appearance = not ice?
BOULTON 1974:
Observed with debris-rich basal ice in situ
REA AND WHALLEY 1984:
…and with ice turning corners
BOULTON 1974:
represent alternating erosion forms
P-forms, origin debate:
FLUVIAL EXPLANATION (Shaw)
- scour mark appearance = analagous
- striated = ice erosion after
- tortuous = meltWATER (ice doesn’t flow like that)
ICE EXPLANATION (Boulton)
- scour marks = fluvial erosion afterwards
- striated = can not be formed by meltwater
- tortuous appearance = evidence of ice flowing like that IN SITU
Intermediate landforms
Roche moutonees
Crags and tails
Channels
Tunnel valleys
Ice marginal channels
Pro glacial channels
Intermediate: roche moutonees =
Asymmetric bedrock bumps w/ abraded stoss faces and quarried lee faces
Vary <1m to 100s m
Shape = stress distribution across bedrock bump
Roche moutonees formation
Tend to form under thin ice with fluctuating Pw (Sugden et al 1992)
Bedrock structure important (Sugden et al 1992, Roberts et al 2005, Lane et al 2014)
- no fractures/bedding planes = harder to erode into classic shape
- oblique to ice flow = asymmetric
- Thick ice pushed down onto bed due to g
- Starts to thin = normal P drops off = meltwater
Are roche moutonees a production of deglaciation or peak glaciation?
DEGLACIATION?
Or immature pre-glacial landforms?
Whaleback =
Cousin of roche moutonees
No pluck phase, smooth
Coupled to bed during formation = unable to produce cavity to promote plucking
Intermediate: crags and tails
e.g. Edinburgh Castle/Royal Mile
= resistant bedrock crag with less resistant tapering tail
Tail length due to pressure shadow - due to velocity?
Intermediate: nye channels
Cut directly into bedrocks
- v high subglacial Pw
- water in isolation and its erosive power
10-100s m long, 10s m wide
Follow hydraulic gradient (ice surface slope)
Local topography important
= linked cavity networks
- anastomosing patterns/shallow depressions
What can nye channels be used for?
Reconstructing subglacial meltwater drainage and equipotential contours (Booth and Wallet 1993)
Intermediate: tunnel valleys
Similar to nye channels but >100km long, 4km wide
Can be in isolation/dendritic/anastomosing
Commonly:
- flat bottomed (i.e. dissipate completely into area of ~flat sea bottom)
- steep sided
- infilled with sedimentary sequence
N.B. huge meltwater drainage at bank-full conditions
Tunnel valley debate
- S-S meltwater drainage over deformable bed towards end of glacial stage (Shoemaker 1986)
- Catastrophic meltwater drainage
- e.g. N Sea lake build up at ice sheet sides = cut off drainage and changes direction - Time-transgressive formation close to ice margin (Kristensen et al 2008)
- end of glaciation meltwater forced out up reverse slope = supercooling
- freeze in water and sediment = reverse clinoforms infill valley
- need water going into body of certain elevation
Clinoform =
Bedding form often associated with deltas
Intermediate: ice marginal (lateral meltwater) channels
Drain and cut channels alongside ice margin and sub-marginally
Preferably alongside cold based ice
What can lateral meltwater channels be used for?
