Drumlin Shape Research Articles

Factors that contribute to the elongation of drumlins beneath the Green Bay Lobe, Laurentide Ice Sheet

AbstractDrumlin shape has been hypothesized to correlate with ice‐flow duration and slip speed, but modern‐day analogues and the Coulomb nature of till render the basis of these correlations in question. The evolution of flow‐parallel subglacial landforms is of importance for ice flow because the form drag they provide may be a dominant factor in regulating glacier slip speeds. Here we examine the relationship between drumlin shape and cumulative slip displacement (i.e. time‐integrated slip speed) as a dominant glaciological control on drumlin shape. First, a new method is developed that allows slip speed to be estimated for deformable bedded glaciers along a flow line from an ice surface profile. Then, reconstructed surface profiles for ice margin chronologies of the Green Bay Lobe (GBL) are used to construct and estimate the spatially varying cumulative slip displacement for use in comparison with drumlin elongation ratios. We focus on a sector of the GBL near the central flow line where the geology is simple and glaciological controls are likely to dominate bedform development. Using Bayesian statistical analysis, a positive and statistically robust relationship between cumulative slip displacement and drumlin elongation ratio is found. Our analysis indicates that drumlin shape could be used to infer palaeo glacier slip speeds if time under the ice can be well constrained and geologic influences are minimal. These findings also suggest that drumlin‐supplied drag could decrease with increased cumulative slip displacement in the absence of rigid geologic features that fix drumlin positions.

Groundwater Flow Under a Paleo‐Ice Stream of the Scandinavian Ice Sheet and Its Implications for the Formation of Stargard Drumlin Field, NW Poland

AbstractSubglacial groundwater flow was an integral part of glaciological systems of past ice sheets, but its impact on the origin of active‐ice landforms remains unexplored. Using numerical experiments, we attempt to constrain groundwater flow dynamics under a major paleo‐ice stream of the southern Scandinavian Ice Sheet and its impact on the formation of the Stargard drumlin field. Flow models show a total reorganization of groundwater dynamics under the advancing ice stream to a depth of up to ~200 m. A mosaic of intervening groundwater recharge and discharge areas originates, whereby the same areas may experience multiple shifts in flow directions. A prominent time‐ and space‐transgressive pressure pump recharges groundwater in a subglacial zone up to about 20 km within the ice margin and discharges it in front of the ice sheet. The simulated groundwater flow pattern suggests that drumlins occupying a portion of the ice stream area occur preferentially where groundwater upwells and discharges at the ice/bed interface. Consistent with the geological composition, a possible drumlin‐forming mechanism involves an excess of pressurized water under the ice reducing the strength of the subglacial deposits and facilitating glacial erosion, erosion by turbulent meltwater flows or both, streamlining antecedent deposits into drumlin shapes. This study emphasizes the importance of causal feedback between subglacial groundwater flow and the ice sheet dynamics and suggests an impact of the former on the formation of streamlined subglacial landforms in general.

A case study in the New York Drumlin Field, an investigation using microsedimentology, resulting in the refinement of a theory of drumlin formation

The formation of drumlins remains a major enigma. It is accepted that drumlins form under active temperate ice most likely within a soft sediment deforming bed at ephemeral ‘sticky points’. These ‘sticky points’ likely lead to the causative mechanism around which sediment nucleation occurs. The critical question is under what conditions and where and how do ‘sticky spots’ form. A comparative investigation of a drumlin and mega-flute in the New York Drumlin Field, as a case study, demonstrates that rheological and sedimentological tills from these different forms are similar. It is not, therefore, rheological change alone that must account for drumlin shape and form but likely the advective subglacial basal sediment flux rate at the ice bed. The rate of sediment motion between the upper interface at the ice–sediment bed boundary, and the lower immobile sediment at depth is crucial. At the lower décollement between the mobile and immobile sediment units, within the deforming sediment package, proto-drumlin nucleation is likely to occur and develop into a streamlined form. The trigger mechanism for such a perturbation is a derivative of sediment rheology and sediment flux rate driven by the overlying ice stresses. Recent evidence from Antarctica lends credence to this new hypothesis that can be related to all drumlins formed under temperate, soft sediment deforming bed conditions.

