Subglacial Landforms Formed By Ice Or Sediment Flow

This category of landforms is divided into those which have been ice-moulded and those that have not. Ice-moulded landforms are significant because they provide information about the direction and velocity of glacier flow.

9.2.1 Ice-Moulded Subglacial Landforms

Three broad families of ice-moulded subglacial landforms (bedforms) have been identified on the basis of size (Figure 9.15). Although each may be genetically distinct, these are as follows.

Ebb Slack Graph

Figure 9.15 Schematic representation of principal spatial frequencies and lengths of streamlined subglacial landforms. The data suggest that there are three populations of subglacial bedform: (i) flutes; (ii) drumlins and megaflutes; and (iii) megascale glacial lineations. [Reproduced with permission of John Wiley & Sons Ltd from: Clark (1993) Earth Surface Processes and Landforms, 18, figure 6, p. 9]

Figure 9.15 Schematic representation of principal spatial frequencies and lengths of streamlined subglacial landforms. The data suggest that there are three populations of subglacial bedform: (i) flutes; (ii) drumlins and megaflutes; and (iii) megascale glacial lineations. [Reproduced with permission of John Wiley & Sons Ltd from: Clark (1993) Earth Surface Processes and Landforms, 18, figure 6, p. 9]

1. Flutes. Typically these are low (< 3 m), narrow (< 3 m), regularly spaced ridges which are usually less than 100 m long and are aligned parallel to the direction of ice flow (Figure 7.8B). They have a uniform cross-section and usually start from either: (i) a large boulder; (ii) a collection of boulders; or (iii) a bedrock obstacle. They are typically composed of lodgement till, although they may also contain fluvial sands and gravels. Clusters of boulders may occur within the body of the flute. They are a common landform in front of many glaciers today.

2. Drumlins, megaflutes and rogen (ribbed) moraines. Drumlins are typically smooth, oval-shaped or elliptical hills composed of glacial sediment (Figures 9.16 and 9.17). They are between 5 and 50 m high and 10-3000 m long. They have length to width ratios that are less than 50. They are composed of a variety of materials, including: (i) lodgement till; (ii) bedrock; (iii) deformed mixtures of till, sand and gravel; and (iv) undeformed beds of sand and gravel. They tend to occur in distinct fields or drumlin 'swarms' and are not uniformly distributed beneath a glacier. The formation of drumlins by small glacial read-vances suggest that drumlins may form rapidly. In contrast to flutes, Megaflutes

Glacial Erosion
Figure 9.16 Oblique aerial photograph of a drumlin swarm in Langstrathdale northern England. Ice flow was from left to right. [Photograph reproduced with permission from: Cambridge University Collection of Aerial Photographs]

are taller (< 5 m), broader and longer (> 100 m) and are distinguished from drumlins by having a length to width ratio in excess of 50. Their long axis is parallel to the direction of basal ice flow and they typically have a uniform cross-section. Occasionally they may start from a large bedrock obstacle, but most do not. Rogen (ribbed) moraines have a variety of morphological forms but generally consist of ridges formed transverse to flow, although sometimes they show reshaping parallel to ice flow. 3. Megascale glacial lineations (MSGL). In recent years, much larger (megascale) lineations composed of glacial sediment have been recognised on satellite images. They are typically between 8 and 70 km long and between 200 and 1300 m wide, with 300-5000 m spacing between lineations. On the ground their morphology is often difficult to detect.

Subglacial bedforms may occur superimposed one on top of another, for example with smaller bedforms resting on the backs of larger ones. Megaflutes may be superimposed on the backs of larger drumlins, and in many of the drumlin fields of northern England small drumlins are found on larger drumlins. The orientation of the two sets of bedforms can either be coincident or discordant. In the latter case the bedform patterns provide evidence of changing patterns of ice flow (see

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Landsat Image North And South America
Figure 9.17 Glacial lineations and drumlins on a Landsat satellite image of Patagonia, southern South America. North is at the top of the image, and former ice flow was from SSW to NNE. The image is approximately 50 km wide. [Image courtesy of: N.F. Glasser]

Chapter 12). In a hypothetical ice sheet, ice will flow from the ice divide to the margin and this pattern will be recorded by subglacial bedforms. Within this ice sheet most geomorphological activity will occur where the ice velocity is greatest, close to the equilibrium line (Figure 3.15), and will decrease beneath the ice divide. If the location of the ice divide changes, then the pattern of ice flow within the ice sheet would be reorganised and a new set or population of bedforms will begin to form parallel to the new pattern of ice flow. These would alter or be superimposed on the original set of bedforms (Figure 9.18). Beneath the new ice divide little modification would occur due to low glacier velocity. However, beneath the equilibrium line the old set of bedforms would be quickly eroded and replaced by a new set with an orientation consistent with the new flow pattern. In between these two locations two populations of bedforms coexist in a superimposed or cross-cutting fashion (Figure 9.18). Cross-cutting bedforms therefore hold important information about changing patterns of ice flow.

