Drumlins

Investigations have been made of the geomorphology, internal structure, and till fabric of small drumlins in a drumlin field exposed in front of the retreating northern part of Mýrdalsjökull, Iceland. The drumlins either comprise irregular drumlin complexes or they show clearly the shape of typical drumlins with their highest points at the up-glacier ends of streamlined hills. The core of each drumlin consists either of undisturbed glacio-fluvial deposits or glacio-dynamically deformed drift. The distribution of the first type often has a characteristic en échelon arrangement, similar to the interstream areas in the upper reaches of outwash fans. The second type forms a pattern with a predominant trend conforming to the glacier margin; this type is superimposed on overridden ice-margin push-moraine ridges. It is shown that the material in the drumlin cores is only slightly eroded by the glacier that formed the drumlins. The core is mantled by subglacial lodgement till about 0.1–1.5 ...

An exposure in a Late Pleistocene drumlin near Kingscourt, Ireland, provides a good insight into some of the processes that give rise to such subglacial bedforms. The drumlin is located on a ridge, cored by red sandstone of Carboniferous (Namurian) age, which rises to 150 m above m.s.1. The drumlin itself is about 380 m long by 170 m wide and is 20 m in height. It is orientated WNW-ESE. Ice flow direction in the area, as inferred from general drumlin orientation, striae, and erratic dispersal, was NW-SE.

Satellite imagery of Donegal Bay, northwestern Ireland, reveals two streamlined subglacial bedform sets produced during the late Devensian glaciation (ca. 22,000–15,000 yrs B.P.). The first bedform set trends northeast-southwest and records fast ice flow from inland ice domes onto the eastern Atlantic continental shelf. The second bedform set, trending east-west, crosscuts and partially reorientates the first. Morphological and sedimentary evidence show that the second set corresponds to the last phase of fast ice flow (drumlinization) in northwestern Ireland.

Morphological characteristics of the Donegal Bay drumlin field, and sedimentary characteristics of the Mullinasole drumlin, Donegal Bay, support a two-stage interpretation of drumlin evolution. These are the following. (1) Deposition of glaciomarine mud and diamict facies at a tidewater glacier margin. Stratified diamicts record debris flow events and sediment reworking. This facies sequence is erosionally truncated. (2) Deposition of subglacial diamict and sand facies recording ice readvance and drumlinization. Drumlinization (sediment streamlining) reflects ice mass-balance destabilization, episodic: fast ice flow and ice-marginal oscillation, and may be correlated with millennial-time scale climate changes in the circum-North Atlantic.

Sediment-cored drumlins are uncommon across the glacially-eroded granite bedrock of Connemara (west County Galway, western Ireland). This paper describes the internal sediments from two significant drumlin exposures at Ardmore and Ballyconneely. Both drumlins show a lowermost lithofacies association comprising overconsolidated subglacial diamicton with clast lithology, strong fabrics and overfolds indicative of regional east/southeast to west/northwest late Weichselian ice flow. This lithofacies association is overlain at both sites by west-dipping, stratified, graded gravel and diamicton beds that downlap onto laminated silts. These beds reflect episodic debris flows into a leeside cavity developed following creation of relief by deposition and deformation of a subglacial diamicton nucleus located up-ice. Variations in substrate hydrological processes are identified in both drumlins based on the presence of clastic dikes, flame structures and debris flow-deposited gravels which are present at different locations spatially within the drumlin and at different stratigraphic levels. These features reflect the close connection between subglacial hydrological and sedimentary processes and their changes over time as the drumlin form develops.

Examination of microstructures within diamictons from southern Ontario, Canada, and southern Bavaria, and Baden-WiJrttemburg, Germany reveal evidence illustrative of certain subglacial bed conditions. The microstructures from Ontario were obtained from a complex glacial m61ange unit that forms the internal structure of an isolated drumlin/megaflute on the north shore of Lake Erie. The evidence from Germany was taken from individual drumlins within drumlin fields in the Inn-Chiemsee, lller, Rhein and Salzach glacier areas. The structures within the diamicton from Ontario reveal clear evidence of massive shear associated with high externally applied strain. There are kink-band arrays, high angle shear zones, crenulation and sedimentary "dyke" and water escapes structures. In the German diamictons there is a lack of evidence of high levels of bulk deformation. There is, however, a very high carbonate content. Some evidence points to the possibility that these diamictons may contain intraclasts of paleosols. Interpretation of both sets of diamictons suggests that the Ontario sediments were subject to conditions at the bed of an ice mass best typified as a deformable bed. The German sediments may be interpreted as: (1) being so saturated that any deformation must have been by inter-grain slip (pervasive deformation) and thus no structures are recorded; or (2) being already cemented by the carbonates and sufficiently rigid to cause strain to be taken up elsewhere; or (3) were frozen prior to transposition and were again too rigid to suffer deformation; or (4) suffered no deformation whatsoever. The analyses of these two sets of sediments have important implications for subglacial conditions before and during drumlin and other subglacial bedform development.