Automated drumlin shape and volume estimation using high resolution LiDAR imagery (Curvature Based Relief Separation): A test from the Wadena Drumlin Field, Minnesota

Resolving the origin(s) of drumlins and related megaridges in areas of megascale glacial lineations (MSGL) left by paleo-ice sheets is critical to understanding how ancient ice sheets interacted with their sediment beds. MSGL is now linked with fast-flowing ice streams but there is a broad range of erosional and depositional models. Further progress is reliant on constraining fluxes of subglacial sediment at the ice sheet base which in turn is dependent on morphological data such as landform shape and elongation and most importantly landform volume. Past practice in determining shape has employed a broad range of geomorphological methods from strictly visualisation techniques to more complex semi-automated and automated drumlin extraction methods. This paper reviews and builds on currently available visualisation, semi-automated and automated extraction methods and presents a new, Curvature Based Relief Separation (CBRS) technique; for drumlin mapping. This uses curvature analysis to generate a base level from which topography can be normalized and drumlin volume can be derived. This methodology is tested using a high resolution (3m) LiDAR elevation dataset from the Wadena Drumlin Field, Minnesota, USA, which was constructed by the Wadena Lobe of the Laurentide Ice Sheet ca. 20,000years ago and which as a whole contains ~2000 drumlins across an area of ~7500km2. This analysis demonstrates that CBRS provides an objective and robust procedure for automated drumlin extraction. There is strong agreement with manually selected landforms but the method is also capable of resolving features that were not detectable manually thereby considerably expanding the known population of streamlined landforms. CBRS provides an effective automatic method for visualisation of large areas of the streamlined beds of former ice sheets and for modelling sediment fluxes below ice sheets.

Magnetic fabrics of drumlins of the Green Bay Lobe, southeastern Wisconsin

To study if the relief of drumlins of the Green Bay Lobe of the Laurentide Ice Sheet is dominantly the result of depositional or erosional processes, we measured the anisotropy of magnetic susceptibility of ∼2500 samples of the Horicon till from five drumlins in southeastern Wisconsin. These drumlins contain no macroscopic heterogeneities that might otherwise help guide interpretation of deformation kinematics. Ring-shear experiments on this till provided the calibration necessary to relate fabrics based on principal susceptibilities of multiple intact samples to patterns of subglacial till deformation. Fabric strengths, based on orientations of maximum susceptibility and measured from samples collected below the zone of pedogenesis, weaken abruptly at depths shallower than 2 m. The till lacks evidence of systematically divergent or convergent shear related to drumlin shape. Most till has been sheared in directions 6–24° to the east of drumlin long axes, with shear-plane orientations that are unrelated to drumlin morphology. An exception is till at shallow depths (≤2 m) where shearing azimuths and shear-plane orientations agree more consistently with drumlin trends and morphologies. The Horicon till was likely deformed in shear during advance of the lobe. Drumlins formed by erosion of this basal till after glacier flow had shifted to the west, when the Johnstown moraine was being built. A supraglacial till, ≤2 m thick, was draped over some of the drumlins during retreat of the lobe. These results do not support models of drumlin formation in which sediment deposition helps amplify drumlin relief.

On the size and shape of drumlins

We provide a mechanistic explanation for observed metrics for drumlins, which represent their sizes and shapes. Our explanation is based on a concept of drumlin growth occurring through a process of instability, whereby small amplitude wave forms first grow as ice slides over a bed of deformable sediments, followed by a coarsening process, in which the wavelength as well as the relief of the drumlins continues to grow. The observations then provide inferences about the growth process itself.

