Quaren~ry Science Reviews, Vol. 16 pp. 9%107,1997. 0 1997 Elsevier Science Ltd.

Printed in Great Britain. All rights reserved. 0277-3791197 $32.00

PII: SO2773791(96)00023-6

THE RELATIONSHIP BETWEEN DRUMLINS AND OTHER FORMS OF SUBGLACIAL GLACIOTECTONIC DEFORMATION

JANE K. HART Department of Geography, University of Southampton, Southampton SO9 SNH, U.K.

(E-mail: jhart@uk.ac.soton)

Abstract - This paper brings together the field data from 33 drumlins to show the relationship between drumlin formation and other subglacial deforming bed processes. It is shown that there is a drumlin structure continuum: (a) depositional - which are similar to flutes, formed by sediment flowing into the low pressure area behind a large obstacle - and these conditions are mostly found within rock-cored drumlins; (b) deformational - which contain different styles of deformation associated with a relatively weak core, including: stoss-side deformation (including both small and large scale; brittle and ductile deformation), stoss and lee-side deformation, compressive deformation and extensional deformation; (c) erosional - which consist of either truncated stratified sequences, or homogeneous tills with distinct ice flow patterns.

Furthermore, it is suggested that if more sediment enters the deforming layer than can be removed, then there will be net subglacial deposition which will result in the build up of a deforming bed till; but if more sediment leaves the deforming layer then net erosion will occur. Under net erosional conditions, any obstacle to flow may be left behind as a drumlin. Thus although individual drumlins have different internal structures they are formed associated with subglacial net erosion. 0 1997 Elsevier Science Ltd. All rights reserved.

INTRODUCTION

Recent work by Boulton (1979), Boulton and Jones (1979), Alley et al. (1986) and Clarke (1987) has shown that when a glacier moves over a potentially deformable bed there is a coupling between the glacier and its underlying sediment. This may lead to an increase in velocity of the glacier (Boulton and Hindmarsh, 1987), and deformation in the underlying sediment (Hart et al., 1990; Hart and Boulton, 1991). This deformation is known as subglacial glaciotectonic deformation and takes place in the deforming layer beneath the glacier.

These workers have shown how sediment deformation is an intrinsic part of both ice sheet flow, and subglacial erosional and depositional processes. The style of subglacial behaviour depends on a number of parameters: ice sheet velocity, basal shear stress, pore water pressure, effective pressure (ice pressure minus pore water pressure) and the geotechnical properties of the subglacial material.

Drumlins are a typical subglacial landform, and so any theory of subglacial processes must include drumlin forming processes. There has been a great deal of research on drumlin formation, as summarised by Menzies (1984), which has continued recently with the proceedings of three symposia on drumlin formation (Menzies and Rose, 1987, 1989; McCabe and Dardis, 1994). Modem theories of drumlin formation can be summarised into two models; the deformational theory of drumlins (after Smalley and Unwin, 1968; Menzies, 1979, 1989; Boulton, 1987) and the fluvial theory (Shaw, 1983; Shaw et d., 1989). In this paper I look specifically at the

QSR

deformational theory for drumlins, and attempt to show how drumlin formation is related to other subglacial deforming bed processes.

Many drumlin studies have concentrated on the dimensions and locations of the drumlins rather than their internal structure. This is because good exposures of drumlins are rare (see Patterson and Hooke (1995) for a recent review of drumlin studies). Those studies on internal structures that do exist have usually concentrated on only a few drumlins. In contrast, this present study attempts to investigate a very large number of exposed drumlins in different geographical locations, so that general points and not just local features can be investigated. In total 33 were studied, from Iceland, Britain, Ireland and North America. They were also compared with drumlins in south east Wisconsin, as described in the literature (Stanford and Mickelson, 1986; see Fig. 1 and Table 1). Due to space limitations it is impossible to describe the detailed sedimentology of all these drumlins in one publication. Instead, the detail of most of these drumlins are discussed elsewhere and only specific examples are briefly discussed here. In this paper the results and interpretations from the large drumlin study are discussed in detail.