Morphological and morphometric analyses of diamict-dominated and rock-cored drumlins in the Omagh Basin, north central Ireland, identify five drumlin types (classical, shield, barchanoid, fused and superimposed) on the basis of outline morphology and topographic setting. Diamict drumlins are mainly of classical or fused type and are found in lowland and basinal locations. Rock-cored drumlins are more morphologically diverse and occur on high-level plateaus and upland flanks. Morphometric analysis reveals both spatial and statistically significant absolute differences between the elongation ratios and orientations of diamict and rock-cored drumlins. Equifinality of the drumlin form despite variable thicknesses of diamict (from a few metres to>30 m) suggests that substrate streamlining took place in several phases, including eroding bedrock into rock-cored drumlins prior to diamict deposition. This means drumlinization cannot be attributed to a single set of subglacial processes or a single time-interval.

Fláajökull 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.

In western Ireland, within a drumlin formed inside the margin of the late Pleistocene ice sheet, a range of different styles of ductile and brittle sediment deformation is observed within subglacial diamicton (till) and associated subglacial waterlain sediments. At Roonah Point, County Mayo, the structures associated with these different deformation styles include gravel clusters that have loaded subjacent sediments, glacitectonic shears, water escape structures, and clastic dikes. These structures are found for tens of metres laterally within the same diamicton units and the structures may cross-cut or be superimposed upon one another. Based on these properties and geometric relationships, the relative chronology and processes and patterns of subglacial deformation can be reconstructed, including an evaluation of subglacial conditions and pore water pressure regimes. This illustrates the relationship between glacitectonic forcing by ice–bed coupling, and the structural response within subglacial sediments to changing pore water pressure gradients.

Subglacial landforms across various scales preserve the history of movement, deposition and erosion by the last great ice sheets and their meltwater. The origin of many of these landforms is, however, contentious. In this chapter these forms are described both individually and as suites that make up entire landscapes. Their interpretations are discussed with reference to the megaflood hypothesis. A description is provided of individual forms via their size, shape, landform associations, sedimentology and the relationship between landform surfaces and internal sediments. The possible origins of each are then discussed. To simplify the chapter the landforms are categorised by their size (micro, meso, macro and mega), although, importantly, it should be noted that several landforms show similarities across scales. Also discussed is the relevant subglacial hydrology associated with the described forms, especially the volume and discharge rates of megaflood flows, and where water may have been stored prior to the megaflood events.

Múlajökull and Þjórsárjökull are surge-type outlet glaciers of the Hofsjökull ice cap, central Iceland. History of Múlajökull surges are known from 1924, four surges occured since then within interval of 10-20 years. The forefield of Múlajökull comprises the active drumlin field of more than 110 drumlins and therefore is an excellent area for studies of glacial geomorphology, subglacial topography and ice structures. This work describes preliminary results obtained during the expedition to Múlajökull and Þjórsárjökull glaciers in August, 2014.

Subglacial processes beneath the Late Weichselian ice sheet in northwest Ireland are deduced from sediments and structures within drumlins in a bedrock valley at Loughros Beg, County Donegal. Here, a glacially smoothed bedrock surface underlies the drumlins, which are composed on their up-ice side of stacked, angular rafts of local bedrock. Overlying and down-ice from these rafts are down-ice-dipping beds of massive to bedded diamicton that contain sand and gravel interbeds. In a down-ice direction the diamicton matrix coarsens and the beds become laterally transitional to water-sorted gravels. The down-ice end of one drumlin shows a concentrically bedded stratified gravel core aligned parallel to ice flow and resembling the internal structure of an esker. With distance away from this core, the gravels become more poorly sorted with an increase in matrix content, and are transitional to massive to stratified diamicton. A four-stage model describes the formation of drumlins in this sediment-poor setting. The sediments that are located directly above the bedrock represent deposition in a semi-enclosed subglacial cavity. A trigger for this process was the formation of subglacial relief by the thrusting up of bedrock rafts, which created the leeside cavity. Subsequent sediment deposition into this cavity represents a form of feedback (self-regulation), which may be a typical characteristic of subglacial processes in sediment-poor settings.

