Form Of Moraines Research Articles

Реконструкция масштабов и морфологических особенностей плейстоценовых оледенений на Северо-Востоке России

Based on the currently available data, it has been established that the earliest glaciations occurred in the Late Eopleistocene and at the end of the Early Neopleistocene. They had local distribution in the most uplifted parts of the Chersky Range. Their traces are presented in the form of moraines enclosed in sediments of intermontain depressions. Glacial forms of this age have not been identified in the relief of mountains and plains in the North-East of Russia. The Middle Neopleistocene glaciation was the largest, both in area and length of individual glaciers. It was mountain-valley and reticulated; in some areas, mountain-cover. Its time coincides with the stage of activation of the ascending neotectonic movements in the Middle Neopleistocene. Traces of the last two Late Neopleistocene glaciations are well expressed in the relief in widespread exaration, moraine, and fluvioglacial complexes. The first Late Neopleistocene glaciation (Zyryan) was significantly inferior to the one occurred in the Middle Neopleistocene, in all parameters, but it also left large amphitheaters of terminal moraines in the foothill belts. Glacial complexes of the last Late Neopleistocene glaciation are widespread in areas 2-3 times smaller than the previous one and, as a rule, are located within its contours.

Timing and pace of ice‐sheet withdrawal across the marine–terrestrial transition west of Ireland during the last glaciation

ABSTRACTUnderstanding the pace and drivers of marine‐based ice‐sheet retreat relies upon the integration of numerical ice‐sheet models with observations from contemporary polar ice sheets and well‐constrained palaeo‐glaciological reconstructions. This paper provides a reconstruction of the retreat of the last British–Irish Ice Sheet (BIIS) from the Atlantic shelf west of Ireland during and following the Last Glacial Maximum (LGM). It uses marine‐geophysical data and sediment cores dated by radiocarbon, combined with terrestrial cosmogenic nuclide and optically stimulated luminescence dating of onshore ice‐marginal landforms, to reconstruct the timing and rate of ice‐sheet retreat from the continental shelf and across the adjoining coastline of Ireland, thus including the switch from a marine‐ to a terrestrially‐based ice‐sheet margin. Seafloor bathymetric data in the form of moraines and grounding‐zone wedges on the continental shelf record an extensive ice sheet west of Ireland during the LGM which advanced to the outer shelf. This interpretation is supported by the presence of dated subglacial tills and overridden glacimarine sediments from across the Porcupine Bank, a westwards extension of the Irish continental shelf. The ice sheet was grounded on the outer shelf at ~26.8 ka cal bp with initial retreat underway by 25.9 ka cal bp. Retreat was not a continuous process but was punctuated by marginal oscillations until ~24.3 ka cal bp. The ice sheet thereafter retreated to the mid‐shelf where it formed a large grounding‐zone complex at ~23.7 ka cal bp. This retreat occurred in a glacimarine environment. The Aran Islands on the inner continental shelf were ice‐free by ~19.5 ka bp and the ice sheet had become largely terrestrially based by 17.3 ka bp. This suggests that the Aran Islands acted to stabilize and slow overall ice‐sheet retreat once the BIIS margin had reached the inner shelf. Our results constrain the timing of initial retreat of the BIIS from the outer shelf west of Ireland to the period of minimum global eustatic sea level. Initial retreat was driven, at least in part, by glacio‐isostatically induced, high relative sea level. Net rates of ice‐sheet retreat across the shelf were slow (62–19 m a−1) and reduced (8 m a−1) as the ice sheet vacated the inner shelf and moved onshore. A picture therefore emerges of an extensive BIIS on the Atlantic shelf west of Ireland, in which early, oscillatory retreat was followed by slow episodic retreat which decelerated further as the ice margin became terrestrially based. More broadly, this demonstrates the importance of localized controls, in particular bed topography, on modulating the retreat of marine‐based sectors of ice sheets.

Structural and depositional evidence for repeated ice-marginal oscillation along the eastern margin of the Late Devensian Irish Sea Ice Stream

