Landform Assemblages Research Articles

Submarine landforms and the behavior of a surging ice cap since the last glacial maximum: The open-marine setting of eastern Austfonna, Svalbard

Multi-beam swath bathymetry data and Topographic Parametric Sonar (TOPAS) 3.5 kHz sub-bottom acoustic profiles were acquired from the inlet of Hartogbutka, an open-marine setting offshore of Austfonna, northeastern Nordaustlandet, Svalbard, the largest ice cap in the Eurasian Arctic. Using these marine geophysical data, the seafloor morphological and sedimentary evidence of former ice extent and dynamics of the three drainage basins (Basins 3, 4, and 5) adjacent to Hartogbukta were examined. Submarine landforms and landform assemblages that provide evidence of past ice dynamics are described and interpreted in the context of previous studies of surge-affected submarine environments elsewhere in Svalbard. A sequence of four distinct ice-flow episodes in Hartogbukta, from the late Weichselian deglaciation to the present, is identified to explain the morphological evolution of the seafloor. A full-glacial ice sheet extended over Hartogbukta during the late Weichselian, flowing ESE. At intervals throughout the Holocene, the drainage basins adjacent to Hartogbukta probably underwent repeated surges. Most recently, Basins 3 and 4 probably surged as one cohesive ice mass in the late 1800s and Basin 5 most likely underwent a less extensive surge in the 20th century. These ice-flow events are interpreted in the context of the glaciology and known glacial history of Austfonna and the NW Barents Sea. The 200 km-long ice cliffs of eastern Austfonna provide an unusual and important analog for open-marine deposition that would have been much more prevalent under full-glacial conditions where ice sheets in many parts of the Arctic advanced from fjord systems onto the adjacent continental shelves.

Importance of sampling across an assemblage of glacial landforms for interpreting cosmogenic ages of deglaciation

Deglaciation chronologies for some sectors of former ice sheets are relatively poorly constrained because of the paucity of features or materials traditionally used to constrain the timing of deglaciation. In areas without good deglaciation varve chronologies and/or without widespread occurrence of material that indicates the start of earliest organic radiocarbon accumulations suitable for radiocarbon dating, typically only general patterns and chronologies of deglaciation have been deduced. However, mid-latitude ice sheets that had warm-based conditions close to their margins often produced distinctive deglaciation landform assemblages, including eskers, deltas, meltwater channels and aligned lineation systems. Because these features were formed or significantly altered during the last glaciation, boulder or bedrock samples from them have the potential to yield reliable deglaciation ages using terrestrial cosmogenic nuclides (TCN) for exposure age dating. Here we present the results of a methodological study designed to examine the consistency of TCN-based deglaciation ages from a range of deglaciation landforms at a site in northern Norway. The strong coherence between exposure ages across several landforms indicates great potential for using TCN techniques on features such as eskers, deltas and meltwater channels to enhance the temporal resolution of ice-sheet deglaciation chronologies over a range of spatial scales.

Late Quaternary ice flow and sediment delivery through Hinlopen Trough, Northern Svalbard margin: Submarine landforms and depositional fan

The morphology and distribution of submarine landforms in Hinlopen Trough, Northern Svalbard margin, and the upper continental slope beyond, are investigated using swath-bathymetric data, together with side-scan sonar, acoustic profiler records and sediment cores. Sun-illuminated images reveal a number of landform assemblages on the sea floor of Hinlopen Trough and confirm that this depression was occupied by an ice stream during the Late Weichselian glaciation around 20 ka before present. The geomorphology of the inner-shelf is characterised by bedrock drumlins and ice-sculpted bedrock. There is a middle-shelf transition from a rock bed to an unconsolidated sedimentary bed that contains highly-attenuated drumlins and megaflutes formed within a dark grey, matrix-supported diamict, interpreted as subglacial till. Outer-shelf landforms are mainly iceberg ploughmarks. The geomorphological imprint identified from Hinlopen Trough is similar to that of many former ice streams; the trough is dominated by elongate landforms orientated parallel to the inferred flow direction and is lacking in transverse features indicative of inter-ice stream locations. A characteristic down-flow progression in landform elongation is also observed, suggesting an increase in ice velocity across the continental shelf. The lack of grounding-zone features imaged from swath-bathymetric data implies that deglaciation was probably rapid within the trough. A large depositional fan exists on the upper continental slope beyond the trough, characterised by debris flow deposits relating to the down-slope transport of glacigenic sediment. Order of magnitude calculations of the volume of glacigenic sediment on the continental slope indicate that the ice stream provided high rates of debris delivery of around 5–7.5 m/ka. This suggests that Hinlopen Trough ice stream represented a major route for the transfer of ice and debris to the Hinlopen Fan on the northern continental margin of Svalbard during the Late Weichselian glaciation.