The use of integrated sedimentological and geophysical methods in drumlin research – a case study of Pigeon Point, Clew Bay, Northwest Ireland

ABSTRACTDespite significant advances over the past decades, our understanding of drumlin formation and associated ice‐bed processes is still incomplete. In this paper, we present the integrated use of geomorphological, sedimentological and geophysical techniques as a powerful means to force a breakthrough towards solving the drumlin enigma.We report on investigations of the anatomy of the Pigeon Point drumlin, Clew Bay, Ireland. We found that the bulk of the landform, which displays a classical drumlin shape, consists of silty‐clayey diamicton showing evidence of deformation, hydrofracturing and comminution. The unit is interpreted as a sub‐glacial traction till/comminution till. The thin unit overlying this basal till consists of silty‐sandy diamicton, and is interpreted as a para‐glacially modified melt‐out till. The partly cemented third unit consists of stratified, massive to graded sands and gravels. Its contact with the sub‐glacial traction till consists of a series of concave shapes, which suggests that it was deposited in meltwater channels that flowed in sub‐glacial cavities and that cut laterally into the drumlin.We propose that highs in the undulating rockhead relief, as shown in the seismic profile, have provided nuclei which initiated drumlin formation. This idea is supported by the observation of local detached bedrock slabs that grade upwards into a comminution till. In the long profile, very high normalized induced polarization (IP) values form a wedge‐shape, which is interpreted as a set of conjugate thrusts, or a ‘pop‐up’ structure. The structure is positioned directly above one of the undulations in the bedrock, suggesting a direct relationship. The high values are thought to reflect the presence of pre‐existing clays, which were sheared into the till, thus forming linings in the thrust features. It is concluded that glacitectonic processes, notably differential bedrock weathering and thrusting, have played a key role in the formation of this drumlin. Copyright © 2011 John Wiley & Sons, Ltd.

The topography of drumlins; assessing their long profile shape

AbstractThe literature on drumlins is pervaded by the notion that their longitudinal profile is usually highly asymmetric with a stoss steeper end and a lee gentler end, and that this can be used to infer the palaeo ice flow direction. The idea is built up in many papers covering more than a century of research. However, most of the early published papers were qualitative in nature and more recent quantitative papers appear to challenge the ubiquity of the classical stoss–lee shape. Here the shape of the drumlin longitudinal profile of 29,238 drumlins in the British Isles has been analysed. Drumlins were grouped into flowsets for which a palaeo ice flow direction was inferred independently of the drumlin shape. Given such ice flow directions it was then possible to identify the upflow and downflow ends of each drumlin and to trace and extract its longitudinal profile. Results indicate that the highest point of the drumlin is usually found around half way along the profile. The mean slope angles at the stoss and lee ends were found to be very similar. Finally, the average profile built from all mapped drumlins appears almost symmetric. In conclusion, drumlin longitudinal profile is far from being the classically asymmetric shape widely reported in text‐book descriptions. Drumlins are mostly symmetrical in shape with a very slight tendency towards the classical shape. Highly asymmetric drumlins do exist but are rare and with an almost equal numbers of drumlins that are the reverse of this. These findings are verified both at the entire population level and the flowset level. This demonstrates that the idealized stoss–lee shape of classically‐asymmetric drumlins is not representative of the population and should be replaced with that of mostly symmetric bumps. Drumlin shape is not a good indicator of the palaeo ice flow direction. Copyright © 2010 John Wiley & Sons, Ltd.

