Drumlin Formation Research Articles

Absent drumlins beneath southern lobes of the Laurentide Ice Sheet: A new hypothesis based on Des Moines Lobe dynamics inferred from landforms

AbstractSpatial distributions of drumlin fields encode information about ice sheet dynamics. No drumlins formed beneath the most lobate parts of the Laurentide Ice Sheet's southern margin, in South Dakota, Iowa and Illinois, whereas ice lobes to the northeast generally produced drumlins. This pattern may have resulted from northerly ice overriding permafrost. Here we propose a new hypothesis for this pattern by constructing a LiDAR‐based landform map and applying a model of drumlin formation to account for the absence of drumlins beneath the largest of the ice sheet's southern lobes, the Des Moines Lobe. Broad belts of hummocky topography, ice‐walled lake plains, doughnuts and minor moraines, which together cover 90% of the lobe's upland area in Iowa, attest to widespread ice stagnation, as does the lobe's scarcity of eskers. Most stagnation topography is subtle, with insufficient relief to have obscured drumlins that might have formed before stagnation. Minor moraines are crevasse‐squeeze ridges diagnostic of surging, and their ubiquity indicates that during surging, the lobe's soft bed was weak nearly everywhere. End moraines are generally parallel to a minor moraine setup‐glacier, implying that surge‐driven advances were more numerous than indicated by the three major end moraines of the lobe. In the only physically based model of drumlin formation that includes surging, till deposition occurs during surges when effective pressure is uniformly low, whereas drumlins develop during quiescent flow between surges, when basal slip and low‐pressure R‐channels create the spatial gradients in effective pressure necessary to sculpt drumlins by differential erosion. Landforms of the lobe, however, indicate stagnation and down‐wasting during quiescence, without significant basal slip or hydraulic potential gradients necessary for R‐channels. We hypothesize that for the three southernmost lobes of the Laurentide Ice Sheet, surging followed by widespread down‐wasting of stagnant ice prevented drumlin formation, whereas beneath northern lobes it was permitted by climatically‐forced ice advance.

Drumlin formation within the Bustarfell drumlin field, northeast Iceland: integrating sedimentological and ground‐penetrating radar data

ABSTRACTDrumlins are important bedforms of former glaciated landscapes as they demonstrate past ice‐flow directions and elucidate processes that operated at the ice/bed interface. Recently mapped drumlins and other streamlined subglacial bedforms in northeast Iceland reveal the flow‐sets of cross‐cutting palaeo‐ice streams that were active within the Iceland Ice Sheet (IIS) during and following the Last Glacial Maximum. Here we study the Bustarfell drumlin field within the Vopnafjörður–Jökuldalsheiði flow‐set. The internal architecture of two drumlins was investigated using sedimentological analysis and ground‐penetrating radar (GPR, 50 and 100 MHz) to illuminate subglacial processes that contributed to drumlin formation, as well as the history and dynamics of the IIS. On the stoss side of one of the drumlins, two subglacial traction till units were identified, separated by a thick unit of deformed glaciofluvial sand and gravel. The core of glaciofluvial material suggests that the drumlin formed around well‐drained patches (sticky spots) in the subglacial bed that retarded the ice flow locally through increased basal drag and encouraged till deposition. Furthermore, our GPR data indicate a combination of erosional and depositional processes. We suggest that the glaciofluvial sediments were deposited as small ice‐marginal fans on the Bustarfell plateau, possibly during the Bølling–Allerød interstadial, and that the drumlins were formed around these fans during a subsequent readvance during the Younger Dryas.

