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Abstract: Drumlins 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 & Sons Ltd. [ABSTRACT FROM AUTHOR]

ABSTRACT Using the LiDAR-derived Swedish national height model we have identified previously undescribed shallow streamlined glacial bedforms, small-scale drumlins, on the Närke plain in south-central Sweden. These drumlins could only be detected with high-resolution LiDAR, due to both their subtle size and forest cover. In this area the ice margin receded in a subaqueous environment with a proglacial water depth in the order of 100 m during the last deglaciation. As indicated by the configuration of marginally formed De Geer moraine ridges draping the drumlinoids, the receding ice margin formed deeply indented calving bays. These were located around subaqueous outlets of the subglacial melt-water drainage, with their apex position marked geomorphologically by beaded esker ridges. The mapped small-scale drumlins are aligned perpendicular to the reconstructed ice sheet margin and suggest formation along flow lines adjusted to the configuration of these calving embayments as they propagated up-flow with ice margin retreat. Based on these geometric relationships we argue that the emplacement of the drumlins was near-marginal, ∼7.7-1 km from the margin, on a short timescale (∼5-35 years). [ABSTRACT FROM AUTHOR]

This paper generates a new data set of streamlined glacial bedforms in southern Sweden, which is used both to test conclusions from previous work on streamlined bedform morphometrics and to advance our knowledge of streamlined bedforms on the Scandinavian Shield. The data set consists of streamlined glacial bedforms in southeast Sweden mapped from the new LiDAR-derived Swedish National Height Model, which has a pixel resolution of 2.0 m with a vertical resolution of 1 cm. We have mapped 10,311 features; of the mapped features, 135 are known to have an unconsolidated sediment core, 2120 a bedrock component, and 8055 whose core composition is unknown. The extracted morphological variables are then subjected to a univariate and bivariate analysis. We find that the extracted characteristics broadly fit into the lower end of the modal and median value spectrum of similar bedforms from around the world. The distribution of the variables is found to be log-normal to a first-order approximation. The covariant relationships between height and length, width and area are examined after the variables have been log-transformed and are found to be significant, if not particularly strong. Rock-cored features are found to have a longer modal length than soft-cored features, which suggests that reconstructions of past flow velocities from streamlined landforms need to closely consider core-type. Additionally, we find no support for a derived scaling law for streamlined features by plotting length, width, and elongation ratio against one another. [ABSTRACT FROM AUTHOR]

Ground-penetrating radar (GPR) is becoming a commonly applied technique in geomorphology. However, its use in the study of subglacial bedforms has yet to be fully explored and exploited. This paper presents the results of a GPR feasibility study conducted on a drumlinized terrain in Cumbria, UK, where five drumlins were investigated using multiple radar antenna frequencies. The site was selected for the presence of nearby bedrock outcrops, suggesting a shallow drumlinized diamict-bedrock contact and a permeable lithology. Despite the clayey sediment and unfavourable weather conditions, a considerable penetration depth of ~12 m was achieved when using a 50 MHz antenna, with a separation of 1 m, trace spacing of 1 m and 128-fold vertical stack. Results indicate that the drumlinized diamict is in direct erosional contact with the bedrock. While the internal drumlin geometry is generally chaotic on the stoss side, evidence of layering dipping downflow at an angle greater than the drumlin surface profile was found on the lee side. The inter-drumlin areas comprise ~4 m of infill sediment that masks part of the original drumlin profile. Overall, this study indicates that GPR can be deployed successfully in the study of glacial bedform sedimentary architecture. [ABSTRACT FROM AUTHOR]

Abstract: Drumlin relief is a key parameter for testing predictions of models of drumlin formation. Although this metric is commonly described in textbooks as being of the order of a few tens of metres, our critical review of the literature suggests an average value of about 13m, but with much uncertainty. Here we investigate a large sample of drumlins (25,848) mapped from a high resolution digital terrain model of Britain, which allowed the identification of extremely shallow drumlins. Results indicate that most drumlins have a relief between 0.5 and 40m (with a surprisingly low average value of only 7.1m) a mode of 3.5–4m, and with 41% of all drumlins characterized by a relief <5m. Drumlin relief is found to never exceed 7% of the width and is positively correlated with this parameter, possibly indicating that drumlins need a large base to stand against the flow of the ice. Drumlin relief is also positively correlated with the length, which shows that drumlins do not grow in length by redistributing sediments from their summits to their downflow (lee) end, as previously hypothesised. [Copyright &y& Elsevier]

