Box 96 Drumlins And Subglacial Deformation

Evidence for the theory of drumlin formation by subglacial deformation is provided by Boyce and Eyles (1991). These authors studied the Peterborough drumlin field in central Canada, which was formed beneath a lobe of ice at the margin of the former Laurentide Ice Sheet during the last glacial cycle. They examined the morphology and internal composition of the drumlins along a line parallel to the direction of glacier flow (a flow line). Along the flow line the drumlins change from elongate to oval forms close to the limit of the ice lobe. The elongated drumlins are composed of till resting on bedrock, whereas the oval forms are eroded from outwash sediments. The drumlins with cores of outwash sediments are mantled with till, which was derived from the deformation and incorporation of underlying till to form a deformation till. A transition therefore occurs along the flow line from depositional to erosional forms close to the ice limit. The down-ice evolution of drumlin form was interpreted by Boyce and Eyles (1991) as a function of the time available for subglacial deformation during the advance of the ice lobe. The duration of deforming bed conditions was greatest up-glacier, where the drumlins are elongated and where all the pre-existing sediment has been eroded and incorporated into the deformation till that forms the drumlins. Towards the limit of the ice lobe, where subglacial deformation will only have occurred for a short period, the drumlins are more oval in shape with cores of undeformed sediment. There was insufficient time here to produced highly streamlined forms or for subglacial deformation to cut down and remould the outwash sediment. These observations not only support the subglacial deformation model of drumlin formation but provide rare insight into the internal composition of drumlins and illustrate how their morphology can vary along a glacier flow line.

Schematic drumlin stratigraphy i

Subglacial Erosion

66 km C

66 km C

ra Till

Ü Outwash

¡^1 Early glacial deposits

Source: Boyce, J.I. and Eyles, N. (1991) Drumlins carved by deforming till streams below the Laurentide ice sheet. Geology, 19, 787-90. [Modified from: Boyce and Eyles (1991) Geology, 19, figure 2, p. 788.]

and contains material in transport. Beneath this there is a slowly deforming horizon (Bx horizon). Below this, the sediment is not deforming but is stable (B2 horizon; Figure 8.13). The boundaries between the three horizons should not be considered to be planar because the thickness of each horizon will vary with the changing properties of the sediment. For example, the presence of the slowly deforming horizon (Bx) is dependant on the rheology or stiffness of the sediment. If the sediment is stiff and not easily deformed this horizon may be absent. Sediment rheology is controlled by a range of variables, of which pore-water pressure is of particular importance. Pore-water pressure is the pressure of the water within the pores or interstices within the sediment and helps to determine intergranular friction. If the pore-water pressure is high individual grains of sediment are pushed further apart and the friction between one grain and the next is reduced. The lower the level of intergranular friction the more easily the sediment will deform. Fine-grained sediments tends to have a higher pore-water content and pressure than coarse sediments and will therefore deform more easily. Pore-water pressures may also be reduced by increasing the effective normal pressure imposed on a sediment, because this tends to drive off water, provided it can drain away. This increases intergranular friction within the sediment. The shape and size of individual grains of sediment also help determine intergranular friction.

The nature of the boundary between the A and B horizons may either be erosional or depositional depending upon whether the glacier is experiencing extending or compressional flow. As we saw in Section 8.1.1, where a glacier is experiencing extending flow the deforming layer may downcut (erode) into the sediment pile beneath, assimilating new sediment (B horizon) into the deforming layer (A horizon; Figure 8.13). In this case the junction between the A and B horizons is erosional. In areas of compressive flow the deforming layer will grow by the accumulation of till transported laterally in the deforming layer from up-ice areas. In summary, therefore, the nature of the deforming layer is a function of compressive or extending ice flow (Figure 8.6) and of variation in the rheology of the deforming sediment beneath the glacier.

Boulton developed a semi-quantitative flow model for the deformation of the rapidly deforming A horizon on the basis of field observations (see Box 3.4). Using this model he was able to predict how the rapidly deforming A horizon would become moulded around an obstacle to form a drumlin. Figure 9.20A shows the flow lines within a layer of soft deforming sediment. There are two zones of enhanced sediment flow either side of the obstacle and a zone of slower flow in its lee. This pattern of sediment flow produces a sheath of soft sediment around the core as shown in Figure 9.20B. The sediment within this sheath is not stationary, although the shape of the sheath is, because sediment is added at the up-glacier side and removed down-glacier. If the glacier decays and/or the stress field beneath it changes then the deforming A horizon will become stationary around the core to form a drumlin.