DEFORMATIONAL HYPOTHESIS OF DRUMLIN FORMATION

The deformational hypothesis for drumlin formation of Smalley and Unwin (1968), Menzies (1979) and Boulton

93

94 Quaternary Science Reviews: Volume 16

FIG. 1. Location of the sites: (a) North Uist, U.K.; (b) Skye, U.K.; (c) North west Wales, U.K.; (d) Clew Bay, Co. Mayo, Ireland; (e) Renvyle, Connemara, Ireland; (f) Vestari-HagafellsjGkull, LangajGkull, Iceland; (g) Northern New York State,

U.S.A.; (h) south east Wisconsin, U.S.A. (after Stanford and Mickelson, 1986).

(19871, suggest that drumlins form around a more competent obstruction within the deforming layer. The more competent mass which provides the obstruction for material is known as the core, whilst the surrounding sheath is known as the carapace. The core can be made either of a clast of hard or soft rock, or a more competent mass of till.

Boulton (1987) suggests that the presence of more competent inhomogeneities within the deforming layer or steps in the dicollement surface (base of the deforming layer) will lead to the development of a sheath fold. He suggests that this is a typical subglacial fold and has been observed in association with subglacial deformation (Hart and Boulton, 1991; Hart and Roberts, 1994). Boulton’s model shows the following effects within the deforming layer associated with the core: (i) divergence followed by convergence of the sediment flow around the core; (ii) a low velocity zone immediately up-glacier of the core; (iii) an acceleration of sediment flow around the up-glacier flanks of the core, reaching a maximum at its maximum diameter; (iv) decelerating flow around the down-glacier flanks of the core; and (v) a low velocity area in the lee position. In this model the drumlin becomes more elongated over time. The core becomes deformed into a syncline whilst the carapace folds around it in the form of a sheath fold, which is rooted on the proximal side of the core. These folds can become ‘de-rooted’ and move as boudins within the deforming layer. He concludes by suggesting that drumlins can be regarded as glacial boudins exposed on the surface whilst boudins within the deforming layer are regarded as ‘mini-drumlins’.

He suggests that if the core is weak, the sediment may be deformed into a sheath fold which is typical of subglacial deformation, and thus the resultant drumlin form reflects this fold. If the core is strong there will be an undeformed core with a thin carapace. The core/carapace interface will be an erosional surface and the drumlin an erosional landform. Thus, the drumlins are formed by longitudinal extension.

However, there are two main problems with this theory: (a) although Boulton (1987) acknowledges these two types of drumlins - deformational and erosional, the former has the most emphasis, which has caused problems since few drumlins have been observed with internal sheath folds; (b) drumlins should occur everywhere that subglacial deformation occurs, but they do not. For example, a large part of east Yorkshire was affected by subglacial deformation during the last glaciation (Boulton and Dobbie, 1993; Eyles et al., 1994; Evans et al., 1995) but there are few drumlins; similarly in Illinois, where there is also good evidence for subglacial deformation (e.g. at Wedron; Johnson and Hansel, 1989) there are no drumlins. Thus, there is a need to separate drumlin formation from other known subglacial deforming bed processes.

It was suggested by Rose (1987) that there is a continuum of streamlined landforms from small highly elongated flutes to larger less elongated drumlins. Any theory of streamlined features needs to be able to explain this continuum. Thus, in this paper the ideas of Boulton (1987) are taken further, by integration with new results about the subglacial processes, and the examples being related to different types of drumlin.

Hart (1995a) has suggested that drumlins associated with a deforming bed can be theoretically divided into three broad classes (Fig. 2).

(a>

(b)

Depositional structure - the presence of a large core will form a cavity or low pressure area into which the deforming bed will flow and stop moving, and thus be ‘deposited’. The resultant drumlin will be similar to a large flute. Till fabric patterns would be expected to show flow into the low pressure area, as indicated in studies of flutes by Boulton (1976), Rose (1989) and Benn (1994; Fig. 2a). Deformational structure - the core will be deformed into the shape of drumlin as suggested by Boulton (1987). The word deformational is used in this

J.K. Hart: Drumlins and Other Forms of Subglacial Deformation 95

TABLE 1. The location of the drumlins in the study

a i-Loch Portain, N. Uist, U.K.

ii-Langlass, N. Uist, U.K. iii-Claddach-Baleshare, N. Uist, U.K.

