Fennoscandian Ice Sheet in MIS 3 – Introduction

Wohlfarth, B. & Näslund, J.‐O. 2010 (April): Fennoscandian Ice Sheet in MIS 3 – Introduction. Boreas, Vol. 39, pp. 325–327. 10.1111/j.1502‐3885.2010.00151.x. ISSN 0300‐9483.     

Submarine push moraine formation during the early Fennoscandian Ice Sheet deglaciation

The glacial history in the topographically confined paleo-ice stream drainage route of Vestfjorden, North Norway, was analysed based on bathymetric data, high-resolution seismology and ¹⁴C AMS-dated gravity cores. The inner part of the fjord is characterised by axial-parallel mega-scale lineations whereas the outer part is dominated by two marginal morainal bank systems. The Værøy (inner) and Røst (outer) marginal moraine systems comprise several transverse, zigzag-shaped ridges. Seismic records show thrusted and folded sediment blocks within the ridges. The landforms are inferred to reflect basal processes and the transition from warm-based (inner fjord) to cold-based (outer fjord) conditions, i.e. fast flow followed by basal freeze-on, sediment deformation and morainal bank formation. The moraines formed during the final part of two paleo-ice sheet re-advances. ¹⁴C AMS dating indicates a maximum age of 13.7 ¹⁴C ka BP (16.2 cal ka BP) for the Røst system whereas the Værøy system is inferred to be slightly older than 12.5 ¹⁴C ka BP (14.5 cal ka BP). This demonstrates that the northern part of the Fennoscandian Ice Sheet behaved in a much more dynamic way during the early deglaciation than previously assumed.     

Modeling the subglacial hydrology of the James Lobe of the Laurentide Ice Sheet

To investigate the drainage conditions that might be expected to develop beneath soft-bedded ice sheets, we modeled the subglacial hydrology of the James Lobe of the Laurentide Ice Sheet from Hudson Bay to the Missouri River. Simulations suggest the James Lobe had little effect on regional groundwater flow because the poorly conductive Upper-Cretaceous shale that occupies the upper layer of the bedrock would have functioned as a regional aquitard. This implies that general northward groundwater flow out of the Williston Basin has likely persisted throughout the Quaternary. Moreover, the simulations indicate that the regional aquifer system could not have drained even the minimum amount of basal meltwater that might have been produced from at the glacier bed. Therefore, excess drainage must have occurred by some sort of channelized drainage network at the ice–till interface. Using a regional groundwater model to determine the hydraulic conductivity for an equivalent porous medium in a 1-m thick zone between the ice and underlying sediment, and assuming conduit dimensions from previous theoretical work, we use a theoretical karst aquifer analog as a heuristic approach to estimate the spacing of subglacial conduits that would have been required at the ice–till interface to evacuate the minimum water flux. Results suggest that for conduits assumed to be on the order of a tenth of a meter deep and up to a meter wide, inter-conduit spacing must be on the order of tens–hundreds of meters apart to maintain basal water pressures below the ice overburden pressure while evacuating the hypothesized minimum meltwater flux.     

Map and GIS database of glacial landforms and features related to the last British Ice Sheet

A review of the academic literature and British Geological Survey mapping is employed to produce a 'Glacial Map', and accompanying geographic information system (GIS) database, of features related to the last (Devensian) British Ice Sheet. The map (1:625 000) is included in a folder and GIS data are freely available by web download (http://www.shef.ac.uk/geography/staff/clark_chris/britice.html). Emphasis is on information that constrains the last ice sheet. The following are included: moraines, eskers, drumlins, meltwater channels, tunnel valleys, trimlines, limit of key glacigenic deposits, glaciolacustrine deposits, ice-dammed lakes, erratic dispersal patterns, shelf-edge fans and the Loch Lomond Readvance limit of the main ice cap. The GIS contains over 20 000 features split into thematic layers (as above). Individual features are attributed such that they can be traced back to their published sources. Given that the published sources of information that underpin this work were derived by piecemeal effort over 150 years, then our main caveat is of data consistency and reliability. It is hoped that this compilation will stimulate greater scrutiny of published data, assist in palaeoglaciological reconstructions and facilitate use of field evidence in numerical ice-sheet modelling. It may also help direct field workers in their future investigations.     