Glacial landsystems of Satujökull, Iceland: A modern analogue for glacial landsystem overprinting by mountain icecaps

Mapping of the surficial geology and geomorphology of the Satujökull foreland of the northern Hofsjökull ice cap in central Iceland provides a clear signature of glacial landsystem overprinting as a result of complex glacier behaviour during the historical period. Landsystem 1 comprises a wide arc of ice-cored moraine and controlled ridges lying outside fluted and drumlinized terrain, and is indicative of polythermal conditions. This landform assemblage commonly marks the historical Little Ice Age maximum limit on the forelands of upland icefields in the arid interior of Iceland and is therefore a record of climatically driven glacier advance. Landsystem 2 contains most of the diagnostic criteria for the surging glacier landsystem and records two separate surges by the western margin of Satujökull in the period since the attainment of the Little Ice Age maximum advance. The occurrence of Landsystem 2 is significant because Satujökull has not been previously regarded as a surging glacier, even though other outlets of Hofsjökull are prone to surging. Landsystem overprinting, especially in response to changing thermal regimes and/or glacier dynamics, and particularly by different flow units in the same glacier, is rarely reported but is crucial to the critical application of modern landsystem analogues to Quaternary palaeoglaciological reconstruction.

Radar images of the bed of the Greenland Ice Sheet

[1] In this paper, we apply radar tomography methods to very-high-frequency, airborne synthetic-aperture radar data to measure the ice thickness field and to construct three-dimensional basal image maps of a 5 × 20 km study area located along the southern flank of the Jakobshavn Glacier, Greenland. Unlike ice radar measurements typically made at nadir, our approach uses radar-echo phase and amplitude measured across an antenna array to determine the propagation angle and signal strength of pixel elements distributed on each side of the aircraft flight path. That information, combined with knowledge of aircraft position and the assumed dielectric properties of the glacier, can be used to measure ice thickness and radar reflectivity across a 3-km wide swath. Combining ice thickness and surface topography data, we estimate basal topography and basal drag. We conclude that the glacier is sliding over the bed. We use the three-dimensional image maps of the bed to inspect the modern subglacial geomorphology and find for the first time beneath the Greenland Ice Sheet assemblages of long ridge-groove landforms that are oriented in the direction of the ice flow. Spatial dimensions (10 to 30 m depths, 150 to 500 m spacing and lengths of 10 km or more) and correlation with the current ice flow direction suggest that these are glacial erosional features similar to mega-grooves observed on deglaciated terrain.

Sorted clastic stripes, lobes and associated gullies in high-latitude craters on Mars: Landforms indicative of very recent, polycyclic ground-ice thaw and liquid flows

