Subglacial Bedforms

Question 1

What are flutes?

Answer 1

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

Question 2

How do flutes form?

Answer 2

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.

Question 3

What do flutes consist mainly of?

Where are flutes formed?

Answer 3

Subglacial till
Cores of deformed, pre existing sediments

Downglacier where they commence at lodged boulders or clast clusters

Question 4

By what two mechanisms do flutes form?

Answer 4

1. pressure drops freezes -> adheres to basal ice > transported forward by ice -> new material added by deformation into cavity at upglacier end.
2. Unfrozen sediment -> new sediment added downglacier end as cavity is transported forward by ice flow. (can freeze if contact pmp of ice)

Question 5

What are the two types of flutes?

Why do you get this difference?

Answer 5

Tapering and parallel

Difference in till rheology or strain response to stress

Question 6

What are tapering flutes?

Answer 6

Typical in Norway

Lower and narrower further down glacier.

Stiff, non-dilatant till

The basal ice contracts by ice deformation and a down lacier decay of flute forms.

Question 7

What are parallel flutes?

Answer 7

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.

Question 8

For flutes to form what must the glacier have?

Answer 8

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.

Question 9

What are the three theories on flute formation?

Answer 9

Deformation theory- Boulton, 1976

Frozen-on theory -Gordon (1992)

Instability theory - Shaw and Freschauf (1971)

Question 10

What is the Frozen-on theory -Gordon low-pressure (1992)?

Answer 10

The material is frozen on to the bed of the glacier and is moved on and deposited.

Question 11

What is the Deformation theory Boulton, 1976?

Answer 11

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.

Question 12

What is the Instability theory?

Answer 12

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

Question 13

What are drumlins?

Answer 13

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)

Question 14

What does being so big mean about drumlins?

Answer 14

They are often preserved.

Question 15

3 different types of drumlin formation models

Answer 15

Deforming bed model
Instability model
Fluvial model

Question 16

What is the deforming bed model?

Answer 16

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.

Question 17

What is the depositional model?

Answer 17

Form in a similar way to flutes
Similar to crag and tails
Normally have rock core
Not very common (6% Hart, 1997)

Question 18

What are deformational drumlins?

Answer 18

Form if the core is relatively weak
Different types of deformation
–Deformation of the core
–Stoss-side deformation

Question 19

What are erosional drumlins?

Answer 19

Majority of drumlins (60%, Hart 1997)

Normally have till core and a layer over the top

Question 20

Deforming bed models

Answer 20

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

Question 21

Hart (1997) relates drumlin formation to other subglacial processes

Answer 21

Concept of sediment flux

More enters than leaves –till deposition

More is removed than enters –erosion of drumlins

Question 22

Hart (1997) model

Answer 22

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.

Question 23

Instability model

Answer 23

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

Question 24

Clark (2010) tries to fit instability ideas into sedimentology

Answer 24

Erosional drumlins –erosional shaping by till –drumlin clone

Depositional drumlins -Agglomeration at obstacles –obstacle drumlins

New type emergent drumlins form due to instabilities

Question 25

Mega Scale Glacial Lineation formations

Answer 25

Extreme form of drumlins from high attenuation (Boulton, 1987, Hart, 1997, Clark 1993)
–Thus formed under fast ice, i.e. palaeoice-streams

Groove-ploughing hypothesis (Clark et al 2003)
–Bedrock topography shape the ice, which moves downstream into soft sediments, cuts grooves in the till, and the till pushed into ridges.

Catastrophic floods (Shaw et al, 2000, 2008)

Instability models
–Spiral flows in basal ice (Schoofand Clark, 2008)
–Rills in the basal hydrological system (Fowler, 2010) –extension instability drumlin theory

Question 26

Ice velocity and bedforms

Concept faster the ice…

Answer 26

the more elongated the bedform.

Question 27

What are depositional drumlins?

Answer 27

Drumlins made entirely out of till.

Question 28

What are erosional drumlins?

Answer 28

Outside of the drumlin bears no resemblance to the inside of the drumlin.

Question 29

What are the three types of drumlin?

Answer 29

Depositional
Erosional
Deformational

Question 30

What is the deforming bed model?

Answer 30

Drumlins are like large flutes- some kind of obstacle in the way.
Object more stable and till moves around it.
Core and carapace

Question 31

Why are drumlins made up of unsorted material?

Answer 31

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.

Question 32

What is the orientation of a drumlin?

Answer 32

The steep slope faces up the valley and the more gently sloping ‘lee slope’ faces down valley.

Question 33

How do drumlins form?

Deforming bed model

Core and carapace

Answer 33

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.

Question 34

How else can drumlins form?

Answer 34

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.