Drumlins and mega-scale glacial lineations as a continuum of subglacial shear marks: A LiDAR based morphometric study of streamlined surfaces on the bed of a Canadian paleo-ice stream

We employ and further develop quantitative analytical techniques used to map roughness on fault planes, to map and classify drumlins and mega-scale glacial lineations (MSGLs) on the bed of a terrestrially-terminating Late Wisconsin paleo ice stream lobe exposed near Peterborough in Southern Ontario, Canada. Geological data show these bedforms are erosional remnants of older sediments, principally coarse-grained proximal outwash, capped by a thin (<2 m) upper till. Our earlier work ascribed the formation of drumlins and MSGLs to differential erosion of antecedent sediment by a thin layer of deforming subglacial debris (the Erodent Layer Hypothesis) which we now test using new geostatistical and unsupervised machine learning techniques applied to large LiDAR-derived digital topographic datasets. Analysis of more than 1739 streamlined bedforms mapped by Curvature-Based Relief Separation, reveals a statistically-validated continuum of eight morphotypes (M1 to M8) ranging from drumlins to grooved multi-crested drumlins to MSGLs. Furthermore, cluster analysis of the entire morphotype population, reveals three statistically-validated groups that define three streamlined surface types (S1 to S3) which record progressive erosional lowering of the ice stream bed, as drumlins evolve into multi-crested morphotypes, and ultimately MSGLs. Calculation of surface roughness using a technique called Median Absolute Differences (MAD) shows the frictional drag of mega-scale glacially lineated surfaces (S3) is 60% less than drumlinized surfaces (S1). Our findings support earlier ideas that drumlins and MSGLs are ‘longitudinal shear marks’ akin to those cut by extended frictional wear on fault planes, to which can now also be added analogous streamlined surfaces cut by wear debris mobilized at the base of landslides, ejecta blankets and pyroclastic flows, and on engineered man-made surfaces.

An erosional origin for drumlins of NW Poland

AbstractDrumlins are landforms essential to understanding of ice sheet movement over soft beds, sediment transport along the ice/bed interface, and the formation of a wide range of glacial deposits. Although investigated more than any other glacial landform, the origin of drumlins remains contentious. Using high‐resolution LiDAR imagery and field data, we investigate the geomorphology and internal composition of one of the biggest drumlin fields in the North European Lowland. The Stargard drumlin field consists of over 1300 drumlins and related streamlined subglacial bedforms in a terminal part of a major Weichselian palaeo‐ice stream of the southern Scandinavian Ice Sheet. The drumlins are typically 600‐800 m long, 200‐250 m wide, 3‐6 m high and have axial elongation ratios ~2 but in some cases exceeding 15. Several subzones inferred from drumlin morphometry exist reflecting different ice flow dynamics. The most elongated drumlins occur in areas where ice moved down‐slope and where thick fine‐grained deposits of low hydraulic conductivity occur in the substratum. The largest portion of land occupied by drumlins and the greatest frequency density of drumlins occur where the ice moved up‐slope. Stargard drumlins are composed of a wide variety of glacial deposits including various types of tills and meltwater sediments, which range from undisturbed to heavily deformed. There is no correlation between the deposits in the drumlins and the drumlin forms indicating that the deposits pre‐date the drumlinizing process. It is suggested that the drumlin field was generated by a combination of direct glacial erosion and subglacial meltwater erosion by removing antecedent material from the inter‐drumlin areas and streamlining the resultant bumps. Our data support the search for a unifying theory of drumlin formation and suggest erosion as the most plausible single mechanism generating drumlin landscapes. © 2019 John Wiley &amp; Sons, Ltd.

The philosopher in the kitchen: the role of mathematical modelling in explaining drumlin formation

Scientific progress does not take place in a linear fashion, but often occurs through sudden advances following years of stagnation, in the course of which quite heated battles between proponents of different viewpoints can occur. In this paper, we review several instances of such debates, and use them to throw light on a current philosophical tussle between proponents of different explanations for the formation of drumlins, and other subglacial bedforms. We conclude that the differences of opinion which emerge in the literature may be more an issue of semantics rather than a fundamental disagreement about process. In addition, we provide a critical review of some recent theories of drumlin formation, highlighting their merits and demerits, as well as defending them against some of the criticism they have received.

