Box 94 Interpreting Landform Assemblages Examples From The Scottish Highlands

The mountains of the British Isles were last occupied by glacier ice during a short sharp return to cold conditions at the end of the last glacial cycle, which is known as the Younger Dryas. Glaciers left a distinctive topography of mounds and ridges that became known as Scottish Hummocky Moraine. These deposits were interpreted in the late 1960s and 1970s as the product of ice stagnation; a theory consistent with emerging evidence of the rapidity of climate warming after the Younger Dryas. This model became the dominant paradigm and the geometry of these former ice bodies were mapped out on the basis of the extent of such deposits and palaeoclimatic inferences made on the basis of these palaeoglaciological reconstructions. This non-genetic landform interpretation was challenged in the late 1980s and early 1990s by several different glacial geologists who started to see Scottish Hummocky Moraine as an assemblage of ice-marginal landforms that could be deciphered to reveal a picture of active rather than passive deglaciation (Bennett, 1994). Since then the focus has been on interpreting this landform assemblage and deciphering its palaeoglaciolo-gical significance. In recent years these models have become increasingly sophisticated demonstrating the glaciological potential of Britain's upland areas (Benn and Lukas, 2006). The case study demonstrates how paradigms shift though time and the importance of the objective appraisal of field evidence.

Sources: Benn, D.I. and Lukas, S. (2006) Younger Dryas glacial landsystems in north west Scotland: an assessment of modern analogues and palaeoclimatic implications. Quaternary Science Reviews, 25, 2390-408. Bennett, M.R. (1994) Morphological evidence as a guide to deglaciation following the Loch Lomond Readvance: a review of research approaches and models. Scottish Geographical Magazine, 110, 24-32.

9.1.3 Ablation Moraines

Material on the surface of a glacier may become concentrated at the ice margin. This results from the supply of supraglacially transported debris and the transfer of subglacial and englacial material to the ice surface by upward flowing ice and englacial thrust planes (see Section 7.4). As supraglacial debris accumulates it initially accelerates ice melt, because darker surfaces absorb solar radiation more effectively than light surfaces, which are more reflective. However, surface melting is retarded as debris thickness increases because it insulates the ice from surface heating. If the debris cover is sufficiently thick, the glacier margin may become detached from the main body of the glacier and become stagnant, resulting in an ice-cored moraine or ablation moraine. The morphology of ablation moraines is controlled by the distribution of debris on and within the glacier, which may be either a product of the glacier structure - thrusts and folds within it - or the result of the surface distribution of supraglacial debris (see Section 7.1). The supraglacial debris distribution is controlled by the location of medial moraines on the glacier and the presence of patches of rock-fall debris.

The type of landform produced depends on two main variables: (i) the debris concentration; and (ii) the nature of the debris supply (i.e. continuous or discontinuous). If the debris concentration on the glacier surface is low and it is concentrated along the ice margin, then discrete moraines will tend to form by dumping and ablation of a narrow belt of buried ice along the ice margin (Figure 9.12). However, if the debris concentration is high and it is spread over a larger part of the glacier an area of hummocky moraine will result (Figures 9.12 and 9.13). Hummocky

A Formation of a single ablation moraine (1)

Thrust planes

A Formation of a single ablation moraine (1)

Thrust planes

Frontal dumping and meltwater flow

C (1) Pulsed supply of (2) Continuous supply of thick supraglacial debris supraglacial debris

Frontal dumping and meltwater flow

Ablation moraine

Ice core

C (1) Pulsed supply of (2) Continuous supply of thick supraglacial debris supraglacial debris

Ice

b

l-íí--;

B Formation of a broad ablation moraine composed of hummocky moraine

Supraglacial debris Ice

B Formation of a broad ablation moraine composed of hummocky moraine

Moraine 1

Moraine 2

D Formation of an ablation moraine by thrusting Thrust plane

Stack of thrust ice blocks

D Formation of an ablation moraine by thrusting Thrust plane

Stack of thrust ice blocks

Moraine morphology reflects ice structure Ice core

Moraine morphology reflects ice structure Ice core

Moraine structure/morphology may be lost when ice core melts a Ablation moraine Medial moraine b Continuous sheet of hummocky moraine

Figure 9.12 The morphology and formation of ablation moraines. (A) The formation of simple ablation moraines by differential ablation of the ice surface. (B) The formation of a complex ablation moraine consisting of belts of hummocks moraine. (C) The distinction between ablation moraines formed from discontinuous and continuous rates of supply of supraglacial debris. (D) Formation of an ablation moraine by thrusting.

Moraine 1

Moraine 2

Figure 9.13 Hummocky moraine in front of Ossian Sarsfjellet, Svalbard [Photograph: N.F. Glasser]

moraine -an irregular collection of mounds and enclosed hollows - is formed by the meltout of ice-cored debris. It may form a continuous sheet of irregular hummocks if the debris on the ice surface is continuous or alternatively discrete belts of hummocky moraine may result if the debris on the glacier surface is not continuous or of a constant thickness (Figure 9.12). This situation can arise if the supply of debris is either pulsed, due for example to discontinuous episodes of rockfall activity, or controlled by the spatial pattern of thrusts within the glacier (Figure 9.12). The morphology of these moraines may reflect the englacial structure of the glacier, at least while they retain their ice core (Figure 9.12). This morphology is often lost, however, when the ice core ablates and the size of the moraine is radically reduced. It is important to note that hummocky moraine can form by other mechanisms as well (see Section 9.2.1).

The processes responsible for the de-icing of ice-cored moraines have been monitored at Kotlujokull over a number of years (Figure 9.14). Here melting along the bottom surface of the ice cores occurs at an annual rate of 250 mm and involves a complex array of processes, including the development of sinkholes at the toe of dead-ice blocks in response to local melting. These initiate retrogressive rotational sliding or backslumping of the ice-cored slopes, and the formation of distinct edges and areas of exposed ice accelerate the rate of melting. Topographic inversion occurs in the initial, but not later, phases of de-icing, where general lowering of the surface occurs.

Figure 9.14 Sedimentological model for the de-icing of ice-cored terrain based on observations at Kotlujokull, Iceland. [Reproduced with permission of Blackwell Publishing Ltd from: Kj^r and Kruger (2001) Sedimentology, 48, figure 12, p. 948]
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