1 - NF927, 720 Peacock, 1984, 1991 2 and 3 - NF930, 716 4 - NF937, 717 5 - NF950, 723 NF837, 653 NF805, 628

b Sligachan, Skye, U.K. NG466, 333 Benn, 1992

C North west Wales Dinas Dinllel - SH436, 563 Hart, 1995a Dinas Dinlle2 - SH435, 559 Beaumaris - SH469,765 Lleiniog - SH610, 790

d Clew Bay, Co. Mayo, Ireland Pigeon Point - L950, 850 Hanvey , 1992 Thomhill - L888, 828 Falduff - L841, 820 Turlin - L805, 820

e Renvyle, Connemara, Ireland A - L654, 640 Synge, 1968; McCabe and B - L665, 638 Dardis, 1992

C - L685, 638 D - L691, 636

f VestariHagafellsj6kul1, Hart, 1995b Langajdkull, Iceland

g Northern New York State, U.S.A. Fairhaven Fairchild, 1929; Slater, 1929; McIntyre Muller, 1974

Broadway Road Larking Road Chimney Bluffs O’Brien Road West Tyre Road Mentz Church Road

h South east Wisconsin, U.S.A. Waukusha Stanford and Mickelson, 1986 Sullivan

context to apply to large scale deformation directly associated with drumlinization rather than the forma- tion of a subglacial till deposit.

(c) Erosional structure - the core will represent an erosional remnant of the deforming layer. If the core is made of till then the till fabric will indicate flow down and glacier, whilst the fabric from the upper carapace till may flow in the shape of the drumlin (Fig. 2~). If the drumlin is made of a layered sequence, then the sequence will be truncated at the edges.

It has been shown by Hart (1994) that till fabric analysis is useful in identifying deforming bed tills. The eigenvalue analysis of Mark (1973) and Dowdeswell and Sharp (1986) provide an indication of the degree of clustering of the data around the maximum (SI) and minimum (S3) axis. It was shown that in general, weak fabrics are indicative a deforming bed till with a thick deforming layer; whilst a strong fabric indicates either a

deforming bed till with a thin deforming layer or a lodgement till.

THE DRUMLIN SITES

In this study the following features were investigated at the drumlin sites: sedimentology; glaciotectonic deforma- tion, presence of a core and/or carapace; and till fabric analysis where appropriate. The results from the field study indicated that the drumlins could be further sub- divided into different descriptive types, and these are shown in Table 2. I shall discuss the best example of these different types in turn.

Rock-Cored Drumlins

Rock-cored drumlins are a specific class of drumlin because they have an identifiable and usually unmoveable

Quaternary Science Reviews: Volume 16

Drumlins a) depositional

m A

A A A

A A A A A A

c

FIG. 2. Theoretical drumlin types: (a) depositional, (b) erosional, (c) deformational.

core. One example of a rock-cored drumlin is Uist-LP2 from Loch Portain, North Uist, which was described by Peacock (1984, 1991). This drumlin is 2 m high by 6 m long and is oriented ESE/WNW, and was formed as ice flowed from the Outer Hebrides ice centre. Above the rock core is a grey homogeneous diamicton (Fig. 3). Three till fabrics were taken within this till, which are shown in Fig. 3 and Table 3. The strengths of the fabrics within the drumlins were very high and they are oriented around the rock core.

The fabric strengths of the till are typical of a lodgement till or a deforming bed till formed within a thin deforming layer (Hart, 1994), and the divergence of flow around the rock-core is similar to a flute. Unfortunately, no exposure was available at the extreme end of the drumlin to see if divergence occurred (as suggested by Rose, 1989).

Thus, I suggest this drumlin formed in a similar way to a flute, that is, the rock-core represents an obstacle behind which a low pressure area develops, into which subglacial sediment flows and is deposited. Thus these drumlins are depositional drumlins and represent large-scale flutes.

If it is assumed that these rock-cored drumlins are formed in the way suggested above then they may be related to other features. If a cavity forms behind clasts in association with high pore water pressure (deforming bed conditions) the cavity will simply fill with the deforming bed till. However, under low pore water pressure (rigid bed conditions) the cavity may remain open and fill with either: (a) fluvial sediments and produce the lee-side stratification sequences suggested by Dardis and McCabe (1983) and Dardis and Hanvey (1994); or (b) melt-out sediments and thus represent a crag and tail feature.