Production and preservation of the smallest drumlins

Few very small drumlins are typically mapped in previously glaciated landscapes, which might be an important signature of subglacial processes or an observational artefact. One hundred and forty-three newly emergent drumlins, recently sculpted by the Múlajökull glacier, have been mapped using high-resolution LiDAR and aerial photographs in addition to field surveying. In this paper, these are used as evidence that few small drumlins (e.g., height H 4 m, width W 40 m, length L 100 m) are produced; at least, few survive to pass outside the ice margin in this actively forming drumlin field. Specifically, the lack of a multitude of small features seen in other landforms (e.g., volcanoes) is argued not to be due to (i) Digital Elevation Model resolution or quality, (ii) mapper ability in complex (i.e., anthropogenically cluttered or vegetated) landscapes or (iii) post-glacial degradation at this site. So, whilst detection ability must still be at least acknowledged in drumlin mapping, and ideally corrected for in quantitative analyses, this observation can now be firmly taken as a constraint upon drumlin formation models (i.e., statistical, conceptual or numerical ice flow). Our preferred explanation for the scarcity of small drumlins, at least at sites similar to Múlajökull (i.e., ice lobes with near-margin drumlin genesis), is that they form stochastically during multiple surge cycles, evolving from wide and gentle pre-existing undulations by increasing rapidly in amplitude before significant streamlining occurs.

A New Model of Drumlin Formation

Observations from the surge-type glacier Múlajökull in Iceland underpin new modeling results that suggest the glacier’s drumlins grow during quiet intervals of normal flow between glacial surges.

A Theoretical Model of Drumlin Formation Based on Observations at Múlajökull, Iceland

AbstractThe drumlin field at the surge‐type glacier, Múlajökull, provides an unusual opportunity to build a model of drumlin formation based on field observations in a modern drumlin‐forming environment. These observations indicate that surges deposit till layers that drape the glacier forefield, conform to drumlin surfaces, and are deposited in shear. Observations also indicate that erosion helps create drumlin relief, effective stresses in subglacial till are highest between drumlins, and during quiescent flow, crevasses on the glacier surface overlie drumlins while subglacial channels occupy intervening swales. In the model, we consider gentle undulations on the bed bounded by subglacial channels at low water pressure. During quiescent flow, slip of temperate ice across these undulations and basal water flow toward bounding channels create an effective stress distribution that maximizes till entrainment in ice on the heads and flanks of drumlins. Crevasses amplify this effect but are not necessary for it. During surges, effective stresses are uniformly low, and the bed shears pervasively. Vigorous basal melting during surges releases debris from ice and deposits it on the bed, with deposition augmented by transport in the deforming bed. As surge cycles progress, drumlins migrate downglacier and grow at increasing rates, due to positive feedbacks that depend on drumlin height. Drumlin growth can be accompanied by either net aggradation or erosion of the bed, and drumlin heights and stratigraphy generally correspond with observations. This model highlights that drumlin growth can reflect instabilities other than those of bed shear instability models, which require heuristic till transport assumptions.

Using the size and position of drumlins to understand how they grow, interact and evolve

AbstractDrumlins are subglacial bedforms streamlined in the direction of ice flow. Common in deglaciated landscapes, they have been widely studied providing rich information on their internal geology, size, shape, and spacing. In contrast with bedform investigations elsewhere in geomorphology (aeolian and fluvial dunes and ripples for example) most drumlin studies derive observations from relict, and thus static features. This has made it difficult to gain information and insights about their evolution over time, which likely hampers our understanding of the process(es) of drumlin formation. Here we take a morphological approach, studying drumlin size and spacing metrics. Unlike previous studies which have focussed on databases derived from entire ice sheet beds, we adopt a space‐for‐time substitution approach using individual drumlin flow‐sets distributed in space as proxies for different development times/periods. Framed and assisted by insights from aeolian and fluvial geomorphology, we use our metric data to explore possible scenarios of drumlin growth, evolution and interaction. We study the metrics of the size and spacing of 36 222 drumlins, distributed amongst 71 flow‐sets, left behind by the former British‐Irish Ice Sheet, and ask whether behaviour common to other bedform phenomena can be derived through statistical analysis. Through characterizing and analysing the shape of the probability distribution functions of size and spacing metrics for each flow‐set we argue that drumlins grow, and potentially migrate, as they evolve leading to pattern coarsening. Furthermore, our findings add support to the notion that no upper limit to drumlin size exists, and to the idea that perpetual coarsening could occur if given sufficient time. We propose that the framework of process and patterning commonly applied to non‐glacial bedforms is potentially powerful for understanding drumlin formation and for deciphering glacial landscapes. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley &amp; Sons Ltd.