Fluctuations of the Fennoscandian Ice Sheet recorded in the anisotropy of magnetic susceptibility of periglacial loess from Ukraine

The azimuth of imbrication of minimum magnetic susceptibility axes in the youngest loess from Ukraine defines prevailing wind directions during aeolian sedimentation. It changes along the studied sections. These changes can be directly correlated with the fluctuations of the Fennoscandian Ice Sheet. The northern and northeastern winds noted in the loess succession separated by a period when southwestern to southeastern winds were predominant may be correlated with two main phases of ice‐sheet advance during the Last Glacial Maximum. The ice‐sheet advances towards the areas of loess deposition generated katabatic winds that influenced aeolian sedimentation in the periglacial zone. A period of relatively stable wind directions during a younger phase of the Last Glacial Maximum was interrupted by periods with more chaotic wind regime most probably caused by fluctuations of the Fennoscandian Ice Sheet during its retreat from the peri‐Baltic part of Europe. These intervals occur where initial soils developed. The distribution of anisotropy of magnetic susceptibility axes defined along the periglacial loess sections from central and eastern Europe can serve to constrain fluctuations of the Fennoscandian Ice Sheet.     

The Bothnian Sea ice stream: early Holocene retreat dynamics of the south‐central Fennoscandian Ice Sheet

The Gulf of Bothnia hosted a variety of palaeo‐glaciodynamic environments throughout the growth and decay of the last Fennoscandian Ice Sheet, from the main ice‐sheet divide to a major corridor of marine‐ and lacustrine‐based deglaciation. Ice streaming through the Bothnian and Baltic basins has been widely assumed, and the damming and drainage of the huge proglacial Baltic Ice Lake has been implicated in major regional and hemispheric climate changes. However, the dynamics of palaeo‐ice flow and retreat in this large marine sector have until now been inferred only indirectly, from terrestrial, peripheral evidence. Recent acquisition of high‐resolution multibeam bathymetry opens these basins up, for the first time, to direct investigation of their glacial footprint and palaeo‐ice sheet behaviour. Here we report on a rich glacial landform record: in particular, a palaeo‐ice stream pathway, abundant traces of high subglacial meltwater volumes, and widespread basal crevasse squeeze ridges. The Bothnian Sea ice stream is a narrow flow corridor that was directed southward through the basin to a terminal zone in the south‐central Bothnian Sea. It was activated after initial margin retreat across the Åland sill and into the Bothnian basin, and the exclusive association of the ice‐stream pathway with crevasse squeeze ridges leads us to interpret a short‐lived stream event, under high extension, followed by rapid crevasse‐triggered break‐up. We link this event with a c. 150‐year ice‐rafted debris signal in peripheral varved records, at c. 10.67 cal. ka BP. Furthermore, the extensive glacifluvial system throughout the Bothnian Sea calls for considerable input of surface meltwater. We interpret strongly atmospherically driven retreat of this marine‐based ice‐sheet sector.     

Refining the ice flow chronology and subglacial dynamics across the migrating Labrador Divide of the Laurentide Ice Sheet with age constraints on deglaciation

The Laurentide Ice Sheet was characterized by a dynamic polythermal base. However, important data and knowledge gaps have led to contrasting reconstructions in areas such as the Labrador Ice Divide. In this study, detailed fieldwork was conducted at the southeastern edge of a major landform boundary to resolve the relative ice flow chronology and constrain the evolution of the subglacial dynamics, including the migration and collapse of the Labrador Ice Divide. Surficial mapping and analysis of 94 outcrop‐scale ice flow indicators were used to develop a relative ice flow chronology. ¹⁰Be exposure ages were used with optical ages to confine the timing of deglaciation within the study area. Four phases of ice flow were identified. Flow 1 was a northeasterly ice flow preserved under non‐erosive subglacial conditions associated with the development of an ice divide. Flow 2 was a northwest ice flow, which we correlate to the Ungava Bay Ice Stream and led to a westward migration of the ice divide, preserving Flow 2 features and resulting in Flow 3's eastward‐trending indicators. Flow 4 is limited to sparse fine striations within and around the regional uplands. The new optical ages and ¹⁰Be exposure ages add to the regional geochronology dataset, which further constrains the timing of ice margin retreat in the area to around 8.0 ka. Copyright © 2019 The Authors. Journal of Quaternary Science Published by John Wiley & Sons Ltd.     