Self-organised patterns of stone stripes, polygons, circles and clastic solifluction lobes form by the sorting of clasts from fine-grained sediments in freeze–thaw cycles. We present new High Resolution Imaging Science Experiment (HiRISE) images of Mars which demonstrate that the slopes of high-latitude craters, including Heimdal crater – just 25 km east of the Phoenix Landing Site – are patterned by all of these landforms. The order of magnitude improvement in imaging data resolution afforded by HiRISE over previous datasets allows not only the reliable identification of these periglacial landforms but also shows that high-latitude fluviatile gullies both pre- and post-date periglacial patterned ground in several high-latitude settings on Mars. Because thaw is inherent to the sorting processes that create these periglacial landforms, and from the association of this landform assemblage with fluviatile gullies, we infer the action of liquid water in a fluvio-periglacial context. We conclude that these observations are evidence of the protracted, widespread action of thaw liquids on and within the martian regolith. Moreover, the size frequency statistics of superposed impact craters demonstrate that this freeze–thaw environment is, at least in Heimdal crater, less than a few million years old. Although the current martian climate does not favour prolonged thaw of water ice, observations of possible liquid droplets on the strut of the Phoenix Lander may imply significant freezing point depression of liquids sourced in the regolith, probably driven by the presence of perchlorates in the soil. Because perchlorates have eutectic temperatures below 240 K and can remain liquid at temperatures far below the freezing point of water we speculate that freeze–thaw involving perchlorate brines provides an alternative “low-temperature” hypothesis to the freeze–thaw of more pure water ice and might drive significant geomorphological work in some areas of Mars. Considering the proximity of Heimdal crater to the Phoenix Landing Site, the presence of such hydrated minerals might therefore explain the landforms described here. If this is the case then the geographical distribution of martian freeze–thaw landforms might reflect relatively high temperatures (but still below 273 K) and the locally elevated concentration of salts in the regolith.

Controlled moraine development and debris transport pathways in polythermal plateau icefields: examples from Tungnafellsjökull, Iceland

AbstractA modern landsystems analogue for plateau icefield glaciation is presented, based upon Tungnafellsjökull in the arid, cold interior of Iceland. Landform mapping and intensive clast form analysis on three separate forelands occupied during the Little Ice Age reveal variability in the morphology of moraines and the degree of clast modification, which is interpreted as a function of the complex interplay between: (a) the efficacy of glacial erosion; (b) passive versus active debris transfer mechanisms; and (c) the volume of debris provision for ice‐marginal freeze‐on mechanisms. A steep outlet bounded by rockwalls produced a landform assemblage and debris transport signature typical of valley glaciers, whereas two unconfined outlets produced a landform‐sediment imprint typical of polythermal conditions. This comprises subglacial bedforms, relating to the initiation of warm‐based ice by strain heating below icefalls at the plateau edge, terminating at arcs of ice‐cored (controlled) moraine representative of frozen snout conditions. Patterns in clast form data on the unrestricted forelands reflect the characteristics of glacier dynamics and thermal regimes at the margin of a plateau icefield, where bedrock steps of sufficient size to initiate strain heating and bedrock abrasion and plucking are located only a short distance from the marginal freeze‐on conditions of a snout creating debris‐rich folia and controlled moraine. Additional englacial debris may well have been carried to the glacier margin through the development of ogives and englacial thrusting at the base of the icefall, thereby explaining the occurrence of striae on clasts that have otherwise been little modified by glacial abrasion. Copyright © 2010 John Wiley & Sons, Ltd.

The first stages of erosion by ice sheets: Evidence from central Europe

In almost all former and presently glaciated areas of the world, glaciers have modified or even transformed pre-glacial terrain during the many cold stages of the Pleistocene. In consequence, the early stages in the development of glacial landscapes have been overprinted or erased by later phases of erosion. The Sudetes in central Europe provide exceptional opportunities to examine the inception of glacial erosion. Evidence of a long geomorphic evolution before glaciation, with the development of etch surfaces, deep weathering covers and the preservation of Neogene kaolinitic sediments, provides a direct analogue to other lowland crystalline terrains as existed immediately prior to Pleistocene glaciation. Large granite intrusions exist in the Sudetes and its Foreland that support hills that range in size from small tors to large domes. These terrains experienced only thin ice cover for short periods when the Scandinavian ice sheet reached its Pleistocene maximum limits in Marine Isotope Stage (MIS) 12 (Elsterian) at 440–430 ka and MIS 8 (Early Saalian) at 250–240 ka. In each of four study areas beyond, at and within the limits of glaciation we have mapped Glacial Erosion Indicators, glacial landform assemblages that indicate progressive glacial modification of the pre-glacial granite terrain. We find that glacial erosion increases with distance from the ice margin, due to greater ice thickness and longer ice cover, but has had only limited impact. On hills, regolith was stripped by moving ice, tors were demolished, and blocks were entrained. However, indicators of more advanced glacial erosion, such as lee-side cliffs and glacial streamlining, are absent, even from granite domes that lay beneath ~ 500 m of ice. The survival of tors beneath ice cover and the removal of tor superstructure by glacial erosion are confirmed. The presence of glacially-modified tors in areas covered only by Elsterian ice implies that the tors existed before 440 ka. Moreover, the contrast between blockfield-covered slopes found above Elsterian and Saalian glacial trimlines on summits of granite, gabbro and basalt hills in the Sudetic Foreland and glacially-stripped surfaces at lower elevations implies only limited regeneration of blockfields since 430 and 240 ka. The first stages of roche moutonnée formation are recognised in the exhumation of tor stumps but development of lee-side steps remains very limited. Granite domes retain a pre-glacial morphology and local examples of hill asymmetry are determined by structural, rather than glaciological controls. Development of roches moutonnées and asymmetric and streamlined hills by glacial erosion has not occurred in the Sudetes despite the passage of largely warm-based ice sheets 80–500 m thick that persisted for 5–20 ka.