Late Devensian glacigenic sediments and landforms along the north-west coast of Wales document the advance and subsequent retreat of the eastern margin of an Irish Sea Ice Stream that met, coalesced and ultimately uncoupled from ice radiating outwards from the adjacent Welsh Ice Cap centred over Snowdonia. Across the boundary between the two former ice masses is a set of sediment–landform assemblages that reflect rapidly changing erosional and depositional conditions during ice interaction. From the inner part of the ice-stream the assemblages range outwards, from a subglacial depositional assemblage, characterised by drumlin swarms; through a subglacial erosional assemblage, marked by prominent bedrock scours and large subglacial rock channels; through an ice-marginal assemblage, identified by closely spaced, glaciotectonised push moraines and intervening marginal sandur troughs; into a freely expanding proglacial sandur and lacustrine delta assemblage. The ice-marginal assemblage provides evidence for numerous oscillatory episodes during retreat and at least 20 ice-marginal limits can be identified. At least 11 of these display multiple criteria for identifying readvance and, in the ideal case, is characterised by a moraine form built by localised tectonic stacking of diamict to the rear, fronted by a clastic wedge of ice-front alluvial fan gravel and intercalated flow till. The distribution of sediment–landform assemblages suggests a highly dynamic, convergent ice-stream flow pattern, with high ice velocity, a sharply delineated lateral shear margin, pervasive ice-marginal glaciotectonic deformation and a tightly focused ice-marginal sediment delivery system; all signature characteristics of contemporary ice streams.

Landscape evolution degrades the geologic signature of past glaciations

Glaciers on land leave behind characteristic landforms and bedrock topography. The spatial extent of this signature and dating of the erosional and depositional features has led to a good understanding of temporal and spatial shifts of past glaciers and climate. At best the landscape contains considerable information on past glacial processes in the form of moraines and other depositional landforms. However, evidence suggests that all unconsolidated landforms degrade, manifested in the general degradation and smoothing of the topography over time. To determine the relative amount and pattern of degradation on glacial moraines we use a well-documented moraine degradation model that uses both local slope and a proportionality constant to determine the diamicton transport rate. This formulation results in diffusion type topographic smearing and smoothing of the initially angular topographic features. More specifically, diamicton is eroded at the crest of a moraine, transported downhill, and deposited on the gentler lower flank. Collectively this produces moraine cross-profiles that closely resemble observations in the field. By assuming an initial constant boulder frequency per unit volume of the diamicton, the model tracks the surface lowering and the related exhumation of boulders to the moraine surface. This produces a tightly constrained set of testable predictions: (1) modelled moraine cross-profile; (2) modelled spatial pattern of surface boulders (no boulders on the lower flank); and (3) relative frequency of surface boulders (smallest frequency of surface boulders at the inflection point, highest frequency at the crest). These characteristics were measured in the field of alpine, tropical, and Antarctic moraines. The correspondence between model predictions and field data is remarkably good, and strongly supports our understanding of the time dependent evolution of glacial landscapes. These results highlight the evolving nature of glacial landforms and indicate the complete removal of original moraine surfaces and postglacial emergence of spatial pattern in boulder frequency.

Bedforms of the Keewatin Ice Sheet, Canada

By compiling glacial bedforms on a map that covers most of one sector of the Laurentide Ice Sheet, it is possible to make some suggestions about their genesis based largely on spatial relationships. It can be concluded that drumlins and ribbed moraine form at the base of actively flowing ice under similar dynamic conditions. For either landform to exist, however, there must have been enough sediment available in the base of the glacier to leave or form a feature large enough to be recognizable. The presence or absence of sufficient load is related to the geology of the glacier bed and has little to do with regionally changing dynamics of the ice-water system. Likewise, given sufficient load, it is evident that whether drumlins formed or whether ribbed moraine formed in a certain area is a function of the physical nature of the load which is, again, related to geology of the source outcrops. Whether the physical characteristics come into play after the sediment has been released from the ice and is being reshaped by basal drag, streamlining, etc., or whether the nature of the load while entrained changes the behaviour of the basal part of the ice is unclear. Physical characteristics of the basal sediment load have apparently promoted internal thrusting of coherent slabs of entrained debris and ice to form ribbed moraine on melting, whereas drumlins may reflect moulding of plastic subglacial debris or erosional streamlining of both the unconsolidated glacial substrate and bedrock. The observation that many eskers cross drumlin fields at nearly right angles to their orientation suggests that conditions producing streamlining and those pertaining to subglacial drainage are separated in time and circumstance. The general occurrence of drumlins and eskers throughout the sediment-rich portions of the Keewatin Ice Sheet, from Zone 1 to its edge, is difficult to reconcile with the restriction and intimate association of these forms with ribbed moraine almost exclusively in Zone 2. Because such a zonal relationship exists to some extent around other ice divides, at least in Labrador/Nouveau Quebec and Newfoundland, it seems that some condition changed or existed in this zone throughout or at some specific time during the existence of the Keewatin Ice Sheet. Possibly around the last ice divides, reactivation of the ice sheet by alimentation associated with climatic deterioration may have promoted flow of thin, brittle ice. This may have “shattered” the glacier, forming stacked thrust plates of ice and debris. With subsequent stagnation, the thrust plates may have melted out without substantial deformation.