The planar shape of drumlins

The asymmetry of the planar shape of drumlins is an established paradigm in the literature and characterizes drumlins as resembling tear drops with a blunt (bullet-shaped) stoss end and a tapering (pointed) lee end. It is widely cited and never been seriously questioned. In this paper, the planar shape of 44,500 drumlins mapped in various regional settings from drumlin fields in North America and Northern Europe were objectively analysed by means of Geographic Information System tools. Two parameters were considered. The first (denoted here as As pl) focuses on the relative position of the point of intersection between the axes of the maximum length and the maximum width. It is defined as the distance between the upstream (i.e. beginning of the drumlin) and the intersection point (measured along the longitudinal axis) divided by the entire length of the long axis. Results indicate that the intersection point of the majority of drumlins (64%) is very close to the longitudinal midpoint (0.33 < As pl < 0.66). The second parameter ( As pl _A ) is defined as the ratio between the area of the upstream half of the drumlin to that of the entire drumlin. Results show that for most drumlins (81%), the upper half area is almost as large as the down-half (0.45 < As pl _A < 0.55). Taken together, these results concordantly indicate that drumlin planar shape has a strong tendency to be longitudinally symmetric and that the long-established paradigm of their plan form is false.

Origin of drumlins on the floor of Lake Ontario and in upper New York State

Upper New York State, USA contains one of the largest drumlin fields (12,000 km 2) on the North America continent consisting of some 10,000 drumlins located between Lake Ontario in the north and the Finger Lakes; drumlin-like bedforms have also recently been discovered in deep water (∼ 200 m) on the nearby floor of the Rochester Basin in Lake Ontario. The New York State drumlin field was produced under the Laurentide Ice Sheet 18,000 years ago when it extended southwards to terminate against the Allegheny Plateau. Outcrops of drumlin cores exposed by erosion along the Lake Ontario shoreline provide windows into the stratigraphy and sedimentology of the northern part of the field. Sedimentological description of 10 coastal drumlin cores, combined with quantitative morphometric analysis of 1487 drumlins, shows that they are the product of subglacial erosion of till sheets separated by glaciolacustrine deposits. A two-phase subglacial model for drumlin formation is suggested consisting of initial aggradation of extensive till deposits during ice advance followed by erosion and streamlining possibly accompanying stillstand of the ice margin and building of moraines. Systematic changes in drumlin morphometry and spacing are evident, ranging from small numbers (< 15% of total bedforms) of widely separated, spindle-shaped forms (Length/Width ratios between 6 and 18) to more numerous (< 40%) broader forms (L/W ratios between 3 and 6) to most numerous (> 60%) closely spaced, ovoid drumlins (L/W ratios between 1 and 3). Drumlin shape appears to be systematically related to sediment thickness available for erosional streamlining; elongate spindle forms occur on the crests and lee sides of bedrock escarpments where sediment cover is thin. Larger ovoid drumlins occur in topographic depressions where sediment thickness is greater. Offshore streamlined forms on the floor of the deep water Rochester Basin in Lake Ontario have long axes at right angles to the trend of onshore drumlins. These are composed of rock (rock drumlins) and have an orientation structurally controlled by a newly identified inlier of Proterozoic Grenville Province metasediments.

The internal sediment architecture of a drumlin, Port Byron, New York State, USA

An exposure within the central portion of a large drumlin at Port Byron, New York State, USA, part of the large New York drumlin field, reveals a sequence of steeply dipping cemented sands and gravels of proglacial, ice-contact deltaic origin overlain by a thin till veneer. The sands and gravels appear to have been deposited within the proximal proglacial environment during a late retreat phase of the Laurentide Ice Sheet sometime prior to being overridden by subsequent ice and drumlinized. During deposition of the ice-contact delta, escaping subglacial regelation-meltwater permeated the proximal deltaic sediment pile and calcium carbonate was released, in a series of pulses, to form pore-occluding calcite cement within the sand and gravel porespaces. The calcium carbonate precipitated into the sands and gravels due to a reduction in hydrostatic pressure and CO2 outgassing of the meltwater as it exited from beneath the ice sheet. Once cemented, these deltaic sediments were considerably stronger and acted afterward as an obstacle around which the future ice advance streamed and, in turn, produced the characteristic drumlin shape. In overriding the ice-contact deltaic sediments, the ice sheet emplaced a thin layer of till which exhibits syndepositional deformation features indicative of being emplaced as a deforming bed layer beneath the advancing ice sheet. Micromorphological analysis of the overlying till shows that no interstitial or intraclastic calcite occurs within the till.