Numerical simulations of drumlin formation.

We summarize the present form of the instability theory for drumlin formation, which describes the coupled subglacial flow of ice, water and sediment. This model has evolved over the last 20 years, and is now at the point where it can predict instabilities corresponding to ribbed moraine, drumlins and mega-scale glacial lineations, but efforts to provide numerical solutions of the model have been limited. The present summary adds some slight nuances to previously published versions of the theory, notably concerning the constitutive description of the subglacial water film and its flow. A new numerical method is devised to solve the model, and we show that it can be solved for realistic values of most of the parameters, with the exception of that corresponding to the water film thickness. We show that evolved bedforms can be three-dimensional and of the correct sizes, and we explore to some extent the variation of the solutions with the model's parameters.

Origin of the active drumlin field at Múlajökull, Iceland: New insights from till shear and consolidation patterns

Abstract Stratigraphic and morphologic data previously collected from the forefield of Mulajokull, Iceland, suggest that its recent surge cycles are responsible for the formation of drumlins there and that their relief reflects both deposition on drumlins and erosion between them. We have tested these ideas and aspects of leading models of drumlin formation by studying past patterns of bed deformation and effective stress in basal tills of the glacier's forefield. Patterns of till strain indicated by the anisotropy of magnetic susceptibility (AMS) of ∼2300 intact till samples indicate that till was deposited during shear deformation, with shearing azimuths and planes that conform to the drumlin morphology. Thus, till deposition occurred as drumlins grew, in agreement with LiDAR data indicating that the degree of aggradation of the glacier forefield is largest in areas subjected to the most surges. Previously described unconformities on the drumlin flanks, however, indicate that drumlin relief at Mulajokull has resulted, in part, from erosion. Given that the last surge deposited a till layer both on and between drumlins, a reasonable hypothesis is that erosion between drumlins occurred during normal (quiescent) flow of the glacier between surges. Densities of till samples, analyzed in conjunction with laboratory consolidation tests, indicate that effective stresses on the bed during such periods were on the order of 100 kPa larger between drumlins than within them, an observation consistent with subglacial channels at low water pressure occupying interdrumlin areas. Transport of sediment by turbulent flow in these channels or high effective stress adjacent to them causing enhanced till entrainment in ice or increased depths of bed deformation would promote the sediment flux divergence necessary to erode areas between drumlins. The observation that effective stresses were higher between drumlins than within them is the opposite of that presumed in leading models of drumlin formation. Moreover, the lack of AMS-fabric evidence of longitudinal compression in drumlin tills does not support some models of drumlin formation that invoke negative till-flux gradients in a deforming bed.

Progressive formation of modern drumlins at Múlajökull, Iceland: stratigraphical and morphological evidence

The drumlin field at Múlajökull, Iceland, is considered to be an active field in that partly and fully ice‐covered drumlins are being shaped by the current glacier regime. We test the hypothesis that the drumlins form by a combination of erosion and deposition during successive surge cycles. We mapped and measured 143 drumlins and studied their stratigraphy in four exposures. All exposures reveal several till units where the youngest till commonly truncates older tills on the drumlin flanks and proximal slope. Drumlins inside a 1992 moraine are relatively long and narrow whereas drumlins outside the moraine are wider and shorter. A conceptual model suggests that radial crevasses create spatial heterogeneity in normal stress on the bed so that deposition is favoured beneath crevasses and erosion in adjacent areas. Consequently, the crevasse pattern of the glacier controls the location of proto‐drumlins. A feedback mechanism leads to continued crevassing and increased sedimentation at the location of the proto‐drumlins. The drumlin relief and elongation ratio increases as the glacier erodes the sides and drapes a new till over the landform through successive surges. Our observations of this only known active drumlin field may have implications for the formation and morphological evolution of Pleistocene drumlin fields with similar composition, and our model may be tested on modern drumlins that may become exposed upon future ice retreat.