272 Landforms of Glacial Deposition on Land A

Ice flow phase I

Ice flow phase II

An area of deformable sediment experiences overriding by ice flow I followed by flow II

a ^^^^^^^

Greater elapse of time or increased velocity

An ice-moulded landform -megaflute, drumlin or megascale glacial lineation -develops in response to ice flow I. No modification of parent landform may result from subsequent ice flow II

Superimposition of smaller forms upon parent landform

Substantial breaching or attenuation of parent landform may occur with or without drumlinised elements

^

Total reorganisation of sediment into the new orientation

Subglacial Erosion

Figure 9.18 Pattern of cross-cutting relationships within subglacial bedforms formed by two phases of ice flow beneath an ice sheets. (A) Types of observed cross-cutting or superimposed relationships found beneath former ice sheets. (B) Distribution of types of cross-cut bedforms in relation to ice geometry and mass balance velocity (see Figure 3.15). Under the ice divide a single population of bedforms is present, which belong to the first phase of ice flow. At the ice margin a single population of bedforms exists which reflects the second phase of ice flow. Between these two extremes two populations of bedforms coexist. [Reproduced with permission of John Wiley & Sons Ltd from: Clark (1993) Earth Surface Processes and Landforms, 18, 1 -29]

Figure 9.18 Pattern of cross-cutting relationships within subglacial bedforms formed by two phases of ice flow beneath an ice sheets. (A) Types of observed cross-cutting or superimposed relationships found beneath former ice sheets. (B) Distribution of types of cross-cut bedforms in relation to ice geometry and mass balance velocity (see Figure 3.15). Under the ice divide a single population of bedforms is present, which belong to the first phase of ice flow. At the ice margin a single population of bedforms exists which reflects the second phase of ice flow. Between these two extremes two populations of bedforms coexist. [Reproduced with permission of John Wiley & Sons Ltd from: Clark (1993) Earth Surface Processes and Landforms, 18, 1 -29]

The formation of subglacial bedforms has interested and intrigued successive generations of glacial geologists and has been the subject of numerous research papers. Despite this interest there is still no single accepted theory for their formation. There is, however, a general consensus about the formation of glacial flutes. Their formation is explained by the presence of a large boulder or bedrock obstacle at their up-ice end. As the ice flows around the obstacle a cavity or area of low pressure will form in its lee. The pressure on the sediment imposed by the ice will be higher on either side of this area of low pressure. Consequently subglacial sediment may flow along this pressure gradient - high to low pressure - forming a linear ridge in the lee of the boulder (Figure 9.19). The flute grows in length, down-ice, as

Ice flow

Ice flow

Glacial Flow Model
cavity

Low-pressure

Low-pressure

Ice Gouges
Figure 9.19 The formation of a glacial flute.

the low-pressure area extends in front of the sediment ridge. Recent observations suggest that subglacial meltwater flow within the cavity may accentuate the morphology of the flute by eroding sediment along its flanks. Although this model is widely accepted, not all flutes have boulders or bedrock obstacles at their up-ice ends. There are two possible explanations: (i) the boulder was there and has been subsequently removed by ice flow; or (ii) there was never a boulder at the head of the flute and an alternative explanation for the formation of these flutes is required, perhaps in the same way that megaflutes and drumlins form.

In contrast to the formation of flutes there is little consensus about the formation of drumlins, megaflutes and ribbed moraines. The range and diversity of these landforms is so great, particularly in the context of their internal composition, that some researchers have suggested that there may not be a single mechanism responsible for their formation. This concept is known as equifinality: different processes form the same morphological products. The acceptance of such an idea should, however, be consequent only upon our failure to find a universal theory. A general model for the formation of drumlins, megaflutes, MSGL and ribbed moraines must be able to explain the following factors.

1. Variables in the theory must be able to account for the different subspecies of subglacial landforms: drumlins, megaflutes, MSGL and ribbed moraines.

2. It must account for the different composition and structure of drumlins, mega-flutes, MSGL and ribbed moraines. In particular it must explain the presence of the three main types of drumlin core commonly found: (i) bedrock; (ii) till; and (iii) bedded sands and gravels.

3. It must account for the spatial distribution of bedforms: why do they only occur beneath certain parts of an ice sheet?

4. It must account for the rapid rates of landform creation observed at modern glacier margins.

Most attention has focused on explaining the formation of drumlins. Numerous models, hypotheses and explanations exist but most attention is now focused on the role of subglacial deformation as the most likely explanation. It is important to note, however, that this is not the only idea currently proposed within the literature. In particular the possibility that glacial bedforms are produced by subglacial floods has been a persistent idea over the past decade (Box 9.5), although many consider this to be an 'outrageous' rather than plausible hypothesis.

Observations of subglacial deformation beneath Brei9amerkurokull in Iceland were used in the late 1980s by Geoffrey Boulton and Richard Hindmarsh to develop a flow law with which to describe the deformation of subglacial till. Although this flow law has been widely disputed and amended since its publication, it forms the basis for several models of drumlin formation. The basic concepts of this theory are reviewed below, but it is important to note that it remains simply one of many models (Box 9.6).

When a glacier flows over soft deformable sediment Boulton suggests that three horizons may be identifiable. The surface horizon (A horizon) is rapidly deforming

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  • Isidora
    What sre the subglacial landforms formed by ice?
    3 years ago
  • faruz
    How are drumlins formed?
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  • Edgardo
    What is a flute glacial bedform?
    11 months ago
  • rhoda
    Do flutes and drumlins form the same way?
    9 months ago
  • Benjamin
    Where to find subglacial sediment?
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