Glacial geomorphology: Towards a convergence of glaciology and geomorphology

This review presents a perspective on recent trends in glacial geomorphological research, which has seen an increasing engagement with investigating glaciation over larger and longer timescales facilitated by advances in remote sensing and numerical modelling. Remote sensing has enabled the visualization of deglaciated landscapes and glacial landform assemblages across continental scales, from which hypotheses of millennial-scale glacial landscape evolution and associations of landforms with palaeo-ice streams have been developed. To test these ideas rigorously, the related goal of imaging comparable subglacial landscapes and landforms beneath contemporary ice masses is being addressed through the application of radar and seismic technologies. Focusing on the West Antarctic Ice Sheet, we review progress to date in achieving this goal, and the use of radar and seismic imaging to assess: (1) subglacial bed morphology and roughness; (2) subglacial bed reflectivity; and (3) subglacial sediment properties. Numerical modelling, now the primary modus operandi of ‘glaciologists’ investigating the dynamics of modern ice sheets, offers significant potential for testing ‘glacial geomorphological’ hypotheses of continental glacial landscape evolution and smaller-scale landform development, and some recent examples of such an approach are presented. We close by identifying some future challenges in glacial geomorphology, which include: (1) embracing numerical modelling as a framework for testing hypotheses of glacial landform and landscape development; (2) identifying analogues beneath modern ice sheets for landscapes and landforms observed across deglaciated terrains; (3) repeat-surveying dynamic subglacial landforms to assess scales of formation and evolution; and (4) applying glacial geomorphological expertise more fully to extraterrestrial cryospheres.

Glacial landforms of extreme size in the Keewatin sector of the Laurentide Ice Sheet

Assemblages of glacial landforms of a ‘mega-scale’ are here identified in the Keewatin sector of the Laurentide Ice Sheet. Large till ‘belts’ or ‘ridges’, apparent only on satellite imagery and lying beneath the drumlins, flutes and ribbed moraine which comprise the known regional glacial landform record, form extensive and coherent patterns throughout the Keewatin region. Planform and crestline mapping from remotely sensed imagery yields a mapped population of >2500 individual landforms, whose dimensions are on average ∼10 km long and ∼1.5 km wide. Based on analysis of their morphology and morphometry, their spatial arrangement and pattern, and comparison with analogues and reference populations of glacial landform types, we interpret three morphological groups of different genetic origin. Two of these are examples of currently known landform types: i) a set of heavily overprinted, i.e. non-pristine, mega-scale glacial lineations, feeding from the heart of the Keewatin region north into Queen Maud Gulf; and ii) a 350 km long moraine zone, overrun by later ice flow paths, and likely associated with the terminal position of an ice sheet prior to the final deglacial episode. A third group, comprising a significant number of the Keewatin population, does not fit any existing category of glacial landforms. Here we report a major new finding: subglacial bedforms, of a mega-scale, transverse to the palaeo-ice flow direction. Mega-scale transverse bedforms have not been previously reported from any palaeo-(or contemporary) ice sheet. Close spatial integration with the ribbed moraine population in Keewatin suggests a similar mode of genesis. The Keewatin landforms indicate there is a fundamental transverse organisation of till at a scale beyond that of conventional transverse bedforms (ribbed moraine), and with as yet unknown implications for our understanding of subglacial processes and ice-bed coupling.