Deglaciation history of the southwestern Fennoscandian Ice Sheet between 15 and 13 14C ka BP

Analysis of 2D and 3D seismic records from the continental shelf off western Norway, in combination with chronological constraints from ¹⁴C dates, has led to a model for the glacial development in these shelf areas between c. 15 and 13 ¹⁴C ka BP. On the shallow Måløy Plateau adjacent to the Norwegian Channel, iceberg scours are preserved below a prominent moraine ridge, which by correlation to the Norwegian Channel indicate ice retreat at c. 15 ¹⁴C ka BP. Subsequently, the ice advanced across the scoured surface and deposited a till sheet before stabilizing to deposit a prominent moraine, termed the Bremanger Moraine. Based on location on the shelf, seismic stratigraphy, morphology and C dates the Bremanger Moraine is correlated with a significant moraine on the continental shelf off Trøndelag. We suggest that these features are products of a regional glacial event, the Bremanger Event, dated to <15–13.3 ¹⁴C ka BP. The Bremanger Event is probably a result of the deteriorating climatic conditions in the NE Atlantic during Heinrich event 1.     

东南极内陆格罗夫山地区冰厚及冰下地形特征

中国第30次南极科学考察队格罗夫山分队(CHINARE30, 2013-2014年)利用雪地车载深层探冰雷达在东南极格罗夫山地区开展了测线总长度超过200 km大范围、高分辨率的冰厚及冰下地形调查, 获得了哈丁山北部和萨哈罗夫岭与阵风悬崖之间详细的冰厚及冰下地形特征. 通过对雷达数据分析表明, 哈丁山北部区域平均冰厚为580 m, 最大冰厚超过1 000 m, 出现在该区域的东北方向, 而东南方向冰厚相对较小; 萨哈罗夫岭与阵风悬崖之间区域的平均冰厚为610 m, 最大冰厚超过1 100 m, 该区域槽谷发育十分成熟, 槽谷形态近似呈U型. 通过对雷达剖面影像的筛选和分析, 推测在格罗夫山地区可能存在2个液态冰下湖泊.     

Retreat pattern of the Cordilleran Ice Sheet in central British Columbia at the end of the last glaciation reconstructed from glacial meltwater landforms

The Cordilleran Ice Sheet (CIS) covered much of the mountainous northwestern part of North America at least several times during the Pleistocene. The pattern and timing of its growth and decay are, however, poorly understood. Here, we present a reconstruction of the pattern of ice‐sheet retreat in central British Columbia at the end of the last glaciation based on a palaeoglaciological interpretation of ice‐marginal meltwater channels, eskers and deltas mapped from satellite imagery and digital elevation models. A consistent spatial pattern of high‐elevation (1600–2400 m a.s.l.), ice‐marginal meltwater channels is evident across central British Columbia. These landforms indicate the presence of ice domes over the Skeena Mountains and the central Coast Mountains early during deglaciation. Ice sourced in the Coast Mountains remained dominant over the southern and east‐central parts of the Interior Plateau during deglaciation. Our reconstruction shows a successive westward retreat of the ice margin from the western foot of the Rocky Mountains, accompanied by the formation and rapid evolution of a glacial lake in the upper Fraser River basin. The final stage of deglaciation is characterized by the frontal retreat of ice lobes through the valleys of the Skeena and Omineca Mountains and by the formation of large esker systems in the most prominent topographic lows of the Interior Plateau. We conclude that the CIS underwent a large‐scale reconfiguration early during deglaciation and was subsequently diminished by thinning and complex frontal retreat towards the Coast Mountains.     