Streamlined subglacial bedforms on the Närke plain, south-central Sweden – Areal distribution, morphometrics, internal architecture and formation

Abstract A flow set of close to 1000 drumlins has been mapped by means of LiDAR-derived digital elevation models and investigated by trenching. The area is situated on the SW part of the Narke plain and its surrounding uplands in south-central Sweden, which was deglaciated in the early Preboreal in a glacioaquatic setting. We find that there is considerable morphological difference in drumlin distribution patterns over crystalline basement areas compared to streamlined terrain over Palaeozoic sedimentary rock basement. The former area is characterized by thin Quaternary drift and the drumlins are all of the rock-cored type, built due to active deposition of sediment around obstacles to glacier flow. The latter area is characterized by deep Quaternary drift and the drumlins are more elongate and also larger in all dimensions, as compared to rock-cored drumlins. Irrespective of these geomorphological differences on local landscape scale we find that drumlin morphometric values remain part of a morphological continuum at the regional scale. Based on the internal sediment architecture as revealed in two cross-drumlin sections we find that the soft-cored drumlins were formed by compressional constructive deformation, along with excavational deformation along the flanks of the emerging drumlins, which shaped the separating troughs. Intermediate-type drumlins are those that demonstrate a coupling between underlying Palaeozoic sediment strata in areas of shallow drift sheet. These are the result of differing rheological response between incorporated sedimentary rock and a deforming bed below the ice-bed interface. An overall conclusion is that we find geomorphic and architectural compositional differences between the drumlins and the flowset they form. We can closely relate these differences to contextual geological variations with respect to basement type and drift depth. We argue that drumlin formation is better explained not by one single ‘unifying’ process but rather a set of processes (erosion/deformation/deposition) by which the subglacial system finds an efficiency equilibrium whereby an obstacle is shaped so that it enhances flow with a minimum of drag, i.e. the typical streamlined form is the result of a positive feedback cycle that tends towards efficiency. This approach argues that equifinality is playing a significant role in drumlin formation with a plurality of initial conditions and processes which, when combined with a uniformity of specific overriding processes and mechanical properties, results in a diversity of morphology and core type at the meso-scale and a continuum of landforms at the macro-scale.

Submarginal drumlin formation and late Holocene history of Fláajökull, southeast Iceland

ABSTRACTFláajökull is a non-surging outlet glacier draining the south-eastern part of the Vatnajökull, southeast Iceland. Fláajökull was stationary or advanced slightly between 1966 and 1995 and formed a prominent end moraine. Glacial retreat since then has revealed a cluster of 15 drumlins. This study focuses on the morphology and sedimentology of the drumlins. They are 100–600 m long, 40–130 m wide, and have cores of glaciofluvial sediment or till. The drumlins are draped by ~1 m thick, massive subglacial traction till. The glacier forefield is characterized by a number of arcuate and saw-tooth, terminal and recessional moraine ridges, overridden moraines with fluted surfaces, and glaciofluvial outwash. Some of the drumlins extend towards the 1995 end moraine but terminate abruptly at the moraine and are not observed in front of it. This suggests that they were formed sub-marginally during the 1966–1995 terminal position. The sedimentary structure of the drumlins is best explained by the sticky spot model. Dating and dendrochronological analyses of birch logs found on the surface of one of the drumlins indicate that the valley was forested about 2100 calendar year BP, after which the glacier started to reform, possibly due to an abrupt change in climate.

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.

Subglacial processes from drumlins in Clew Bay, western Ireland

AbstractInterpretation of subglacial processes and environments can be usefully informed by the stratigraphy and structures of sediments preserved within drumlins, and the down‐ice variability of these sediments. Drumlins in Clew Bay, western Ireland, were formed by westward late Pleistocene ice flow onto the Atlantic shelf, but the depositional processes and environments of these sediments remain uncertain. This study describes and interprets the drumlin stratigraphy and sediment properties and structures from two drumlins on the south side of Clew Bay. Drumlin sediments are dominated by massive to stratified diamicton deposited subglacially by deformation of flows of varying rheology. Folds, shears and clastic dikes within the sediments suggest active subglacial water flow related to variations in ice flow and ice‐bed coupling. The distal ends of the drumlins are dominated by stratified diamicton and gravel units with sandy interbeds. These sediments reflect deposition in a leeside subglacial cavity formed coevally with bedform development. A model is proposed that can account for sediment stratigraphic patterns and drumlin formation. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Quantifying Eroded Sediment Volume during Drumlin Formation in Simcoe County