Variability in Neoglacial Moraine Morphology and Composition, Storbreen, Jotunheimen, Norway: Within-Moraine Patterns and their Implications

The nature and origin of the outermost 'Little Ice Age' moraine in front of a NE-flowing glacier (Storbreen, Jotunheimen) are investigated by analysing the variability along the length of the moraine of a range of characteristics (crossprofile form, surface composition, clast form and quartz sand grain characteristics). Concentrated application of a range of techniques in relation to a single moraine has enabled inferences to be made about glacial transport paths of debris in the moraine and processes of moraine-building. Moraine form is consistent within lateral and latero-terminal sections. Origin is attributed to a combination of some pushing of valley-side debris and dumping of glacially-transported material, in which the ratio of modified subglacial to unmodified supraglacial/englacial debris increases downglacier. A larger southern than northern side of the outer moraine is attributed to a difference in the quantity of debris being supplied from headwall areas, which are steep and heavily-eroded on the southern side but low-angled and vegetated on the northern side.

The variability of soil development on Preboreal moraine ridge crests, Breiseterdalen, southern Norway

The common assumption that moraine ridge crests provide a uniform soil-forming environment is tested by recording soil depth and degree of horizon development on 13 sections of Preboreal moraine ridge crest. Considerable variability is not related to parent material or vegetation. The shallowest Brown Soils develop on steep, exposed ridge crests where the duration of translocation potential is at a minimum. Podzols develop in sheltered locations where snow accumulates. Moraine ridge crests do not, therefore, provide a uniform soil-forming environment, and where degree of soil development is used as an indicator of relative age, the influence of moraine form must be taken into account.

A preserved drumlinized moraine built up by an “old till”, in Västerbotten county, northern Sweden

Abstract The aims of the research reported in this paper have been to describe the composition of a moraine mainly built up by a till unit deposited by an earlier ice flow, and to discuss the conditions necessary for the preservation of the form. The lower dominating till unit, characterized by large-scale fissility, was deposited from the SW, probably during the middle Weichselian stadial or in the beginning of the latest stadial. The upper unit, deposited by the main ice flow from the NW, shows very small-scale fissility. However, only a slight reorientation of clasts in the lower unit is visible. The positive form might have been preserved because of its position in relation to the ice divide and the ensuring slight erosion. Another more likely factor may be the presence of a till accumulation (ramp) in front of the moraine form. This ramp seems to have been adapted to divert the ice flow over the moraine and the rather steep hill behind it. This implies that the basal ice round the moraine must have b...

The distribution of local bedrock material in some moraine forms form the inner part of northern Sweden

An inventory of bedrock material has been made from different levels in some moraine types situated in the areas around Storvindeln and Storavan. The results show that moraine forms which have a curved morphology transverse to the direction of ice flow, always have a high content of local bedrock material independent of till thickness. In many cases the local bedrock material is represented more in the upper levels of the moraine. Large transverse ridges with less curvature have a highly variable bedrock content. Drumlins may lack local bedrock material over a distance of many kilometres. The variable distribution of local bedrock material in different moraine types has been interpreted as a function of ice pressure, temperature, and movement during deglaciation.

Deglaciation models from the Swedish West Coast

Deglaciation processes within different rock relief types are discussed. The lower parts of the fissure‐valley landscape in western Sweden were covered by the late‐glacial sea at deglaciation, while the rock plateaux between the valleys formed an arctic archipelago. The glacial movements, deposition activity and recession were intimately dependent on the variations of the topography and on the buoyancy of the seawater in the valleys. The opinion that a piedmont glaciation existed in eastern Halland during the deglaciation stage has been corroborated concerning areas above the marine limit. In the valleys below this limit the ice margin, however, was straight or slightly concave. The western part of the South Swedish Highland, situated high above the marine limit, is characterized by a zonal deglaciation; zone by zone of the ice margin was detached from the actively moving ice and became immobile. Subglacially formed eskers appear together with glaciofluvial deltas which formed extramarginally in ice‐dammed lakes. The moraine forms are often dominated by 1–2 km long drumlins with rock cores. Where the ice diverged over a convex bedrock basement, Rogen‐like moraine ridges, radial as well as transverse, were formed during the deglaciation stage when the ice was stagnating.