Reconstructing the last Irish Ice Sheet 2: a geomorphologically-driven model of ice sheet growth, retreat and dynamics

The ice sheet that once covered Ireland has a long history of investigation. Much prior work focussed on localised evidence-based reconstructions and ice-marginal dynamics and chronologies, with less attention paid to an ice sheet wide view of the first order properties of the ice sheet: centres of mass, ice divide structure, ice flow geometry and behaviour and changes thereof. In this paper we focus on the latter aspect and use our new, countrywide glacial geomorphological mapping of the Irish landscape (>39 000 landforms), and our analysis of the palaeo-glaciological significance of observed landform assemblages (article Part 1), to build an ice sheet reconstruction yielding these fundamental ice sheet properties. We present a seven stage model of ice sheet evolution, from initiation to demise, in the form of palaeo-geographic maps. An early incursion of ice from Scotland likely coalesced with local ice caps and spread in a south-westerly direction 200 km across Ireland. A semi-independent Irish Ice Sheet was then established during ice sheet growth, with a branching ice divide structure whose main axis migrated up to 140 km from the west coast towards the east. Ice stream systems converging on Donegal Bay in the west and funnelling through the North Channel and Irish Sea Basin in the east emerge as major flow components of the maximum stages of glaciation. Ice cover is reconstructed as extending to the continental shelf break. The Irish Ice Sheet became autonomous (i.e. separate from the British Ice Sheet) during deglaciation and fragmented into multiple ice masses, each decaying towards the west. Final sites of demise were likely over the mountains of Donegal, Leitrim and Connemara. Patterns of growth and decay of the ice sheet are shown to be radically different: asynchronous and asymmetric in both spatial and temporal domains. We implicate collapse of the ice stream system in the North Channel – Irish Sea Basin in driving such asymmetry, since rapid collapse would sever the ties between the British and Irish Ice Sheets and drive flow configuration changes in response. Enhanced calving and flow acceleration in response to rising relative sea level is speculated to have undermined the integrity of the ice stream system, precipitating its collapse and driving the reconstructed pattern of ice sheet evolution.

Reconstructing the last Irish Ice Sheet 1: changing flow geometries and ice flow dynamics deciphered from the glacial landform record

The glacial geomorphological record provides an effective means to reconstruct former ice sheets at ice sheet scale. In this paper we document our approach and methods for synthesising and interpreting a glacial landform record for its palaeo-ice flow information, applied to landforms of Ireland. New, countrywide glacial geomorphological maps of Ireland comprising >39,000 glacial landforms are interpreted for the spatial, glaciodynamic and relative chronological information they reveal. Seventy one ‘flowsets’ comprising glacial lineations, and 19 ribbed moraine flowsets are identified based on the spatial properties of these landforms, yielding information on palaeo-ice flow geometry. Flowset cross-cutting is prevalent and reveals a highly complex flow geometry; major ice divide migrations are interpreted with commensurate changes in the flow configuration of the ice sheet. Landform superimposition is the key to deciphering the chronology of such changes, and documenting superimposition relationships yields a relative ‘age-stack’ of all Irish flowsets. We use and develop existing templates for interpreting the glaciodynamic context of each flowset – its palaeo-glaciology. Landform patterns consistent with interior ice sheet flow, ice stream flow, and with time-transgressive bedform generation behind a retreating margin, under a thinning ice sheet, and under migrating palaeo-flowlines are each identified. Fast ice flow is found to have evacuated ice from central and northern Ireland into Donegal Bay, and across County Clare towards the south-west. Ice-marginal landform assemblages form a coherent system across southern Ireland marking stages of ice sheet retreat. Time-transgressive, ‘smudged’ landform imprints are particularly abundant; in several ice sheet sectors ice flow geometry was rapidly varying at timescales close to the timescale of bedform generation. The methods and approach we document herein could be useful for interpreting other ice sheet histories. The flowsets and their palaeo-glaciological significance that we derive for Ireland provide a regional framework and context for interpreting results from local scale fieldwork, provide major flow events for testing numerical ice sheet models, and underpin a data-driven reconstruction of the Irish Ice Sheet that we present in an accompanying paper – Part 2.