In reviewing hypotheses of subglacial bed formation it becomes apparent that a close relationship must exist between the development of these bedforms and subglacial hydraulic conditions. A speculative classification of subglacial temperate beds is presented. Subglacial beds are subdivided into "H", "Q", and "M" beds. "H" beds are typically hard beds in which Upper interface meltwater activity dominates. "'M" beds are those in which soft or mobile subglacial debris conditions prevail and in which advective pervasive flow of saturated debris occurs. In "Q" beds both of the other bed types occur but are of a spatially and temporally transitory nature. Thus this latter bed type may be the most common beneath ice masses. It is under an "M"-bed state that many drumlin subglacial bedforms and associated forms are thought to develop. Discussion on the processes involved in mobile debris entrainment and the effect of changing rheological parameters are developed. It is argued that such bedform initiation occurs due to anisotropic rheological variations within the mobile debris setting up a 'feedback effect". Such conditions are thought to be found most likely at the Lower interface of a fast moving ice mass possibly typical of an ice stream. In using empirical analyses of mobile debris conditions a limited set of relationships between subglacial meltwater flux rates, basal ice velocity, debris porosity and bed width are established with reference to mobile debris thicknesses. Using this relationship, a set of possible parameter values that would allow bed formation to be achieved are presented. The impact of bed formation under these conditions upon possible sedimentological facies and deformation structures is postulated. The wider effect of "M"-bed conditions on subglacial sedimentology is introduced and the need to investigate subglacial hydraulic and bed conditions under past glaciations from the evidence of their glacial sediments and landforms emphasised.

Middle and Upper Pleistocene till beds with stratified glaciofluvial and glaciolacustrine deposits constitute the structure of the drumlins in the Saadjärve Drumlin Field. The only interglacial deposits of the area belong to the Karuküla Formation (Holsteinian) at the distal end of the drumlin field. The differences in grain-size distribution, mineralogical and chemical composition are regarded as confirmation of the crucial role of compositional characteristics in determining the stratigraphy of these tills. Analysis shows that the Upper Sangaste (Elsterian) till, the oldest in the area, as well as the Valgjärve (Lower Weichselian) and Võrtsjärve (Upper Weichselian) tills have distinct compositional characteristics. The Lower and Upper Ugandi (Saale glaciation) tills have no lithological differences; thus there may be only one Saalian till in the area. The stratigraphy and distribution of tills in the Saadjärve Drumlin Field proves an accretionary origin of these drumlins. They started to form during Saale glaciation and the formation process lasted for several glacial advances.

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...

An exposure in a Late Pleistocene drumlin near Kingscourt, Ireland, provides a good insight into some of the processes that give rise to such subglacial bedforms. The drumlin is located on a ridge, cored by red sandstone of Carboniferous (Namurian) age, which rises to 150 m above m.s.1. The drumlin itself is about 380 m long by 170 m wide and is 20 m in height. It is orientated WNW-ESE. Ice flow direction in the area, as inferred from general drumlin orientation, striae, and erratic dispersal, was NW-SE. The drumlin is composed of diamicton (with four constituent facies), containing large-scale (up to 20 m long and 1.5 m high) slabs of the sandstone bedrock, which have been displaced tens of metres by ice dragging. The thin diamicton matrix is sand-rich with few clasts greater than pebble size. It contains green sandstone erratics from west and northwest of the study site along with clasts of weathered Namurian sandstone which have been sheared from local bedrock. The diamicton attains a maximum thickness of 5.4 m. The slabs are confined to the basal 2.5 m of the diamicton. Thus the drumlin is essentially rock cored. Parts of the matrix are interpreted as injection sediments that have been squeezed into fractures and voids in the bedrock, and between rock slabs, under high porewater pressures, when the ice began to displace fractured substrate. Deformation structures at a variety of scales are seen in the matrix. Deformation, erosion and deposition were all important in the formation of the drumlin. Much of the upper part of the diamicton has been sheared, subsequent to initial deposition, by ice action. The sheared units are uppermost in the stratigraphic sequence. The sheafing is the last glacial process apparent in the drumlin sediment and may have been contemporaneous with drumlinisation. Both the squeezing process, which deposited the iajections within the matrix, and the deflection in flow of ice were influenced by the obstructing bedrock ridge which, ia this case, is also responsible for the anomalous orientation of the feature.