Drumlins Showing Evidence of Erosion

Many drumlins that show evidence of erosion represent the opposite end of a drumlin forming continuum. Many are composed entirely of homogeneous till with till fabrics oriented down-glacier (and not associated with the drumlin form), or they may show truncated stratified internal sequence.

One particular example of a drumlin composed of homogeneous till is from Sligachan on the Isle of Skye, U.K. This is a small drumlin (3 m high by approximately 120 m long; Fig. 4), and is adjacent to the drumlin described by Benn (1992). The lower part of the drumlin is composed of grey homogenous till which Benn suggests from scanning electron microscope analysis has evidence of pervasive shear and there may be a possible boulder pavement towards the top of the till, which could be further evidence for deforming bed conditions (Clark, 1991; Hicock, 1991). Similarly, the low strength eigenvalues (SI=O.601, S3=0.062) are further evidence that this is a deforming bed till. The fabric is oriented with the ice flow direction. Benn (1992) also recorded the till fabrics at two points in the lower diamicton, one in the centre of the drumlin which yielded Sl, S3 results of 0.645, 0.057, respectively, and one at the eastern side of the drumlin with SI, S3 results of 0.613, 0.024. The fabrics are both oriented NNW along the drumlin axis.

This lower till is overlain by an upper, more sandy brown homogeneous till. This has an average Sl, S3 values of 0.562, 0.053 which are also typical of a deforming bed till. The fabric is oriented in the general shape of the drumlin (divergent flow). This is overlain by a brown sandy diamicton with discontinuous sandy lenses which Benn (1992) has suggested represents a supraglacial drape.

I suggest the lower grey diamicton represents a pre- drumlin deforming bed till (as it is oriented in the direction of ice flow) whilst the upper brown diamicton either represents the carapace (as the fabric flows around the drumlin) or a post-drumlin supraglacial drape. In either explanation the drumlin was formed from the erosion of a pre-existing till sheet.

Drumlins Showing Evidence of Deform&ion

There are many types of deformation associated with drumlins, these are shown in Table 2 and are discussed below. These are intermediate between totally deposi- tional and totally erosional drumlins.

Stoss-side deformation

This is the simplest style of deformation associated with drumlins (Fig. 5). This can consist of clasts being stacked up on the stoss-side of the drumlin as seen at Pigeon Point drumlin, Clew Bay, Ireland (Hanvey, 1992; see Fig. 5a, O-90 m horizontally along the section); stoss- side large thrust blocks as seen at Dinas Dinlle, north Wales (Hart, 1994; Fig. 5b, O-22 m horizontally along the section), or stoss-side ductile subglacial deformation

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98 Quaternay Science Reviews: Volume 16

7654321 m

supported diamicton

FIG. 3. Loch Portain drumlin - an example of a rock cored depositional drumlin.

observed at Turlin drumlin, Clew bay, Ireland (Fig. 5c, 200-410 m horizontally along the section).

Pigeon Point is on the western edge of Clew Bay, north of Westport. This drumlin is 35 m high and 200 m long and oriented WSW/ESE (Hanvey, 1992). At this site there are a series of subhorizontal diamicton units which have been truncated on the lee-side (Fig. 5a, 80&900 m horizontally along the section). These consist of a lower homogeneous grey diamicton (facies i) which has been interpreted by Hanvey (1992) to have been deposited in a subglacial environment, and two upper brown stratified diamicton units (facies ii - sandy; facies iii - gravel-

rich) whose origin she suggests is subaqueous. Also within the sequence are large channel-shaped units of high energy gravels (e.g. 550-750 m horizontally along the section). At the stoss-end of the drumlin, local bed rock has been plastered on to form a very angular rock breccia (Fig. 5a, O-90 m horizontally along the section).