Abstract: Investigation of drumlins is significant to both glaciology and palaeoglaciology but the sheer diversity of their composition and internal structure is often cited as a major obstacle towards a satisfactory (unifying?) explanation of their formation. This paper presents the first systematic survey of the vast literature on this subject, with the aim of concisely summarising observations and identifying any emergent patterns or commonality that theories of drumlin formation should be able to explain. Results confirm that investigations are often limited by availability of suitable sediment exposures (40% of studies report data from <5 drumlins and 44% do not specify sample size), although borehole data and geophysical techniques can alleviate this problem. However, it is clear that the constituents of drumlins are incredibly diverse in terms of their composition (e.g. a range of lithologies, clast shapes, sizes and fabrics); structure (e.g. sediments that are sorted, homogeneous, surface conformable, unconformable); and evidence of deformation (e.g. ranging from pervasive, to non-pervasive/limited, to absent). Despite this diversity, our review leads us to suggest that drumlin composition can be simplified to five basic types: (i), mainly bedrock, (ii), part bedrock/part till; (iii), mainly till; (iv), part till/part sorted sediments; and (v), mainly sorted sediments. This is a potentially significant step, in that it reduces the oft-cited complexity of drumlin composition and provides a more realistic goal for theories or numerical models of drumlin formation to target. The various types can occur within the same drumlin field, which leaves us with two possible implications for drumlin formation. (1) Different types of drumlin are formed/seeded by different processes, despite being morphologically similar (equifinality?) — investigation of drumlin composition may, therefore, reveal diagnostic processes/explanations for these different types of drumlin and we argue that bedrock ‘drumlins’ are an example. (2) A single process occurs across large parts of the ice-bed interface to create drumlinised terrain in a variety of sediments — investigation of drumlin composition may, in this case, simply reflect pre-existing sediments but, importantly, the way in which the drumlin-forming mechanism modifies/is modified by them. We argue that the latter, simpler, explanation applies to all other types of drumlin (excluding purely bedrock forms) and conclude that the diversity in drumlin composition is no obstacle to a single unifying theory. [Copyright &y& Elsevier]

The 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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

Abstract: Despite their importance in understanding glaciological processes and constraining large-scale flow patterns in palaeo-glaciology, there is little consensus as to how drumlins are formed. Attempts to solve the ‘drumlin problem’ often fail to address how they are created from an initially flat surface in the absence of obvious cores or obstacles. This is a key strength of the instability theory, which has been described in a suite of physically-based mathematical models and proposes that the coupled flow of ice and till causes spontaneous formation of relief in the till surface. Encouragingly, model predictions of bedform height and length are consistent with observations and, furthermore, the theory has been applied to a range of subglacial bedforms and not just drumlins. However, it has yet to confront the myriad observations relating to the composition and internal structure of drumlins and this could be seen as a major deficiency. This paper is a first attempt to assess whether the instability theory is compatible with the incredible diversity of sediments and structures found within drumlins. We summarise the underlying principles of the theory and then describe and attempt to explain the main types of drumlin composition (e.g. bedrock, till, glaciofluvial sediments, and combinations thereof). Contrary to a view which suggests that the presence of some sedimentary sequences (e.g. horizontally stratified cores) is inconsistent with the theory, we suggest that one would actually expect a diverse range of constituents depending on the inheritance of sediments that pre-date drumlin formation, the duration and variability of ice flow, and the balance between erosion and deposition (till continuity) at the ice–bed interface. We conclude that the instability theory is compatible with (and potentially strengthened by) what is known about drumlin composition and, as such, offers the most complete and promising solution to the drumlin problem to date. [Copyright &y& Elsevier]

Abstract: Ice sheets flowing across a sedimentary bed usually produce a landscape of blister-like landforms streamlined in the direction of the ice flow and with each bump of the order of 102 to 103 m in length and 101 m in relief. Such landforms, known as drumlins, have mystified investigators for over a hundred years. A satisfactory explanation for their formation, and thus an appreciation of their glaciological significance, has remained elusive. A recent advance has been in numerical modelling of the land-forming process. In anticipation of future modelling endeavours, this paper is motivated by the requirement for robust data on drumlin size and shape for model testing. From a systematic programme of drumlin mapping from digital elevation models and satellite images of Britain and Ireland, we used a geographic information system to compile a range of statistics on length L, width W, and elongation ratio E (where E = L/W) for a large sample. Mean L, is found to be 629m (n =58,983), mean W is 209m and mean E is 2.9 (n =37,043). Most drumlins are between 250 and 1000 metres in length; between 120 and 300 metres in width; and between 1.7 and 4.1 times as long as they are wide. Analysis of such data and plots of drumlin width against length reveals some new insights. All frequency distributions are unimodal from which we infer that the geomorphological label of ‘drumlin’ is fair in that this is a true single population of landforms, rather than an amalgam of different landform types. Drumlin size shows a clear minimum bound of around 100m (horizontal). Maybe drumlins are generated at many scales and this is the minimum, or this value may be an indication of the fundamental scale of bump generation (‘proto-drumlins’) prior to them growing and elongating. A relationship between drumlin width and length is found (with r 2 =0.48) and that is approximately W=7 L 1/2 when measured in metres. A surprising and sharply-defined line bounds the data cloud plotted in E–W–L space, and records a scale-dependent maximum elongation limit (approximated by E max = L 1/3, when L measured in metres). For a given length, for some reason as yet unknown, drumlins do not exceed the elongation ratio defined by this scaling law. We also report and compare our statistics to an amalgamated sample (25,907 drumlins) of measures derived from around 50 published investigations. Any theory must be able to explain the drumlin statistics and fundamental scaling properties reported herein and they thus provide powerful tests for drumlin modelling. [Copyright &y& Elsevier]