Bedform signature of a West Antarctic palaeo-ice stream reveals a multi-temporal record of flow and substrate control

The presence of a complex bedform arrangement on the sea floor of the continental shelf in the western Amundsen Sea Embayment, West Antarctica, indicates a multi-temporal record of flow related to the activity of one or more ice streams in the past. Mapping and division of the bedforms into distinct landform assemblages reveals their time-transgressive history, which implies that bedforms can neither be considered part of a single downflow continuum nor a direct proxy for palaeo-ice velocity, as suggested previously. A main control on the bedform imprint is the geology of the shelf, which is divided broadly between rough bedrock on the inner shelf, and smooth, dipping sedimentary strata on the middle to outer shelf. Inner shelf bedform variability is well preserved, revealing information about local, complex basal ice conditions, meltwater flow, and ice dynamics over time. These details, which are not apparent at the scale of regional morphological studies, indicate that past ice streams flowed across the entire shelf at times, and often had onset zones that lay within the interior of the Antarctic Ice Sheet today. In contrast, highly elongated subglacial bedforms on sedimentary strata of the middle to outer shelf represent a timeslice snapshot of the last activity of ice stream flow, and may be a truer representation of fast palaeo-ice flow in these locations. A revised model for ice streams on the shelf captures complicated multi-temporal bedform patterns associated with an Antarctic palaeo-ice stream for the first time, and confirms a strong substrate control on a major ice stream system that drained the West Antarctic Ice Sheet during the Late Quaternary.

Palaeo‐ice streams and the subglacial landscape mosaic of the North American mid‐continental prairies

The analysis of the glacial landscape of southern Saskatchewan (Canada) through multiple data sets (e.g. digital elevation model, till compositional data) has revealed previously unrecognized subglacial sediment–landform assemblages. A southwest‐trending corridor of mega‐scale till lineations (Maskwa corridor) bounded on each side by hummocky terrain extends from the Canadian Shield to southwestern Saskatchewan. This landform assemblage is clearly cross‐cut by a broad south to southeast trending corridor (Buffalo corridor) consisting of subparallel curvilinear till ridges. The carbonate content of the surface till is spatially consistent within these assemblages, suggesting a strong sediment–landform relationship. The two corridors are interpreted as the product of palaeo‐ice streams. The Maskwa palaeo‐ice stream flowed up the regional slope and across preglacial valleys, indicating it was thick and stable. Narrow dispersal trains extending across as well as down‐glacier from the Athabasca Basin suggest that the Maskwa palaeo‐ice stream extended far into the ice sheet across contrasting shield and platform terrains. In comparison, the Buffalo palaeo‐ice stream was thinner and largely controlled by subglacial geology and topography. Its catchments were located at the Canadian Shield boundary and the system was oriented along‐slope. It experienced lateral shifts and it was fed by a network of tributaries. The glacial dynamics shift from the Maskwa to the Buffalo system occurred at about 13.5 14C kyr BP. The Buffalo system later evolved into thin outlet lobes until final deglaciation of the area. The proposed model has implications for ice‐sheet reconstruction and the assessment of till properties in the prairies and in similar terrains.