The landform at Dinas Dinlle 1, north west Wales, is 25 m high and 400 m long and is oriented in a N/S direction. Saunders (1968) and Harris et al. (1995) have argued that this feature is a push moraine, however Hart (1995a) has argued that it is a drumlin because of its shape, i.e. that is one of a number of elongated hills

J.K. Hart: Drumlins and Other Forms of Subglacial Deformation 99

TABLE 3. Till fabric data associated with the drumlins

Location Sl s3

Loch Portain 2, N Uist, U.K. 1 0.680 0.043 2 0.786 0.024

Turlin, Eire 1 0.562 0.057 2 0.750 0.006 3 0.668 0.003

O’Brien Rd, NY, U.S.A. 1 0.557 0.039 2 0.564 0.037 3 0.570 0.064 4 0.670 0.028

Sligachan 1 0.655 0.071 2 0.546 0.027 3 0.569 0.053 4 0.577 0.079

oriented down ice. The internal sediment comprises two subglacial tills (Rhudd and Llwyd Diamictons) with an intervening outwash sand and gravel (Dinas Dinlle sand and gravel). The lower till (Rhudd diamicton) was deposited by the Irish Sea ice sheet. It has a poor exposure so its depositional origin has not been established. The upper till (Llwyd diamicton) has been interpreted as a deforming bed till with subglacial fluvial units (Hart, 1995a) formed once the Irish Sea and Welsh ice sheets joined. These three units have been deformed into an anticline, and on the stoss side there are large thrust blocks of upper till and inter-till stratified beds (Fig. 5b). Another drumlin that has a stoss-side thrust sheet, is at Falduff, Clew Bay, western Ireland. The drumlin has a structure similar to that exposed at Turlin (Fig. 5c), but has distinct stoss-side thrusts (average dip 17”) within the proximal homogeneous diamicton.

The Turlin drumlin is located on the western end of Clew Bay, north of Louisburgh. This drumlin is 50 m high by 800 m long and is oriented W/E. The internal structure is composed of a layered till sequence that also shows erosion at the lee-side (Fig. 5c, O-30 m horizontally along the section). At the base there is a fine-grained stratified diamicton, which is overlain by a homogeneous grey diamicton, which has a medium strength fabric (M3) and is oriented in the same direction as the drumlin. There is also a boulder pavement at the base of the homogeneous diamicton. The stoss-side of the drumlin is composed entirely of homogeneous grey diamicton. At the base overlying the hard rock bed the fabric is strong (M2), whilst within the diamicton the fabric is weaker (Ml). The orientation of the fabrics in the stoss-side appear to mirror the shape of the drumlin.

I suggest the following interpretation for the three examples. At Pigeon Point drumlin, immediately prior to drumlinization, there were three subhorizontal beds. Once drumlinization occurred these beds were relatively competent and were thus able to resist deformation. Clasts could then be plastered up on the stoss end. This concept of plastering has been observed and suggested at many other drumlins e.g. Muller (1974) and Boulton

(1987). In this example the core may have been the large coarse-grained channel features within the drumlin.

At Dinas Dinlle there were also three subhorizontal sedimentary units prior to drumlinization. It is probable that the large sand and gravel unit behaved as a core. Once drumlinization began the coarse-grained unit was first deformed into an anticline and then large scale thrusting occurred on the stoss-side of this obstruction.

At Turlin there was a two layer drift sequence prior to drumlinization, a lower stratified bed and an upper homogeneous till. This upper till could have been formed under deforming bed conditions because of its low fabric strength, and the presence of the boulder pavement, which Clark (1991) and Hicock (1991) have suggested indicate deforming bed. However, once drumlinization occurred, these two layers became mixed in the stoss end by subglacial ductile deformation, and so the resultant till has both a low fabric strength and has been re-oriented into the flow pattern of the drumlin.

Thus, these three examples of stoss-side deformation all show evidence for the presence of a core. In the case of the Pigeon Point drumlin the core would be the large channel features within the drumlin, at the Dinas Dinlle it is the large unit of sand and gravel, and at Turlin it may be the stratified sediment. They also all show evidence of lee-side erosion and stoss-side deformation. However, this deformation has a different form in each of the three examples. At Pigeon Point it consists simply of the stacking of local rocks onto the core of the drumlin. At Dinas Dinlle it involves the large scale stacking of thrust blocks, whilst at Turlin, deformation is more ductile in nature and affects an even larger area. It could be argued that these three examples show increasing deformation and/or decreasing strength of the core.