In reviewing hypotheses of subglacial bed formation it becomes apparent that a close relationship must exist between the development of these bedforms and subglacial hydraulic conditions. A speculative classification of subglacial temperate beds is presented. Subglacial beds are subdivided into "H", "Q", and "M" beds. "H" beds are typically hard beds in which Upper interface meltwater activity dominates. "'M" beds are those in which soft or mobile subglacial debris conditions prevail and in which advective pervasive flow of saturated debris occurs. In "Q" beds both of the other bed types occur but are of a spatially and temporally transitory nature. Thus this latter bed type may be the most common beneath ice masses. It is under an "M"-bed state that many drumlin subglacial bedforms and associated forms are thought to develop. Discussion on the processes involved in mobile debris entrainment and the effect of changing rheological parameters are developed. It is argued that such bedform initiation occurs due to anisotropic rheological variations within the mobile debris setting up a 'feedback effect". Such conditions are thought to be found most likely at the Lower interface of a fast moving ice mass possibly typical of an ice stream. In using empirical analyses of mobile debris conditions a limited set of relationships between subglacial meltwater flux rates, basal ice velocity, debris porosity and bed width are established with reference to mobile debris thicknesses. Using this relationship, a set of possible parameter values that would allow bed formation to be achieved are presented. The impact of bed formation under these conditions upon possible sedimentological facies and deformation structures is postulated. The wider effect of "M"-bed conditions on subglacial sedimentology is introduced and the need to investigate subglacial hydraulic and bed conditions under past glaciations from the evidence of their glacial sediments and landforms emphasised.

In this study results from traditional field mapping were merged with precise elevation information from airborne LiDAR (Light detection and ranging) surveys. Morphological and sedimentological data provide new results from the Austrian (eastern) part of the Salzach piedmont glacier during times of and shortly after the Last Glacial Maximum (LGM). The variations in meltwater discharge had a major impact on the development of glacial landforms. In areas with high meltwater supply erosional or debris reworking processes play a major role, represented by drainage channels, drumlins and kettled, low relief hummocky moraine with low slope angles. Low discharge areas are associated with distinct depositional forms such as high relief end moraines (up to 30 m) and hummocky moraine (averaging 20 m) with high slope angles. Isolated conical kames may reach heights up to 45 m. Fluvial activity is supposed to rise towards the end of the glacial cycle causing high melting rates and comprehensive debris reworking. The formation of terminal lakes and associated widespread, inorganic lake clays are the last deposits within the study area before the Salzach Glacier completely receded to its main valley. The survey of glacial landforms through the combination of field mapping and high-resolution DEM derived from airborne LiDAR missions gives precise information on transport and deposition during the last glacial cycle of the eastern Salzach Glacier piedmont lobe.

We study the relationship between deformation and sedimentation of the unity one from Turu (1992) and we have concluded that this unity is an ancient drumlin, parcialy eroded, from the final part of the last glacial cycle in the Pyrenees.
Se analiza las relaciones entre la sedimentación y la deformación de los materiales de la unidad 1 de Turu (1992). A partir de estas relaciones y su posición respecto al eje del valle de la Valira del Nord, se llega a la conclusión que se esta delante de un drumlin perteneciente a finales del último ciclo glaciar.

Patterns of glacial bedforms and their internal sediments are described from the Omagh Basin, north-central Ireland. Here, drumlins and ribbed moraines were variously formed, preserved and remoulded during several late Devensian ice flow stages, and their patterns can be used as input into glaciological inversion models. More detailed examination of the morphology and internal sediments of these bedforms shows a complex pattern of reworking driven by local-scale basal thermal and hydrological regime. Sediment lithofacies associations and patterns of erratic carriage can be used to better evaluate ice–bed conditions and basal processes than bedforms alone. The sedimentary evidence suggests that bedforms were unlikely to have been formed during a single glacial episode, as is implicit in glaciological inversion models, and more likely formed during several episodes that cannot be resolved by crosscutting or superimposed bedform relationships alone.