Reconstructing Past Glaciation (lecture 3)

Question 1

What is a trimline?

Answer 1

A trimline is analogous to an erosional ‘tidemark’. It demarcates the transition between ice-moulded, glacially scoured bedrock below (often striated) and the bedrock above influenced by periglacial activity - freeze thaw weathering, structures associated with solifluction and ground ice.
Periglacial weathering limit.

Question 2

What determines the preservation of a tidemark?

Answer 2

Preservation is dependent upon the geology of the bedrock (rock type) - in particular the resistance. What determines this?

Question 3

What rock type best preserve a tidemark?

Answer 3

Resistant, crystalline rocks best preserve this feature.

Question 4

How do trimlines support ice sheet reconstruction?

Answer 4

They reveal the vertical dimensions of the ice sheet - ice sheet thickness.

Question 5

What problems are associated with the use of tidemarks in ice sheet reconstruction?

Answer 5

At the LGM (in North America and Europe) many ice masses engulfed upland areas - the erosional feature would not be present within the landscape.
A thermal boundary that demarcates the separation between warm and cold based ice at the PMP may be erroneously identified as the trimline

Question 6

Give an example where the trimline was used to reconstruct past glaciation. (Ballantyne et al., 2008) case study

Answer 6

Extend of the LGM advance in western Ireland

Question 7

What were the conflicting reconstructions?

(Ballantyne et al., 2008) case study

Answer 7

(1) Ice sheet terminated on land, some areas were left unglaciated
or
(2) Ice sheet extended offshore onto the continental shelf, engulfing save the highest summits - nunataks.
CRN

Question 8

How did the chronology support? (Ballantyne et al., 2008) case study

Answer 8

Surface exposure dates bedrock above trimline as 36-61 kyr BP
Dates below are >24 kyr BP - consistent with post-LGM glaciation

Question 9

Using reconstruction of the minimum altitude, how far did the ice sheet extend offshore? (Ballantyne et al., 2008) case study

Answer 9

25 km

Question 10

How was this 25 km extension verified on the continental shelf? (Ballantyne et al., 2008) case study

Answer 10

Submarine arcuate moraines offshore.

Question 11

What 3 problems are associated with reconstruction of ice sheet margins (lateral extent)?

Answer 11

Geomorphological evidence from previous glacial advances is destroyed by those later - glacial episodes broadly cover the same area - bed substrate becomes stickier following progressive glacial advance.
Landforms modified by periglacial (through meltwater activity during glacier wastage) and para glacial activity - postglacial erosion of subaerial weathering.
Issues arise in interpretation (e.g., moraines in particular).

Question 12

How is lateral extent reconstructed?

Answer 12

Through lateral and terminal (both recessional and end) moraine and ice contact features (e.g., Kame terraces).

Question 13

What determines the presence of terminal moraines?

Answer 13

Rate of retreat, amount of sub glacial sediment, position of entrainment and the topography.

Question 14

What problems are associated with using moraine to reconstruct ice margins?

Answer 14

Maintain steady state for ample time to construct this depositional feature.
Sufficient debris present.

Question 15

What are the issues associated with using moraine to reconstruct ice sheet behaviour (flow dynamics)?

Answer 15

Issue surrounding interpretation - recessional (glacier retreat) or push moraine (reflecting glacier readvance).

Question 16

What’s the role of stratigraphy in reconstructing glaciation? Give an example.

Answer 16

Stratigraphic evidence is used as the conclusive proof of push moraine.
Geomorphological evidence not always present or preserved. Lithostratigraphy and sedimentology records and allows reconstruction of former ice presence e.g., NSL at Sandsend (Robert et al., 2012).

Question 17

What can be used to reconstruct direction of land movement?

Answer 17

Glacial landforms (striae and friction cracks, drumlins and MSL - streamlined bedrock - Roche moutonnées, whalebacks - streamlined ice moulded bedrock) and sediments (till fabrics, erratics and erratic dispersal trains).

Question 18

What are striations (or striae)?

Answer 18

Elongated grooves and scratches produced when clasts embedded in the basal ice (subglacial till) are dragged across the substrate.

Question 19

What determines the size and depth of indentation?

Answer 19

The relative hardness of the clast and the bedrock; and the load.

Question 20

Do striae reveal flow direction?

Answer 20

No they reflect flow orientation because they form parallel to ice flow. N to S or S to N?

Question 21

What three complications arise when interpreting striae or using them in reconstructions?

Answer 21

Not all are related to glacial activity - principle of equifinality. Instead reflect glacio(fluvial), snow-creep and avalanche activity, or geological weakness exploited but subaerial weathering.
Some represent ice movement determined by bedrock irregularities - following curvature of rock outcrop and oblique to ice flow.
Cross-cutting striae - ice divide migration or glacier readvance - striae superimposed upon one another. It is sometimes possible to differentiate e.g., Snowdonia, Wales LGM and Younger Dryas striae.

Question 22

How are friction cracks produced?

Answer 22

Clasts entrained sub-glacially are forced against the underlying substrate.

Question 23

Give two example of friction cracks.

Answer 23

Crescentic gouges and crescentic fractures.

Question 24

How do crescentic gouges and fractures from (in terms of concavity) and how does this relate to ice flow direction?

Answer 24

Crescentic gouges - concave down
Crescentic fractures - concave up
The direction of concavity demonstrates ice flow direction.

Question 25

Are friction cracks reliable in reconstruction?

Answer 25

It can strengthen other lines of evidence but should not be used in isolation as consistent patterns are not always presented.
A consistent pattern should emerge for all ice flow reconstructions.

Question 26

Do Roche moutonnées and whalebacks (streamlined bedrock) give flow direction or orientation?

Answer 26

Roche moutonnées - direction

Whalebacks - orientation

Question 27

How do roche moutonnées form?

Answer 27

Irregular bedrock - an obstruction - compression and abrasion on stoss (upstream) side as the ice deforms (pressure increase - PMP) - plucked and shattered lee side.
Whalebacks - abraded on all sides.

Question 28

What can glacial troughs reveal?

Answer 28

Ice streams may be revealed through glacial troughs (and ice dispersal centres, ice divides) as the trough constitutes the predominant pathway of ice flow.

Question 29

What is the predominant issue with ice direction indicators and interpretation?

Answer 29

When ice flow direction has changed between glacial episodes interpretation can be complicated when landforms from both survive - construct a chronology!
Flow direction or orientation?

Question 30

What are the two types of streamlined sub glacial depositional landforms used in ice sheet reconstruction? Are the genetically distinct? Width ratio?

Answer 30

Drumlins (may potentially transition into) - width ratio: <50:1
MSL - >90:1

Question 31

What other information can drumlins provide?

Answer 31

Major flow arteries - ice streams
Basal ice pressure
Basal sheer stress
Ice flow rate

Question 32

What are indicator erratics?

Answer 32

Far-travelled clasts of known origin.

Question 33

What information do erratics provide?

Answer 33

Local and regional ice movement - used to infer changing ice flow directions.
Lateral and vertical extent of glaciation - much cruder compared to moraine.

Question 34

What are erratic dispersal trains?

Answer 34

Regional areas over which erratics have been dispersed.

Question 35

What is the pattern in [erratic] in dispersal trains?

Answer 35

Decrease in [erratic] with increasing distance from the source - constitutes ‘head’ and ‘tail’ of dispersal train.

Question 36

How are till fabrics used in ice sheet reconstruction?

Answer 36

Ice flow indicator
Clasts in subglacial tills adopt orientation of least resistance to ice flow - this long-axes of coast aligns with direction of ice flow.