Glacier movement, ice structures, and medial moraine form at a glacier confluence, Berendon Glacier, British Columbia, Canada

On Berendon Glacier, Canada, at the asymmetric Y junction of two large valley glaciers, observations of ice velocity, strain rate, and ice structures indicate that the form of a medial moraine is determined by severe lateral compression between the two ice streams. Debris of the moraine is recycled along complex transport paths determined by ice structures. For example, supraglacial debris is precipitated into the confluence zone between the two glaciers forming a subglacial debris reservoir. Debris is evacuated from the reservoir by shearing between the two glaciers and is revealed on the glacier surface as debris dykes and melt-out tills along the contact plane. Important implications regarding the sedimentology of the moraine can be stated. Elsewhere debris is precipitated into transverse crevasses subsequently resulting in transverse till ridges at the glacier surface.Annually-formed wave ogives, bulges in surface ice, and overriding along the glacier contacts result from severe lateral compresssion, and add further diversity to the moraine.Downglacier, confluence ice structures are destroyed by ablation, the pattern of ice flow becomes less complex, unified flow between the two glaciers is established, and a distinct structurally-determined moraine morphology is replaced by one dependent upon the differential ablation of debris covered and clean ice.

Till in Sweden

The article gives a brief summary of till investigations in Sweden, which have long traditions. The main types of till and moraine forms occurring in Sweden are briefly defined and the till stratigrapy is described. A main part of the article is an extensive list of references.

Flow near the Margin of the Barnes Ice Cap, and the Development of Ice-Cored Moraines

Fabric studies suggest that an up-glacier thinning wedge of deformed superimposed ice is present beneath glacial ice at the margin of the Barnes Ice Cap. Where the glacial ice contains debris, the contact between the two is approximately marked by a ridge-shaped ice-cored moraine, 100 to 150 m from the margin. Where the glacial ice contains little or no debris, the surface profile is smooth and convex upward. Measurements of surface flow and ablation rates on both types of margins suggest that a general advance of the glacier is necessary to incorporate such superimposed ice into the margin, and hence, to instigate development of this type of ice-cored moraine. However, once a moraine forms, it may continue to grow in height and move outward without further advance of the glacier as a whole. As the moraine moves outward, the depression up-glacier from it becomes deeper, and the moraine is gradually separated from the glacier. Debris that slumps from the moraine onto forelying superimposed ice may become buried by new superimposed ice and be reincorporated into the margin. During a subsequent advance, this debris layer will be deformed, possibly overturned, and finally brought back to the surface where it will melt to form a new ridge. Such recycling of debris may result in very complex structures in an ice-cap margin. Measurements in boreholes and in a 125-m ice tunnel do not support the hypothesis that discrete shear zones occur at or near the contact between the dirt-bearing glacial ice and the superimposed ice.

Morphology of the Ice-sheet Margin Near Thule, Greenland

Three types of glacier margin are found along the edge of the Greenland ice sheet near Thule: ice cliffs, ramps and ice-cored moraines. Where the glacier margin is perpendicular to prevailing katabatic winds, drifting snow accumulates along it in stagnant wind-drill ice wedges. Upward flow of active ice behind these wedges causes ice originally near the base of the glacier to rise to the surface. Where this basal ice is free of debris, a gently sloping ramp develops. However, where the basal ice contains sufficient debris, a layer of till accumulates on the glacier surface. Ice beneath the till is insulated and a debris-capped ice ridge or ice-cored moraine forms, Ice cliffs occur where the ice-sheet margin is parallel to prevailing winds and is thus swept clear of drifting snow. Although the ice sheet in the Thule area appears to have had a negative mass balance for many years, all three types of glacier margin are believed to be equilibrium forms that can develop and persist on a glacier with a balanced mass budget.Foliation in wind-drift ice wedges generally dips down-glacier but foliation in active ice dips up-glacier. It is inferred that foliation in the wedges was once sedimentary stratification that has been tipped upward and locally overturned.

Morphology of the Ice-sheet Margin Near Thule, Greenland

Three types of glacier margin are found along the edge of the Greenland ice sheet near Thule: ice cliffs, ramps and ice-cored moraines. Where the glacier margin is perpendicular to prevailing katabatic winds, drifting snow accumulates along it in stagnant wind-drill ice wedges. Upward flow of active ice behind these wedges causes ice originally near the base of the glacier to rise to the surface. Where this basal ice is free of debris, a gently sloping ramp develops. However, where the basal ice contains sufficient debris, a layer of till accumulates on the glacier surface. Ice beneath the till is insulated and a debris-capped ice ridge or ice-cored moraine forms, Ice cliffs occur where the ice-sheet margin is parallel to prevailing winds and is thus swept clear of drifting snow. Although the ice sheet in the Thule area appears to have had a negative mass balance for many years, all three types of glacier margin are believed to be equilibrium forms that can develop and persist on a glacier with a balanced mass budget.Foliation in wind-drift ice wedges generally dips down-glacier but foliation in active ice dips up-glacier. It is inferred that foliation in the wedges was once sedimentary stratification that has been tipped upward and locally overturned.