Discussion

In many reconstructions of formerly glaciated landscapes the role of plateau icefield inputs to the mass balance of disconnected and connected valley glaciers has been ignored. Indeed in some areas this misinterpretation of the style of glaciation may be the cause of large variations in reconstructed ELAs and appeals to large gradients in winter precipitation and excessive snow blow effects.

Sissons (1980) reconstructed the Younger Dryas glacier cover in the Lake District, England and needed to invoke three separate snowfall intensity zones, in combination with the effects of

450400350300-

15010050 0

0 20 40 60

% plateau area

Figure 16.18 Data for the Little Ice Age maximum Lyngen glaciers showing the altitudinal shift in equilibrium line altitude (ELA) as a function of the percentage of the plateau icefield area to the combined glacier area, in accumulation area ratio (AAR) calculations. (The two outliers represent the ELAs shifting up very narrow icefalls.)

0 20 40 60

% plateau area

Figure 16.18 Data for the Little Ice Age maximum Lyngen glaciers showing the altitudinal shift in equilibrium line altitude (ELA) as a function of the percentage of the plateau icefield area to the combined glacier area, in accumulation area ratio (AAR) calculations. (The two outliers represent the ELAs shifting up very narrow icefalls.)

Glacier

Area above/below 800 m contour (km2)

Above 800 m Below 800 m

Glacier area (km2)

Per cent area plateau

Sôrfjorddalen - 1

3.28

4.32

7.6

43.2

Fjorddalen - 2

4.71

7.39

12.1

38.9

Tverrfjorddalen - 3

1.00

2.80

3.8

26.3

Storelvdalen - 4

4.65

2.85

7.5

62

Bac'cavuonvag'gi - 5

1.40

2.20

3.6

39

Isfjorden - 6

11.67

2.43

14.1

82.8

Skalsavatnet - 7

5.39

4.41

9.8

55

Isdalen - 8

11.31

4.99

16.3

69.4

Skognesdalen - 9

0.28

2.52

2.8

10

n-Tverrfjorddalen - 10

2.82

6.58

9.4

30

Table 16.2 Plateau and valley glacier area distributions of 0ksfjordj0kelen reconstructed for the Younger Dryas. (Number after name refers to glacier location shown on Fig. 16.19)

Table 16.2 Plateau and valley glacier area distributions of 0ksfjordj0kelen reconstructed for the Younger Dryas. (Number after name refers to glacier location shown on Fig. 16.19)

Figure 16.19 Modelled reconstruction of the Younger Dryas extent of 0ksfjordj0kelen (dark grey). The sea is white and the land submerged by glacio-isostatically controlled higher sea level is shown in light grey. The boxed numbers on the icefield refer to the glacier names given in Table 16.2. Note the radial drainage pattern shown by the outlet glaciers. (After Evans et al., 2002).

Figure 16.19 Modelled reconstruction of the Younger Dryas extent of 0ksfjordj0kelen (dark grey). The sea is white and the land submerged by glacio-isostatically controlled higher sea level is shown in light grey. The boxed numbers on the icefield refer to the glacier names given in Table 16.2. Note the radial drainage pattern shown by the outlet glaciers. (After Evans et al., 2002).

snowblow and aspect, to account for considerable variations in glacier extent and ELAs (the latter varying by 500 m). However, while Sissons envisaged that variations in glacier extent and ELA were primarily climatically driven, recent research in the Lake District suggests a likelihood that the majority of these variations reflect topo-climatic controls (McDougall, 1998, 2001). Specifically, the more extensive 'valley' glaciers mapped by Sissons were actually outlet glaciers of small plateau icefields. Similar to the contemporary examples cited above, the Lake District valley heads contained substantial volumes of glacial sediment in latero-frontal and linear hummocky moraines that extend in many cases onto plateau margins. Evidence of plateau ice is patchy and often restricted to scoured bedrock at plateau edges. McDougall's (1998) palaeo-glacier reconstructions for the English Lake District, incorporating plateau icefields, suggest that ELAs during the Younger Dryas were up to 90 m higher than those calculated by Sissons (1980) based upon an alpine style of glaciation with no plateau ice.

400 350 300 - 250

Z 200

w 150

100 50

400 350 300 - 250

Z 200

w 150

100 50

OAAR 0.5

o

m

• AAR 0.8

V

o

O

O

8

O

O

• •

% plateau area

1 00

Figure 16.20 Equilibrium line altitude (ELA) shifts plotted as a function of plateau area for each 0ksfjordj0kelen Younger Dryas outlet glacier, for the upper and lower accumulation area ratio (AAR) limits. The plateau summit was arbitrarily chosen as bedrock areas above 800 m.

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