Drumlins, despite being well studied, are still incredibly enigmatic in terms of their formation processes. Theories on their origin vary widely, and the evidence presented in this study seeks to assist in this debate. To support an erosional theory for the formation of drumlins, large volumes of sediment would have undergone transport and deposition. The volume of sediment that would have eroded from part of the Peterborough Drumlin Field in Simcoe County was calculated using two methods in a GIS. The limitations of both methods and the geological composition of potential sediment sinks were analysed. The range of eroded sediment volume was calculated to be 3.5–8.3 km3, which could fill between 30.2% and 71.6% of the Lake Simcoe basin. The high volume of sediment that was estimated in this study will have future implications in the drumlin debate and for the feasibility of an erosional process of formation.

Drumlinised glaciofluvial and glaciolacustrine sediments on the Småland peneplain, South Sweden – new information on the growth and decay history of the Fennoscandian Ice Sheets during MIS 3

Dug trenches in sediment-cored drumlins on the Småland peneplain, southern Sweden, occurring intermixed with rock-cored drumlins, revealed sedimentary successions and facies suggesting that the core sediments were primarily deposited proglacially, both in glaciolacustrine settings as delta-front and more distal deeper-water sediments, and as glaciofluvial braidplain sediments. From the relationship between the drumlin sorted-sediment cores and their surficial drape by a till carapace, it is concluded that erosion was the main drumlin-forming process here, reshaping the original sediment morphology into streamlined landforms, after which a more or less thick traction till was deposited over the eroded and/or deformed ice–bed interface. Drumlin formation probably took place during the last deglaciation over the area. The drumlin-core sediments in 14 excavated trenches were dated by Optically Stimulated Luminescence (OSL); most retrieved ages fall into two distinct groups, both within Marine Isotope Stage 3 (c. 60–30 ka). When plotted as mean OSL ages for each site, all but two sites fall either into an older age group, c. 48–56 ka (seven sites), or into a younger age group, c. 28–38 ka (five sites). These age intervals correlate well with the suggested time spans for the Ristinge and Klintholm ice advances in Denmark by ice flowing from the Baltic Basin westwards, terminating in fan-shaped patterns. These interstadial ice advances in the southwestern sector of the Fennoscandian Ice Sheet give rise to enigmatic questions on the physics and mechanics of ice sheet build-up and decay, especially as palaeoenvironmental data for northern Fennoscandia suggest widespread ice-free conditions within roughly the same time fame as the recorded ice advances in the south.

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.

The 1992 Farrington Lecture: Drumlin Bedforms and Related Ice-Marginal Depositional Systems in Ireland

The Irish drumlin bell comprises a wide morphological range of subglacial bedforms which are best developed in lowland zones located mainly in northern and western sectors of the island. Stratigraphic evidence suggests that drumlinisation occurred around 17ka B.P. in response to fast ice flow from major centres of inland ice dispersion onto the continental shelf. Well-defined moraines and morainal banks bordering the drumlin fields indicate that drumlinisation was accompanied by a high basal debris flux to ice sheet margins which was resedimented in a variety of subaquaeous settings. Field observations indicate that the fast ice flow responsible for streamlining and sediment moulding was closely associated with intense marine downdraw, a decrease in basal effective pressures and a general lowering of ice sheet profiles during deglaciation. Deglacial mechanisms of this type help to explain the very rapid termination of the last glacial cycle because downdraw ice-stream drainage basins must expand in area if pulling power lowers the surface faster than ice supply raises it. Sedimentary and structural variability within drumlins indicate that drumlin formation reflects not only a highly dynamic subglacial system but, in some cases, can be related to ice sheet history. The seven main facies associations which have been identified provide a window into a variety of depositional settings which contributed to drumlin formation and drumlinisation. They confirm that drumlins are not simple landforms but have a complex history related to changing glaciological states and ice sheet evolution. The final drumlin forms however are the result of a highly mobile flux of basal debris and meltwater directed subglacially towards ice sheet margins.