An equatorial periglacial landscape on Mars

We present evidence, drawn from new 25 cm/pixel “HiRISE” images of Mars, of landforms indicative of geologically-recent thaw degradation of ice-rich terrain near the Martian equator. We have focused our study on the head-region of Athabasca Vallis, a catastrophic flood-carved outflow channel thought to have been active within the last 2–8 Ma. The geomorphology of the study area is characterised by a series of polygonally-patterned surfaces that are bounded by erosional scarps. The scarps are indented with cirque-shaped niches that appear to be controlled in scale and form by the pre-existing polygonal sculpture. Shallow basins, defined by niche-indented scarps, are also seen and contain pitted “mound and cone” structures that are similar in form to terrestrial pingos. Associated with many of these scarps are dendritic channels and gullies, blocky debris and hummocky terminal deposits. Together, these correspond morphologically to terrestrial “retrogressive thaw slumps” which are landforms typical of thermokarst environments. We have also observed an evolutionary sequence of morphologies beginning with subsidence of polygonised surfaces and ending with scarp bounded basins fed by inter-polygon channels. This is consistent with the former presence of surficial liquid water released by ground-ice thaw and is again indicative of thermokarst. Moreover, the observations of epigenetic polygons and channels, and of polycyclic retrogressive thaw slumps, suggest that thaw and standing water persisted for many seasonal cycles. The assemblage of landforms, the occurrence of polygons and pingo-like forms within erosional basins, and observations of fluvial-like channels cross-cutting pre-existing surface features are consistent with a surface that has evolved significantly since it was originally emplaced. This precludes this terrain being a volcanic (i.e., primary) landscape, as has been suggested in the past. The formation of this landform assemblage requires that there was thaw of ground ice and persistent liquid water at the surface. Given the geologically young flood deposits in which this assemblage occurs, our observations point to a recent period in which Mars’ climate supported thaw conditions in this region.

Marine Isotope Stage 7–6 transition age for beach sediments at Morston, north Norfolk, UK: implications for Pleistocene chronology, stratigraphy and tectonics

AbstractOptically stimulated luminescence age estimates for the Pleistocene beach at Morston, north Norfolk, UK, obtained by the single‐aliquot regenerative‐dose protocol, indicate a Marine Isotope Stage (MIS) 7–6 transition date. The view that the beach is of Ipswichian (MIS 5e) age, held virtually unanimously for the last 75 years, may now be discarded. The extant beach sequence lies up to ∼5 m OD, yet global models suggest that MIS 7–6 sea levels were typically substantially below that of today. The explanation may lie with poorly understood regional tectonic movements. The MIS 7–6 date helps to constrain the ages of glacial deposits that bracket the beach sediments at Morston. The underlying Marly Drift till cannot be younger than MIS 8; this may also be true for the complex assemblage of glaciogenic landforms and sediments, including the Blakeney esker, in the adjacent lower Glaven valley. The well‐established Late Devensian (MIS 2) age of the Hunstanton Till is not compromised by the date of the Morston beach. There is no indication of a proposed Briton's Lane glaciation during MIS 6 times. Copyright © 2009 John Wiley & Sons, Ltd.

Catastrophic middle Pleistocene jökulhlaups in the upper Susquehanna River: Distinctive landforms from breakout floods in the central Appalachians

Widespread till and moraines record excursions of middle-Pleistocene ice that flowed up-slope into several watersheds of the Valley and Ridge Province along the West Branch of the Susquehanna River. A unique landform assemblage was created by ice-damming and jökulhlaups emanating from high gradient mountain watersheds. This combination of topography formed by multiple eastward-plunging anticlinal ridges, and the upvalley advance of glaciers resulted in an ideal geomorphic condition for the formation of temporary ice-dammed lakes. Extensive low gradient (1°–2° slope) gravel surfaces dominate the mountain front geomorphology in this region and defy simple explanation. The geomorphic circumstances that occurred in tributaries to the West Branch Susquehanna River during middle Pleistocene glaciation are extremely rare and may be unique in the world. Failure of ice dams released sediment-rich water from lakes, entraining cobbles and boulders, and depositing them in elongated debris fans extending up to 9 km downstream from their mountain-front breakout points. Poorly developed imbrication is rare, but occasionally present in matrix-supported sediments resembling debris flow deposits. Clast weathering and soils are consistent with a middle Pleistocene age for the most recent flows, circa the 880-ka paleomagnetic date for glacial lake sediments north of the region on the West Branch Susquehanna River. Post-glacial stream incision has focused along the margins of fan surfaces, resulting in topographic inversion, leaving bouldery jökulhlaup surfaces up to 15 m above Holocene channels. Because of their coarse nature and high water tables, jökulhlaup surfaces are generally forested in contrast to agricultural land use in the valleys and, thus, are readily apparent from orbital imagery.