The range of drumlin morphologies may be explained by the deformation of this A horizon around either fixed obstacles or mobile obstacles. The obstacles need not, however, necessarily be visible at the surface but simply provide a rigid or stiffer area within the deforming bed. Three types of obstacle were considered in the

A p. Outline of sheath

A p. Outline of sheath

Outline of sheath formed by the deforming A horizon

Ice flow

Outline of sheath formed by the deforming A horizon

Enhanced flow

Enhanced flow

Enhanced flow

Enhanced flow

Figure 9.20 Drumlins formed by subglacial deformation. (A) Outline of the shape formed by a sheath of soft deforming sediment around a slowly deforming core. (B) The pattern of flow within a deforming layer passing around a rigid cylinder. The progressive deformation of an originally straight transverse line is followed from T to T8. Note the reduced rate of deformation in the lee of the cylinder and the enhanced flow along its flanks. [Modified from: Boulton (1987) in: Drumlin symposium (eds J. Menzies and J. Rose), Balkema, figure 11, p. 49]

Tb Ts

T2 Ti

Figure 9.20 Drumlins formed by subglacial deformation. (A) Outline of the shape formed by a sheath of soft deforming sediment around a slowly deforming core. (B) The pattern of flow within a deforming layer passing around a rigid cylinder. The progressive deformation of an originally straight transverse line is followed from T to T8. Note the reduced rate of deformation in the lee of the cylinder and the enhanced flow along its flanks. [Modified from: Boulton (1987) in: Drumlin symposium (eds J. Menzies and J. Rose), Balkema, figure 11, p. 49]

theory: (i) bedrock obstacles (Figure 9.21); (ii) folds within the B1 horizon (Figure 9.22); and (iii) undeformed areas of sand and gravel (Figure 9.23).

According to Boulton's model, deforming sediment will thicken in front of a bedrock scarp, due to the high effective pressures present, to form drumlinoid noses. These noses form because the high pressures generated by ice flowing against such scarps (see Section 4.6: Figure 4.6) expel water from the sediment and therefore its ability to deform. The reduced rate of deformation (i.e. sediment transport) causes sediment to accumulate. Deforming sediment will also form in

1: Continuous sediment cover

A: Laterally extensive bedrock step: 2-D obstacle

Glacier flow Drumlin

A: Laterally extensive bedrock step: 2-D obstacle

Glacier flow -►

--

B: Bedrock knob: 3-D obstacle

__Drumlin

-► —

B: Bedrock knob: 3-D obstacle

Drumlin detached from obstacle

C: Bedrock knobs and steps in plan

Sediment builds up v r-' y against 2-D scarp

A »T Sediment noses s<\\N. \— Drumlin tails in lee V\\\ of 3-D rock knobs

C: Moving drift patch on a rough surface

0 / JU.

Ice flow \ Drumlins Bedrock Time knobs -►

Figure 9.21 Morphology of a deforming layer moving over an irregular bedrock surface. Accumulations of deforming sediment, drumlins, are static where the sediment supply or cover is continuous, but mobile where the sediment cover is patchy and the supply is therefore discontinuous. [Modified from: Boulton (1987) in Drumlin Symposium (eds J. Menzies and J. Rose), Balkema, figure 27, p. 72]

Figure 9.21 Morphology of a deforming layer moving over an irregular bedrock surface. Accumulations of deforming sediment, drumlins, are static where the sediment supply or cover is continuous, but mobile where the sediment cover is patchy and the supply is therefore discontinuous. [Modified from: Boulton (1987) in Drumlin Symposium (eds J. Menzies and J. Rose), Balkema, figure 27, p. 72]

the lee of bedrock knobs, due to the decrease in flow of the deforming sediment (Figure 9.21). If the supply of deforming sediment is large then the tail of sediment around the bedrock obstacle will remain, like a stationary or standing wave. In this case there is a constant throughput of sediment within the drumlin; it is added upstream as quickly as it is removed downstream. However, if the supply of deforming sediment is small, for example if it is just a patch of deforming sediment, then the drumlin formed around the bedrock knob will move past the obstacle as sediment is removed from the up-glacier face but not replaced. The pattern of drumlins created by bedrock obstacles depends largely on the availability of sediment and the roughness of the bedrock surface (Figure 9.21).

Folding along the boundary between the B1 horizon and the A horizon may provide foci for drumlin formation. If the properties of the B1 horizon vary in the direction of flow, for example if the sediment becomes stiffer down-ice, then its rate of deformation will change (high to low), which may lead to compression and folding. Deformation of the A horizon around such folds may form a drumlin as shown in Figure 9.22. Repeated folding and refolding of the original fold may cause it to be derooted, in the same way that a piece of chewing gum may be stretched and