Sedimentological and GPR Studies of Subglacial Deposits in the Joux Valley (Vaud, Switzerland): Backset Accretion in an Esker Followed by an Erosive Jökulhlaup

During the Würmian glaciation, the Jura ice sheet covered the Joux Valley (Vaud, Switzerland). A geomorphological study reveals many drumlins in this valley. Some are composed of gravels and sand, others of till. Outcrops show that the surface of the sandy-gravel drumlins is a major and sharp erosion surface. Given the lack of shearing structures in sediments below this erosion level, its origin cannot be linked to ice action of the glacier. Very high-energy subglacial meltwater floods (jökulhlaups), probably due to the drainage of subglacial or supra-glacial lakes, are the more likely cause of the erosion. Results of a ground penetrating radar (GPR) survey show the internal structure of one of these sandy-gravel drumlins to depth of 15 m. These GPR data, together with sedimentological observations, indicate that prior to erosion, subglacial sedimentation occurred in closed conduits (eskers) with strong and rapid flow variations. The sediments contain large chute-and-pool structures (high flow energy backset accretion) with dimensions comparable to the conduit width. Therefore, we interpret these sandy-gravel drumlins as portions of eskers, their present drumlin shape being the result of erosion by one or many jökulhlaups.The good preservation of the subglacial meltwater deposits is the result of the closed-basin geometry of the Joux Valley, which limited movement at the base of the glacier. This new contribution to the interpretation of the Joux Valley glacial features underlines the importance of meltwater in sedimentological processes under the Jura ice sheet.

Drumlinization and drumlin-forming instabilities: viscous till mechanisms

AbstractGlacially induced flow naturally tends to thin and extended till cover through shock formation, even in the absence of longitudinal gradients in the applied stress. Thicker till cover has an increased effective pressure at its surface and base, a lower sliding velocity or deformation rate and above a critical thickness, a decrease in wave velocity with thickness, leading to reverse-facing shocks moving downstream. For sliding and for some theologies of internal deformation, a decrease in sediment flux with thickness occurs, implying backward-moving kinematic waves and reverse-facing, reverse-moving shock.Downstream-facing shocks are also formed which move upstream if the till is sliding and downstream if the till is deforming internally. Eventually, shocks coalesce, leaving an upstream-lacing shock for sliding and a downstream-facing shock for internal deformation. It is observed that some drumlins have downstream blunt ends only.Fairly realistic three-dimensional drumlin shapes can be produced from symmetric sediment bodies and barchan shapes can be produced from linear forms perpendicular to the ice-sheet flow.The fact that viscous theories produce drumlinoid forms suggests that on this scale till behaves viscously and the the lower length scale for drumlins represents the plastic/viscous transition scale.

Drumlinization and drumlin-forming instabilities: viscous till mechanisms

AbstractGlacially induced flow naturally tends to thin and extended till cover through shock formation, even in the absence of longitudinal gradients in the applied stress. Thicker till cover has an increased effective pressure at its surface and base, a lower sliding velocity or deformation rate and above a critical thickness, a decrease in wave velocity with thickness, leading to reverse-facing shocks moving downstream. For sliding and for some theologies of internal deformation, a decrease in sediment flux with thickness occurs, implying backward-moving kinematic waves and reverse-facing, reverse-moving shock.Downstream-facing shocks are also formed which move upstream if the till is sliding and downstream if the till is deforming internally. Eventually, shocks coalesce, leaving an upstream-lacing shock for sliding and a downstream-facing shock for internal deformation. It is observed that some drumlins have downstream blunt ends only.Fairly realistic three-dimensional drumlin shapes can be produced from symmetric sediment bodies and barchan shapes can be produced from linear forms perpendicular to the ice-sheet flow.The fact that viscous theories produce drumlinoid forms suggests that on this scale till behaves viscously and the the lower length scale for drumlins represents the plastic/viscous transition scale.