Quantifying subglacial bed roughness in Antarctica: implications for ice-sheet dynamics and history

Glaciated landscapes consist of complex assemblages of landforms resulting from ice flow dynamic regimes and ice-sheet history, superimposed over, and in turn modifying, preglacial topography, lithology and geological structure. Insights into the formation of glaciated landscapes can, in principle, be obtained by analysing modern ice-sheet beds, but terrain analyses beneath modern ice sheets are restricted by the inaccessibility of the bed. It is, however, possible to quantify roughness, the vertical variation of the subglacial interface with horizontal distance, along two-dimensional images of the bed obtained from radio-echo sounding (RES). Here we collate several case studies from Antarctica, where roughness calculations have been used as a glaciological tool to infer basal processes and ice-sheet history over large (>500 km 2) areas. We present two examples from West Antarctica, which demonstrate the utility of bed roughness in determining the presence and extent of subglacial sediments, glacial dynamics and former ice-sheet size. We also present two examples from East Antarctica, which illustrate how roughness provides knowledge of ice-sheet dynamics in the interior and pre-Quaternary ice-sheet histories. In modern ice-sheet settings, characterising bed roughness along RES tracks has the twin advantages of being relatively simple to calculate while producing informative subsurface data, and is especially powerful at furthering understanding when coupled with knowledge of ice flow from field, satellite and modelling investigations. The technique also offers significant potential for the comparison of modern and former ice-sheet terrains, contributing to an improved understanding of the formation and evolution of glaciated landscapes.

Geomorphological Mapping along the Upper Southeast Florida Atlantic Continental Platform; I: Mapping Units, Symbolization and Geographic Information System Presentation of Interpreted Seafloor Topography

Abstract Continental shelf environments along the Florida Atlantic southeast coast offshore Palm Beach County were mapped by interpreting seafloor topography derived from airborne laser bathymetry. The study area covered approximately 270 km2 along 75 km of coast, extending from the shore to about 55 m water depth beyond the shelf break to the upper Florida-Hatteras Slope. Forty-two mapping units were used to depict seafloor features that ranged from bedrock outcrops and coral reefs to different types of morphosedimentary bodies. Background theory and protocols of geomorphological symbolization are provided as a rationale for the seafloor mapping. Geomorphological mapping symbols were developed specifically for this project and collected in a symbol library as part of an ArcInfo GIS extension that was used to produce the maps in a GIS environment. Spatial distribution patterns of landform assemblages evident in the color-pattern mapping units permitted regionalization of shelf environments into three prim...

The formation of sawtooth moraine ridges in Bødalen, western Norway

The forefield of the Bødalsbreen glacier, western Norway, exhibits a prominent set of end moraines. The outermost one, deposited at the Little Ice Age (LIA) maximum (A.D. ∼ 1755), is smooth, continuous and arcuate, thus reflecting an even ice front. In contrast, subsequently formed moraines (deposited A.D. ∼ 1767–1930) show a striking “sawtooth” morphology. In an attempt to elucidate the formation processes responsible for this complex landform assemblage, we have employed several methods of investigation including geomorphological mapping, sedimentological studies, and ground penetrating radar (GPR). In general, the detailed formation processes of all moraines include bulldozing and thrusting. Furthermore, the sediment composition and structure are similar for all the moraines containing recycled diamicton. The remaining major factor influencing the moraine morphology thus seems to be the shape of the glacier front. Due to the distinctive geometry of the Bødalen valley, the former glacier underwent strong changes of its transverse stress field. In the part where the valley was wider, splaying/radial crevasses developed. Further downvalley, where the valley again narrowed, these crevasses were closed. As previous studies have concluded, the change in valley morphology to a large extent explains the distinct morphology of the Bødalen valley moraines. This notion is further corroborated by studying sawtooth or sawtooth-like moraines elsewhere where valley geometry also seems to have been a major contributing factor for moraine morphology.