A: Fixed fold core riri imlin

Rapidly

A: Fixed fold core riri imlin

Rapidly

Softer sediment in B horizon

Resistant sediment in B horizon

Softer sediment in B horizon

Resistant sediment in B horizon

B: Mobile fold core

Drumlin

Core of

B: Mobile fold core

Drumlin

Core of

Figure 9.22 Morphology of a deforming layer around a fold generated at the interface between the A and B horizons. (A) Fixed fold core and static drumlin. (B) Derooted, mobile, fold core and therefore mobile drumlin. [Modified from: Boulton (1987) in: Drumlin Symposium (eds J. Menzies and J. Rose), Balkema, figure 27, p. 73]

Figure 9.22 Morphology of a deforming layer around a fold generated at the interface between the A and B horizons. (A) Fixed fold core and static drumlin. (B) Derooted, mobile, fold core and therefore mobile drumlin. [Modified from: Boulton (1987) in: Drumlin Symposium (eds J. Menzies and J. Rose), Balkema, figure 27, p. 73]

stretched until it finally breaks. Once the fold has been derooted it is able to move in the direction of glacier flow. The drumlin will also be able to migrate.

The final type of drumlin core considered by Boulton was one of undeformed sand and gravel. Figure 9.23 shows a glacier forefield in which coarse gravel is deposited close to the meltwater portal of an ice front. If this ice front was to advance over the outwash sediment it would deform it. The coarse free-draining gravels would be less likely to deform due to low pore-water pressures within them and remain as fixed undeformed cores around which finer grained sediment, less well drained and therefore with higher pore-water pressures, can deform. The core is shaped and eroded by the deformation of the A horizon over its surface and by erosion at the base of this deforming layer. In this way drumlins with undeformed cores of bedded fluvial sands and gravels may be generated (Box 9.5). In areas of strongly extending flow, erosion occurs at the boundary between the A and B horizons as deformation cuts down through the sediment pile. The deforming A horizon may be very thin due to the lateral transport of the deforming sediment assimilated at the A-B horizon. In this case a drumlin may effectively be formed by erosion along the interface between the A and B horizons.

Within this model megaflutes are simply a subtype of drumlin, produced by rapid rates of glacier flow and subglacial deformation, which would tend to produce more elongated forms. Ribbed moraines are considered within this model to be formed by the remoulding of earlier linear bodies of sediment, perhaps formed

Time 1

Portals from which meltwater emerges

Ice margin

Proglacial meltwater streams

Old ice margin

Flow line

New ice margin

Time 1

Portals from which meltwater emerges

Ice margin

Proglacial meltwater streams

Old ice margin

Flow line

New ice margin

Coarse gravel bars

Figure 9.23 Drumlins initiated around cores of stiff undeforming sediment, in this case coarse gravel bars. Time 1: deposition of coarse gravel close to meltwater portals. Time 2: ice advances and subglacial deformation around the coarse gravel produces streamlined drumlins with cores of undeformed gravel. They are analogous to boudins within a highly deformed rock body. [Modified from: Boulton (1987) in Drumlin symposium (eds J. Menzies and J. Rose), Balkema, figure 27, p. 73]

by earlier ice-flow directions, although a range of other explanation have been proposed in recent years (Figure 9.24; Box 9.7).

The strength of Boulton's model lies in the fact that it can explain all the requirements of a general theory, that is the presence of different subspecies of subglacial landforms such as megaflutes, drumlins, MSGL and ribbed moraines. The model also explains the range of different compositions and structure found within these landforms; in particular the presence of drumlin cores composed of: (i) bedrock; (ii) till; and (iii) bedded sands and gravels. Additionally the model also explains the spatial distribution of bedforms: they only occur where subglacial deformation is possible. Finally the model can explain the rapid rates of drumlin formation observed in some studies. The key strength of this model is that it represents an attempt to develop a unified model of drumlin formation by subglacial deformation.

It is not, however, without its opponents. Several researchers have argued that at present there is no direct evidence to suggest that subglacial deformation is a pervasive process beneath ice sheets. To date, direct field observations of subglacial deformation are restricted to fast flowing Antarctic ice streams and glaciers in Iceland and Alaska. Opponents of subglacial deformation point to the absence of widespread evidence of subglacial tectonic structures, such as folds and thrusts within glacial sediments, and to homogeneous till layers and undisturbed sediment sequences. However, as we saw in

Subglacial Erosion

Time 1

Time 2

Time 1

Ice flow 1

Ice centre 1

Ice centre 2

Time 2

Ice centre 2

Ice flow 2

Ice flow 1

Ice centre 1

Figure 9.24 Schematic illustration of the way in which transverse drift ridges may be progressively transformed by deformation into rogen or ribbed moraines and drumlins. Original drift ridges, megascale glacial lineations or large drumlins, may reflect an earlier direction of ice flow and are deformed into ribbed moraine by a new ice-flow direction associated with a shift in the ice divide. [Modified from: Boulton (1987) in Drumlin Symposium (eds J. Menzies and J. Rose), Balkema, figure 28, p. 75]

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