Glacial Sediment

Question 1

What is a till?

Answer 1

• It’s a type of sediment that is only deposited by glaciers.
• Type of diamicton (made of many different sized particles).
• Usually unstratified (no layers) and unsorted (not sorted by size due to energy)
• Usually contains far-travelled material (erratics)

Question 2

What are till pinnacles?

Answer 2

Larger stones protect some of the tills underneath resulting in pillars
Usually has some sort of calcium carbonate component to it (can bare its own weight)

Question 3

Types of Glacial Till

Answer 3

Supraglacial till

Sub-glacial tills

Question 4

Sub glacial till types…?

Answer 4

Melt out till (commonly forms within cavities)

Lodgement till (“lodged” by the ice onto the substrate)

Deformation till (deformation of soft non-glacial sediments)

Question 5

Supra-glacial till types…?

Answer 5

Supra-glacial meltout till (e.g., lateral and medial moraines)

Flow till (from flow of supraglacial material at the ice front)

Sublimation till (form only in very cold regions, not common)

Question 6

What does fabric mean?

Answer 6

Orientation of the stones in a till

Question 7

What supra glacial till?

Answer 7

Debris on the surface of the glacier, the ice melts out and a till remains on the land surface.

Very random in the orientation of the grains - may be slow or fast.

Generally chaotic fabric, also called ablation till.

May be affected by sediment gravity flows (then called flow till).

Question 8

What are flow tills?

Answer 8

Wet sediment sliding down slope.

Any till that has flowed downslope.

Question 9

Lodgement till

Answer 9

Till deposited in the direction of ice flow from a moving glacier.
Friction retardation against a hard rock bed.

Where the glacier is moving up obstacles, then the glacier is in contact with the bed.
The stones could stick through friction.

Question 10

How would you identify something as a lodgement till?

Answer 10

Stoss and lee clasts (minor’s roche mouteneees with a braided side and a jagged side).

Strong fabric in ice direction producing striated clasts

No evidence of deformation

Question 11

What is ploughing?

Answer 11

Stones are getting pushed up against some kind of surface.

Pushes up the material in front of the clasts.

Makes the bed a little bit more active.

Question 12

Deformation till

3 elements to deformation till

Answer 12

Stones are being removed at the ice till interface (an element of meltout).

As the ice moves forward, so the till moves forward (from one glacial environment to another).

You can work out the thickness of this deforming layer.

The base in the deforming layer is changing, material beneath is being deposited because it’s not moving any more. Where there is soft bed material there is potential for sediment to get deformed.

Question 13

Styles of deformation

Answer 13

Low shear strain, vertical strain markers are overturned in the ice direction

Ice wedge casts

Faults

Shows you how much deformation has gone on
What was originally vertical has been overturned

Question 14

Folds and boudinage

Answer 14

When there is a rheological difference between the pre-glacial sediments and the till, boudins will form

Question 15

Boudinage

Answer 15

Extensions of beds of different competance

Question 16

Folds are rare in

Answer 16

shear zones, except in protected low pressure zones, e.g. on the distal side of a boudin.

Question 17

Constructional Deformational Sequence

Answer 17

Thin deforming bed / high advection
Builds up a sequence of preserved layers
Deformation increases upwards
• E.g. Gt Blakenham

Question 18

Excavational Sequence

Answer 18

• Thick deforming bed / lower advection
• Deforming layer moves down into preglacial sediments
• Deformation similar throughout
• e.g. West Runton

Question 19

What is a sediment gravity flow?

Answer 19

Wet sediment sliding down slope easily.

Question 20

What is flow till?

Answer 20

Any till that has flowed downslope.

Question 21

How do cavities form?

Answer 21

If you’re in a hard bedrock environment, the bed of the valley is going to be very irregular.
The glacier has to go up and down these obstacles.
As it does so it makes a cavity behind the obstacle.

Question 22

What does the fact that within the cavity is slightly warmer material?

Answer 22

The material will slightly melt out, and so does the debris.

Melt out till

Question 23

What is simple shear?

Answer 23

Much like a stack of cards if you sheer the top when the bottom stays in place, the whole rest of deck stretches.

This thin zone of deformation is called the shear zone.

Question 24

Clast shape of subglacially transported tills

Answer 24

Sub-glacially transported tills typically have sub-rounded to rounded, striated, faceted and spherical (not elongated) clasts, with a bimodal distribution (silt/clay and coarse).

Question 25

Clast shape of supra glacially transported tills

Answer 25

Supra glacially transported tills typically have angular, non- spherical clasts with a coarse unimodal grain-size distribution (this example from Whistler is not unimodal!)

Question 26

What are melt out tills formed from?

Answer 26

Formed from the melting out of the debris rich basal ice layer without deformation

Debris on the surface is frozen into the ice layer
Slightly warmer in the cavity so that material will slightly melt out and so will the debris (unconsolidated material)

Question 27

Where are melt out tills best preserved?

Answer 27

Best preserved in cavities, in hard rock environments

Flat bedrock environments are unlikely to contain melt out tills

Question 28

What is effective pressure continuum?

Answer 28

The difference between the weight of the glacier and the water pressure in the till

Question 29

Regarding effective pressure, at the rigid bed there is…

Answer 29

High effective pressure

Question 30

Regarding effective pressure, at the deforming bed there is…

Answer 30

Low effective pressure

Question 31

Where does subglacial deformation occur?

What is essential?

Answer 31

In a shear zone.

A wet sediment deforming bed is essential.

Question 32

What happens during shearing?

Answer 32

Irregularities get stretched out and when it does it forms a lamination (line).

If you stretch (attenuate) something so much you produce boudins. when it snaps.

Once boudins form laminations form around them.

Question 33

In a lodgement till, stones are being lodged against the bedrock oriented with the direction.

In the deforming layers the clasts are…

Answer 33

Rotating

Question 34

What are the three zones in deformation till?

Answer 34

Homogoenous zone
Sheared zone
Overturned zone

Question 35

What is a subaqueous till?

What are dropstones?

Answer 35

Debris in the ice of icebergs that are released as the iceberg melts into a waterbody.