The shape of drumlins, their distribution in drumlin fields, and the nature of the sub-ice shaping forces

This paper examines attempts to define and interpret drumlin shape; in particular the Chorley lemniscate and the Reed—Galvin—Miller ellipsoid. It also considers the Koenderink approach to describing shape, which ignores statistically defined shapes and concentrates on arbitrary smooth lumps of three-dimensional space. The basic shape of a drumlin at first sight appears to offer no clue to the rightness of any of the hypotheses or conjectures about drumlin formation. The shape suggests that a flowing fluid erodes the parts of landscape which have a lower shear strength than the adjacent parts. However, by considering whether the fluid is ice (very viscous) or water (less viscous), it is suggested that it is possible to distinguish between a drumlin formed by deformation and a drumlin formed by basal water erosion, by identifying subtle shape differences produced by different flow regimes. We find little indication, either from drumlin shape or disposition of drumlins in fields, that the basal water erosion mechanism causes the formation of drumlins. Observations of the growth and break-up of bubbles rising through a fluid may serve as an appropriate analogy for describing drumlin growth and eventual break-up. This bubble model is useful in suggesting an accretion model for drumlin growth and predicting that the form of a drumlin before break-up is similar to a spherical cap or parabolic drumlin. The bubble model and observation of flow past obstacles at very low Reynolds numbers suggest that a minimum resistance shape is not always a lemniscate. The variation of the critical stress and strength parameters across a drumlin field (essentially perpendicular to the ice-flow direction) suggests that the distribution could be zonal, that across a drumlin field a change in drumlin density should occur. Considering the drumlins as a whole-field phenomenon the “dimpled” surface of the drumlin swarm may actually reduce drag.

Drumlin location as a response to bedrock topography on the southeastern slope of the Fennoscandian Shield

There are several thousand drumlins in Estonia, northern Latvia and on the bottom of the Gulf of Finland. Drumlins occur most frequently behind bedrock elevations, on the slopes of which they form huge crag-and-tail formations, and in marginal areas of depressions where the monolithic glacier disintegrated into tongues which moved at different velocities. A dense net of boreholes and detailed geophysical measurements showed that even small irregularities of the bedrock topography and slight variations in the resistance of bedrock to glacial erosion exerted remarkable influence upon the location, shape and internal structure of drumlins and drumlin-like forms. This accounts for the occurrence of drumlins with distinctly different structures within the same drumlin field. However, the drumlins were formed in a variety of ways and at different times under the effect of glaciers with different thickness. In most cases they seem to have formed near the ice margin under rather thin glacier ice during its rapid advance when the dynamics of the ice responded to topographical peculiarities.

Drumlins carved by deforming till streams below the Laurentide ice sheet

The Peterborough drumlin field (5000 km 2 ) is the largest in central Canada and was formed during readvance of the Laurentide ice sheet shortly after 13,000 B.P. Morphometric analysis of 998 drumlins, combined with study of 6921 borehole logs and downhole geophysical data, has established the distribution of drumlin types and their stratigraphy along a 70 km flow line through the drumlin field. In the north, drumlins are spindle-shaped, composed of till resting directly on bedrock, and were formed by deformation of till into low-pressure zones in the lee of bedrock scarps. Eskers record subglacial drainage by channeled meltwaters flowing on bedrock. To the south, drift thickness increases and eskers are absent; drumlins are larger and more ovoid and are composed of overridden proglacial and glaciolacustrine sediments overlain erosively by deformation till. Till is thin over drumlin crests and thick in interdrumlin swales and records subglacial cannibalization and mixing of overridden sediments; the drumlin form is the product of erosion by streams of deforming till flowing in swales separating cores composed of older sediments. Systematic down-field changes in drumlin shape and size can be related to decreased duration of subglacial deformation toward the limit of the readvance and increased thickness of preexisting sediments available for dissection by subglacial till streams. Absence of eskers in the central and southern parts of the drumlin field suggests that the ice base was drained by mass flow of the deforming substrate. The substantial subsurface data available from the Peterborough area indicate that drumlins are predominantly erosional in origin and are the geomorphological expression of a deforming subglacial bed below the margin of the Laurentide ice sheet.

