Subglacial Landforms Exam Flashcards
What are subglacial landforms?
A generic term for the range of longitudinal and transverse landforms produced at the base of a glacier or ice sheet as a result of active ice flow across a sediment base
(Benn and Evans, 1998).
What are flutes?
They are small, elongated features that orient themselves in the direction of the ice.
They’re usually around 1m x 10m
Normally they have a proximal core and are made of till
They’re found in large groups clustered together
(Morris and Morland, 1976)
How do flutes form?
Rocks may reach the bottom of the glacier and begin to scrape along the earth.
Eventually, boulder-sized rocks become lodged in the ground. Once the boulder is in place, the glacier flows around it.
As a result, a cavity is formed on the down slope side of the boulder. This cavity is then filled with sediments such as till and outwash.
(Morris and Morland, 1976)
What do flutes consist mainly of?
Where are flutes formed?
Subglacial till
Cores of deformed, pre existing sediments
Downglacier where they commence at lodged boulders or clast clusters
(Morris and Morland, 1976)
By what two mechanisms do flutes form?
- pressure drops freezes -> adheres to basal ice > transported forward by ice -> new material added by deformation into cavity at upglacier end.
- Unfrozen sediment -> new sediment added downglacier end as cavity is transported forward by ice flow. (can freeze if contact pmp of ice)
(Morris and Morland, 1976)
What are the two types of flutes?
Why do you get this difference?
Tapering and parallel
Difference in till rheology or strain response to stress
(Morris and Morland, 1976)
What are tapering flutes?
Typical in Norway
Lower and narrower further down glacier.
Stiff, non-dilatant till
The basal ice contracts by ice deformation and a down glacier decay of flute forms.
(Morris and Morland, 1976)
What are parallel flutes?
Typical in Iceland
Constant cross profile for long distances.
Weak, dilating deformation till that deforms more readily than ice (dilitant = material is one in which viscosity increases with the rate of shear strain).
The pressure difference within basal ice grooves have equilibrium via till flow (not ice creep).
Groove closure is prevented and the flute propagates down glacier to the margin.
(Morris and Morland, 1976)
For flutes to form what must the glacier have?
Sufficient energy to deform the sediments.
This energy varies as the ice changes in thickness and velocity (and with the resistance of the sediments themselves also)
Changes in flow direction can be observed in flutes that have enough energy.
(Morris and Morland, 1976)
What are the three theories on flute formation?
Deformation theory- Boulton, 1976
Frozen-on theory -Gordon (1992)
Instability theory - Shaw and Freschauf (1971)
What is the Frozen-on theory -Gordon low-pressure (1992)?
The material is frozen on to the bed of the glacier and is moved on and deposited.
What is the Deformation theory Boulton, 1976?
Flutes form associated with a saturated deforming bed
Till flows into the low pressure area in the lee side of a subglacial clast
Some have clast at the stoss side and have a herringbone fabric pattern.
What is the Instability theory? Shaw and Freschauf (1971)
low-pressure property of the base of the glacier?
Series of obstacles within the till you get a series of flow cells moving sediment away from the troughs to the crest, marked by a herringbone type fabric
What are drumlins?
Drumlins are elongated features made of till that can reach a km or more in length.
One end is quite step, whilst the other end tapers away to ground level. The Stoss end is the steeper of the two ends and used to face into the ice flow.
They may or may not have a proximal core (uncommon)
(Smalley and Unwin, 1968)
What does being so big mean about drumlins?
They are often preserved.
Smalley and Unwin, 1968
3 different types of drumlin formation models
Deforming bed model
Instability model
Fluvial model
(Smalley and Unwin, 1968)
What is the deforming bed model?
If the sediments of the bed are weak they may deform as a result of the shear stress imparted by the overlying ice.
If parts of this deforming till layer vary in relative strength/ there are obstacles, then the stronger stiffer portions will deform less and remain static, whilst the intervening weaker portions will deform more readily and become mobile.
(Smalley and Unwin, 1968)
What is the depositional model?
Form in a similar way to flutes
Similar to crag and tails
Normally have rock core
Not very common (6% Hart, 1997)
What are deformational drumlins?
Form if the core is relatively weak
Different types of deformation
–Deformation of the core
–Stoss-side deformation
(Smalley and Unwin, 1968)
What are erosional drumlins?
Majority of drumlins (60%, Hart 1997)
Normally have till core and a layer over the top
(Smalley and Unwin, 1968)
Deforming bed models
Smalley and Unwin (1968) –parts of the till expand (dilate), and become shaped into drumlins
Boulton (1987) –rigid parts with the till are deformed into drumlins
Boyce and Eyles (1991) Eyles et al 2016•–drumlins are erosional, cut by faster moving till
Hart (1997) relates drumlin formation to other subglacial processes
Concept of sediment flux
More enters than leaves –till deposition
More is removed than enters –erosion of drumlins
Hart (1997) model
Individual drumlin core structures may be formed by different processes but the drumlin shape is result of erosion.
Inhomogeneities in the till act as cores
- Core is strong, material rapidly removed
- Core is weaker so is able to be deformed
- Slower moving till deposited in the lee (low pressure), faster moving till forms the drumlin shape.
Instability model
Hindmarsh 1998, Fowler 2000
Coupled till and till leads to the formation of relief in the till surface, local highs will accumulate till by deposition, local lows will be eroded
But currently these models produce ribs not drumlins
Not related to internal structures observed in the field
Ice velocity and bedforms
Concept faster the ice…
the more elongated the bedform.
What are depositional drumlins?
Drumlins made entirely out of till.
What are erosional drumlins?
Outside of the drumlin bears no resemblance to the inside of the drumlin.
What are the three types of drumlin?
Depositional
Erosional
Deformational
What is the deforming bed model?
Drumlins are like large flutes- some kind of obstacle in the way.
Object more stable and till moves around it.
Core and carapace
Why are drumlins made up of unsorted material?
They are made up of unsorted material because the ice deposited all the material at the same time underneath the glacier as it moved downhill.
What is the orientation of a drumlin?
The steep slope faces up the valley and the more gently sloping ‘lee slope’ faces down valley.
How do drumlins form?
Deforming bed model
Core and carapace
If there is a small obstacle on the ground in the deforming bed, and can build up around it.
Drumlins are formed when the sediment becomes too heavy for the glacier.
The glacier deposits the material, shaping it into streamlined mounds as it flows over the top.
How else can drumlins form?
Drumlins may also form when ice flows over a layer of sediment that is deeper than normal.
The huge overlying weight of the ice causes increased pressure on the sediment beneath it, moulding it into the drumlin shapes.