At some sites deformation was also observed at the lee side (stoss and lee-side deformation), although the amount of deformation was much less. At Kanrawer drumlin in Connemara, Eire (RenA in Tables 1 and 2) this consisted of recumbent folding in the upper beds, and at Thomhill, Clew Bay, Ireland it consisted of open folding and backthntsting. Another form of deformation was compressive deformation of the core. An example of this has already been discussed from Dinas Dinlle 1 where stoss-side deformation was also seen.

Recumbent folded core deformation

The site of Sullivan, south east Wisconsin has been described in detail by Stanford and Mickelson (1986). This site consists of a lower sand and gravel unit and an upper diamicton unit. The sand and gravel has been overturned in the ice sheet direction (Fig. 6). I suggest that the sand and gravel represent the core that was relatively weak during drumlinization and so was overturned into a subglacial fold as suggested by Boulton (1987).

Concentric drumlins

The drumlins of New York state have been described by many researchers as concentric (Fairchild, 1929; Slater, 1929; Muller, 1974; Calkin and Muller, 1992),

100 Quaternary Science Reviews: Volume 16

Q’ N

A 1 ? 3 B

e+u 4

stratified diamicton

3 m

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1 N

__) Pre-drumlin

----) Drumlin till

till

FIG. 4. Section from the Sligachan drumlin, Skye, U.K.

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102 Quaternary Science Reviews: Volume 16

W Ice direction -

C)

S N 25

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that is the internal structure mirrors the external form of the drumlin. An example of one of these is O’Brien Road, near Syracuse, described by Muller (1974) and further investigated by the author. The drumlin at this site is 10 m high and approximately 30 m long. It is composed of a lower and upper massive sandy-silty red till with an intervening bed of interbedded sands, gravels and till (Fig. 7a and b).

Four till fabrics were measured in the upper till unit (Table 3). These showed that the average SI, S3 values were 0.590, 0.042. The fabric data at sites 0B2, 0B3 and OB4 indicate convergent flow within the drumlin, whilst the fabric at OB 1 is indicative of divergent flow. Many of the previous workers e.g. Muller (1974) have suggested that the homogeneous till was a lodgement till; however, the eigenvalues from this site are far too low to be due to lodgement (Dowdeswell and Sharp, 1986; Hart, 1994) but instead are typical of a deforming bed till associated with a thick deforming layer (Hart, 1994).

The stratified units within a subglacial till could have

formed either proglacially associate with a retreat stage or subglacially. Because the stratified units are very thin (0.3 m) and separated by till units it is most likely that they were formed subglacially. This could either be in the form of a sheet flow, canal flow or tunnel flow (Clark and Walder, 1994). Clark and Walder (1994) have suggested that flow in canals is typical of deforming bed tills, whilst Boulton and Hindmarsh (1987) have argued that at very high pore water pressures subglacial flow is channelized and form eskers.

It has been suggested by the earlier workers that the concentric drumlins were formed by the accretion of till over a core. However, at this site there is firstly the problem of the deposition of the stratified sediments, and secondly the till is too weakly oriented to be formed by classical lodgement processes. Instead I suggest that these concentric drumlins were formed after the deposition of the till and the stratified layer, due to subglacial extension acting on layers of different competencies. These layers were broken up into boudins and thus the concentric

J.K. Hart: Drumlins and Other Forms of Subglacial Deformation 103

S N

Ice direction t

A ,, ;::: .‘,‘,‘:,~ .,‘.,.., ::.:‘:::..;::.‘::::.:.ii:..~..: ~~~

pJ Fluvial sediments cl A Diamicton

FIG. 6. Section from the Sulivan drumlin, Wisconsin, U.S.A.

drumlins are formed (Fig. 7~). These also show evidence of erosion on the sides of the drumlins.

ubiquitous with deforming bed conditions, and so I suggest that there are two contrasting styles of subglacial deformation.

Discussion of Deformational Drumlins

The discussion above has shown how there are many different styles of deformation associated with drumlins. I suggest that they represent a continuum of deformational forms, from stoss-side deformation, compressive core deformation, through to subglacial folds and finally extensional deformation. The degree of deformation of the core will depend on its relative strength. This deformation is very often associated with erosion.

Drumlin Structures

It was shown above from the study of 33 drumlins that there were three types; depositional, deformational and erosional, and many drumlins were shown to be a combination of these three basic types. However it can be seen from Table 2 that most drumlins were erosional, but those with weak cores showed evidence of differing degrees of deformation, and those with rock-cores showed evidence of deposition.