Precrags and drumlin fields in Finland

The paper deals with radially oriented moraine ridges-precrags and other drumlins-in Finland, their distribution and location in relation to ice lobes, local topography and bedrock structure. Emphasis is given to precrag ridges, whose occurrence in southwestern Finland was only recently established as a result of more detailed Quaternary geological mapping (1:20,000 and 1:100,000). The short review of drumlins is based both on recent drumlin studies and on those undertaken during past decades. The size and shape of all these streamlined moraine ridges were studied morphologically. The internal structure and material of moraines were determined by drilling and sampling in addition to seismic refraction sounding or ground probing radar surveys. Variations in the size, shape and structure of precrags and drumlins are attributed to the dimensions of the moving continental ice mass and glacial activity in different parts of the country. The orientation of the precrag ridges and the drumlin fields reflects local ice flow patterns during the Late Weichselian glaciation. The precrag field and southwestern Finland is evidently older than the other drumlin fields in Finland and does not contain any other streamlined moraine ridges than precrags. On the basis of the ice-marginal formations and the glacial flow patterns the authors have inferred that the oldest precrag ridges originally developed in a cold-based subglacial thermal zone at the time when the margin of the continental ice was probably still in Estonia, i.e. almost 200 km to the south. The drumlin fields formed again under the warm-based part of the ice sheet in a zone that extended for about 20–100 km inwards from the ice margin and situated in the middle of active ice lobes.

Till Fabric and Deformational Structures in Drumlins Near Waukesha, Wisconsin, U.S.A.

AbstractDeep gravel-pit exposures reveal the distribution and structure of till and underlying sand and gravel in drumlins near Waukesha, Wisconsin. The subglacial sediment is interpreted to have moved laterally into the drumlin sites because the till thickens from the margin to the core of the drumlins, the stone orientation in the till is perpendicular and oblique to ice flow on the drumlin margins, and recumbent isoclinal folds occur in sand on the drumlin margins with axes parallel to the drumlin axes. The resulting accumulations of sediment presented obstacles to ice flow and were streamlined into the minimum-drag drumlin shape by erosion on the margins and by remolding of material in the core of the drumlins. These drumlin nuclei may have formed at spots where there was low effective stress on the bed. The subglacial sediment became mobile as a result of high pore pressure that may have developed as ground water and subglacial melt water were trapped behind a frozen bed at the ice margin. Under certain conditions, however, lateral sediment flow might also have occurred when the sediment was frozen.

Till Fabric and Deformational Structures in Drumlins Near Waukesha, Wisconsin, U.S.A.

AbstractDeep gravel-pit exposures reveal the distribution and structure of till and underlying sand and gravel in drumlins near Waukesha, Wisconsin. The subglacial sediment is interpreted to have moved laterally into the drumlin sites because the till thickens from the margin to the core of the drumlins, the stone orientation in the till is perpendicular and oblique to ice flow on the drumlin margins, and recumbent isoclinal folds occur in sand on the drumlin margins with axes parallel to the drumlin axes. The resulting accumulations of sediment presented obstacles to ice flow and were streamlined into the minimum-drag drumlin shape by erosion on the margins and by remolding of material in the core of the drumlins. These drumlin nuclei may have formed at spots where there was low effective stress on the bed. The subglacial sediment became mobile as a result of high pore pressure that may have developed as ground water and subglacial melt water were trapped behind a frozen bed at the ice margin. Under certain conditions, however, lateral sediment flow might also have occurred when the sediment was frozen.