Reconstructing marginal recession and ice sheet elevation (Greenwood et al 2007)
Example of switching between lateral and subglacial channels
Lake District
Temperate/warm-based
Intermediate: pro-glacial channels
Form by catastrophic draining of lake = channels/gorges
Can exploit previous subglacial (Nye) channels
Discrete spillways
Discrete spillway =
Where water escapes from lakes over low points/cols
Pro-glacial channel example
Channeled scablands Washington State, USA
(Largest are formed by jokulhlaups like this)
Baker and Bunker 1985
Mega-grooves =
Straight grooves // to bedrock troughs
100s to 1000s m
Formed by glacial ice/meltwater = erosion
- Structurally controlled
- Structurally independent
* *diagrams**
Relationship to fast flow e.g. ice streams
Mega-ridges =
Straight ridges // to bedrock troughs
100s to 1000s m
Not necessarily related to fast flow; multiple glacial cycles = same repeated flow paths i.e. positive feedback
NOT diagnostic of “ice stream onset zones”
Ice stream onset zones =
Flow converges = high velocity and bed erosion
= series of subglacial canyons/areas of subglacial scour
Streamlined hard beds concept =
Influence of bedrock properties (hardness/fracture spacing/bedding) on occurence/character of mega-scale bedrooms
Erosion dominated by preferentially focussed abrasion/lateral plucking
Eyles et al 2012/2015
Krabbendam et al 2016
SHOWS THAT ICE STREAMING DOES NOT NEED A DEFORMABLE BED BUT CAN EQUALLY OCCUR ON A SMOOTH HARD BED
Large scale landforms
Rock basins and overdeepenings
Troughs and fjords
Cirques
Large: rock basins and overdeepenings =
Depressions commonly filled with post-glacial lakes
Form by quarrying/abrasion
+ enhanced by meltwater pressure variations (Hooke 1991, Kor et al 1991)
Can also occur in soft sediments (plucking/thrusting/advection of till)
Controls on rock basins and overdeepenings
Glaciological variables
Substratum characteristics (structure/lithology)
Formation of rock basins and overdeepenings
Downhill = extension
Hits bump = compression
Over bump = extension
Basal shear stress mostly over the over deepening
- associated with supercooling freezing sediment back onto glacier base = allows to rethicken
Grounding line instabilities = ice wants to lift
= increase flow velocity/surface fracturing/rate ice wants to flow back as it comes back over over-deepening
Large: troughs and fjords
U-shaped, tend towards asymmetry
Some = pre-existing fluvial valleys/cut deep below SL
Jamieson et al 2008 + Kessler et al 2008 = +ve feedback linked to topographic steering and erosion proportional to ice discharge
How long do fjords take to form?
1.2 Ma
Large: cirques =
flat-floored/overdeepened basin connected to steep back wall by concave slope (Benn and Evans 1998)
Types of cirque
Benn and Evans 1998:
- Simple
- Compound
- Cirque complexes
- Staircase
- Cirque troughs
How do cirques form?
Develop from hollows - progressive back wall retreat and floor downcutting via:
- quarrying
- abrasion
- weathering
- mass movement
Thicken and merge across landscape = ice dome (flows irrespective of topography)
Develop from more than one episode of occupancy (Evans and Cox 1995)
What are landscapes of glacial erosion based upon?
The regional association of landforms
Landscapes of glacial erosion
Areal scouring
Selective linear erosion
Little/no glacial erosion
Alpine
Cirque
Landscapes: areal scouring
Everywhere dominated by glacial erosion
e.g. Canadian Shield NW Scotland
- streamlined bedrock knobs
- roche moutonees
- rock basins
Extensive warm-based erosion/basal sliding
Low relief areas
Landscapes: selective linear erosion
In troughs with intervening plateau surfaces unmodified e.g. Scandinavia
- Fast, warm-based ice in troughs
- Slow, cold-based ice on plateau
Ice converges and moves through pre-existing corridors
Landscapes: little/no glacial erosion
Unmodified pre-glacial landscapes survived glacial occupation
Protected under cold based ice
Characteristic dendritic pattern of periglacial features:
- fluvial valleys
- tors
- blockfields
- patterned ground
Landscapes: little/no erosion debate
Subglacial Thermal Organisation (STO)
Kleman and Glasser 2007
Cold based ice vs no ice = ongoing debate
Exposure dating can help
Landscapes: alpine
In areas subjected to repeated valley glaciation
= networks of glacial valleys/cirques separated by aretes
High relief, tectonically active mountain ranges e.g. Himalayas
Polythermal/warm-based
Aretes =
Ice-free mountain peaks
Landscapes: cirque
Independent cirques incised into upland terrain
Density/orientation/altitude = palaeoclimate indicators (Evans 1977)
Linked/associated with alpine landscapes
Cirques are sensitive indicators of…
Climate regime
CASE STUDY: Jakobshavn Isbrae, W Greenland FACTS
Sugen 1974 = landscapes of areal scour and selective linear erosion
Roberts et al 2005 = reconstruct ice sheet/stream evolution from various scales of erosional landforms
- Roche moutonees evolved over multiple cycles = changing striae direction (during deglaciation when ice became trapped into fjords)
- Whalebacks and RMs shape changed over time = different phases of glacial cycle
- Landscape has multiple glacial cycles locked within
CASE STUDY: Jakobshavn Isbrae, W Greenland STAGES
- 1st advance
- thick, fast ice
- increased velocity lowers viscosity = suppresses bed separation
- WB - Deglaciation
- thinning ice, velocity fluctuations
- bed separation as cavities open = plucking
- penetration of meltwater/air to base
- Was modified to shorter RMs - 2nd advance
- thick, fast ice
- RMs smoothed = multi-way smaller WBs over macro perturbations