(a) If more sediment enters a given subglacial area than is removed (negative sediment flux), then the sediment will build up, and cores within the deforming mass will be surrounded with till, and a subglacial till sequence will develop (Fig. 8a). Hart et al. (1990) have shown that there are two typical styles of subglacial deformation: (a) at the margin, the deforming layer is thin (due to low basal shear stresses) and there is a large amount of sediment being moved into the area. This results in the base of the deforming moving upwards through the sequence, with one style of subglacial deformation being superimposed on the other (constructional deforma- tion); (b) in contrast, further up-glacier (typically in the area near the equilibrium line) the deforming layer thickens, causing the base of the deforming layer to cut down into the underlying sediments (excavational deformation).

DISCUSSION OF THE RELATIONSHIP OF DRUMLIN FORMATION TO OTHER

SUBGLACIAL DEFORMING BED PROCESSES

It was discussed above that drumlin formation is not

(b) However, if more sediment is removed from a given subglacial area than enters (positive sediment flux), then any cores will be left behind as erosional remnants (Fig. 8b).

Thus the individual structures of the drumlin cores may be formed by different processes but the drumlin shape is the result of net subglacial deforming bed erosion

Thus, a very typical drumlin sequence would be similar to that seen at Sligachan, Skye: (a) net subglacial deposition of a deforming bed’ till; (b) then a change in

104 Quaternary Science Reviews: Volume 16

0’Briai-f Road

ifiid sediments

_--_ I’

’ \ \ : \ I

\ \

‘\

??Stratified 66ditIWtIt El Massive dimktm with @bfIiC 66Uld @6X8

FIG. 7. (a) Photograph of the O’Brian Road drumlin, New York, U.S.A. (b) Schematic section from the O’Brian Road drumlin. (c) The development of concentric drumlins.

J.K. Hart: Drumlins and Other Forms of Subglacial Deformation

Erosion

105 , ,,;<;%‘ _,,t:,. ,.-1

. ,:..; ._,‘(.

_,‘,-l_‘,‘, Ice f_‘-,:, I

’ :-, ,; -’ : . I ;‘_ .’ * G .“,I, \;. ,., ; ,: \ .

,I \I. I

0 deforming layer a

a

FIG. 8. (a) Subglacial soft bed deposition-till formation. (b) Subglacial soft bed erosion-drumlins.

subglacial conditions so that there is net subglacial erosion. If the sediment is completely homogeneous then all the sediment may be removed. However, this is unlikely because there will probably be inhomogeneities in the drift that would act as cores and lead to drumlinization. In this way, drumlins can form without an intervening retreat but merely a change in subglacial deforming bed conditions.

In order for more sediment to be removed (positive sediment flux) one or both of the following processes must occur. (a>

(b)

Reduction in sediment supply. This may be due to changes in subglacial pore water pressure, changes in geotechnical properties of the sediment, or changes in the basal shear stress, resulting in the subglacial sediment becoming more rigid further up-glacier. Velocity increase. If the velocity increases, sediment will be removed more quickly than it can be replaced from up-glacier.

I suggest that although there are three types of internal structures, that the external shape of the drumlin is formed due to erosion. That is, the core of the drumlin represents a stable part of the deforming layer, and the surrounding sediment is removed by deformation down- glacier and may form push moraines at the glacier margin (Hart, 1995b).

This results in a drumlin structure continuum (shown in Fig. 9) which may be related to the behaviour of the deforming layer. A one end of the continuum is the erosional structure drumlin (Fig. 9(i)), which has a relatively competent core and where the internal part of the drumlin is unrelated to the drumlin structure. This may form where the deforming layer is cutting down into underlying sediment, and material is being rapidly removed, so that there is no sediment build up in the low pressure shadow area.

If the core is weaker, then a deformational structure drumlin will form (Fig. 9(ii)). The style of deformation

may be in the form of a boudin if the drumlin core sediment is weaker than the surrounding sediment or a sheath fold if it is stronger. Intermediate between the erosional structure drumlin and the deformational struc- ture drumlin is the drumlin with brittle stoss-side deformation (Fig. 9(ii)). Within this type of drumlin, most of the internal sediment was deposited prior to the drumlinizing event, in a similar way to the erosional drumlin.

At the other end of the spectrum is the depositional structure drumlin where slower moving till has been deposited in the low pressure shadow, but faster moving till may have eroded the drumlin into its form (Fig. 9(v)). This may reflect situations where the deforming bed is thin and there is a (relatively) high sediment input (to allow sediment build up in the low pressure shadow area). The ductile stoss-side deformation may be intermediate form between the deformational structure drumlin and the depositional structure drumlin (Fig. 9(iv)), as in this drumlin most of the internal sediment is related to the drumlin forming event.

If this interpretation is correct then the interplay between net sediment flux and styles of deformation can be related (see Table 4). This would explain why depositional drumlins and flutes are frequently found at the glacier margin (e.g. Vestari-Hagafellsjiikull, Iceland; Hart, 1995b) where the deforming layer is thin and there is a high sediment input from up-glacier; and erosional drumlins are found up-glacier.

Assuming that the streamlined bedforms are associated with deforming bed processes, then the overall shape of the drumlin must also be related to the processes within the deforming layer. Many workers (Hollingworth, 1931; Boulton, 1987; Boyce and Eyles, 1991; Clark, 1994) have argued that the elongation of the drumlin can be related to the velocity of the ice sheet and shear strain, and Hart (1995b) has argued that the height is due to the thickness of the deforming layer. Hart and Smith (submitted) have

erosional structure

i

brittle stoss-side deformational structure deformational structure

ductile stoss-side deformational structure

ii iii iv

FIG. 9. The drumlin structure continuum.

depositional structure

V

106 Quaternary Science Reviews: Volume 16

TABLE 4. The relationship between deforming bed till and drumlin formation

Movement of the base of Net sediment flux the deforming layer Resultant landformsediment

Negative Upwards through the sequence plus Till (constructional deformation / deposition) high sediment influx

Negative Downwards through the sequence Till (excavational deformation / deposition) Positive Upwards through the sequence plus Drumlin - depositional structure

high sediment influx Positive Downwards through the sequence Drumlin - erosional structure

argued that the resultant drumlin shape is related both to the complex interplay of these two factors. Thus, the presence and nature of drumlins in the geological record can be used to reconstruct past glacier dynamics.

CONCLUSION

From a study of a large number of drumlins it has been suggested that there is a drumlin structure continuum: (a) depositional, which are mostly (fixed) rock cored; (b) deformational, which include stoss-side deformation (brittle and ductile deformation), stoss and lee-side deformation, compressional deformation, sheath folds and concentric deformation; and (c) erosional. These all form associated with net subglacial deforming bed erosion (when more sediment is removed than enters a given subglacial section) in contrast to subglacial deposition (when more sediment enters than is removed) which results in the build up of subglacial deforming bed till. It is further suggested that the depositional structure drumlins reflect the base of the deforming layer rising upwards through the sequence (associated with marginal conditions) and the erosional drumlins are associated with the base of the deforming layer moving downwards into underlying sediments (associated with more up-glacier conditions). The deforming bed till will only show visible evidence of deformation if it has either undergone relatively low shear strain, or if it moves over an inhomogeneous bedrock (Hart and Boulton, 1991). Most deforming bed till is homogeneous, and its identification includes the study of till fabric, clast arrangements, the nature of the base of the till and micromorphology (Hart, 1994, 199%; Hicock and Dreimanis, 1992 ; van der Meer, 1994). Similarly the drumlin will only show evidence of deformation if the core is relatively weak or has moved over homogeneous bedrock, and so most drumlins show no visible evidence of deformation.

It is further suggested that subglacial erosion occurs associated with either an increase in ice sheet velocity or decrease in subglacial sediment supply. Thus drumlins are not only indicative of deforming bed conditions; but can be used to reconstruct both the nature of the deforming bed and the glacier.

ACKNOWLEDGEMENTS

I would like to thank Kirk Martinez for his help in the field and Dave Mickelson for showing me field sites in south east

Wisconsin. I would also like to thank Alan Bum and Tim Aspden and their colleagues in the Cartographic Unit, Depart- ment of Geography, University of Southampton for excellent figure reproduction and Mary Hendy for proof reading. This work was funded by a University of Southampton Grant 92/2.

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