Geomorphology,sedimentology and minimum exposure ages of streamlined subglacial landforms in the NW Himalaya,India

Streamlined subglacial landforms that include drumlins in three study areas, the upper Chandra valley around Chandra Tal, the upper Spiti Valley and the middle Yunam Valley of the NW Himalaya of India were mapped and studied using geomorphic, sedimentological and geochronological methods. These streamlined subglacial landforms include a variety of morphological types, including: (i) half egg‐shaped forms; (ii) complex superimposed forms; (iii) dome‐shaped forms; (iv) inverse forms; and (v) flat‐topped symmetrical forms. Sedimentological data indicate that subglacial deformational processes are responsible for the formation of the streamlined subglacial landforms in the Chandra Tal and upper Spiti Valley study areas. In contrast, streamlined landforms in the middle Yunam Valley are the result of melt‐out and subglacial erosional processes. In the Yunam Valley study area, 11 new cosmogenic ¹⁰Be surface exposure ages were obtained for boulders inset into the crests of streamlined subglacial landforms and moraines, and also for a bedrock surface. The streamlined landforms date to 8–7 ka, providing evidence of an early Holocene valley glaciation, and older moraines date to ～17–15 and 79–52 ka, representing other significant valley glacial advances in the middle Yunam Valley. The subglacial landforms in the Chandra Valley provide evidence for a ≥300‐m‐thick Lateglacial glacier that advanced southeast, overtopping the Kunzum Range, and advancing into the upper Spiti Valley. The streamlined subglacial landforms in these study areas of the NW Himalaya highlight the usefulness of such landforms in developing glacial chronostratigraphy and for understanding the dynamics of Himalayan glaciation.     

Assessment of paleo-recharge under the Fennoscandian Ice Sheet and its impact on regional groundwater flow in the northern Baltic Artesian Basin using a numerical model

The study investigates the mechanism of glacial meltwater recharge under the Fennosciandian Ice Sheet during the last glacial maximum (LGM) and its impact on regional groundwater flow in the northern Baltic Artesian Basin (BAB) in Estonia and Latvia. The current hypothesis is that a flow reversal occurred in the BAB due to subglacial recharge during the LGM. This hypothesis is supported by an extensive dataset of geochemical and isotopic measurements in the groundwater of northern Estonia, exhibiting significant depletion in δ¹⁸O with respect to modern precipitation. To verify the consistency of this hypothesis and better understand groundwater flow dynamics during the LGM period, a numerical model is developed for this area. Two cross-sectional models have been created across the northern BAB, in which groundwater flow and the transport of δ¹⁸O have been simulated from the beginning of the LGM to present-day. Several simulations were performed with different subglacial boundary conditions, to investigate the uncertainty related to subglacial recharge of meltwater during the LGM and the subsequent flow reversal in the northern BAB. Several simulations provide a satisfying fit between computed and observed values of δ¹⁸O, which means that the hypothesis of subglacial recharge of meltwater is consistent with δ¹⁸O distribution. The numerical model suggests that preservation of meltwater in northern Estonia is controlled by confining layers and the proximity to the outcrop area of aquifers, located in the Gulf of Finland. The results also suggest that glacial meltwater has been preserved under the Baltic Sea in the Gulf of Riga.     

Morphology of Younger Dryas subglacial and ice-proximal submarine landforms, inner Vestfjorden, northern Norway

The sea-floor morphology of two pronounced across-fjord bedrock thresholds located at the mouths of Ofotfjorden and Tysfjorden, northern Norway, has been analysed based on swath bathymetry and seismic data. The Younger Dryas ice front was located here during the recession of one of the large palaeo-ice streams of the Fennoscandian Ice Sheet. The thresholds are several kilometres long and wide, rising to several hundred metres above the adjacent sea floor, and the slopes are steep, up to 25°. The Ofotfjorden threshold is draped by acoustically discontinuous to chaotic sediments partly infilling the bedrock relief. A pattern of well-developed, subglacial bedforms (e.g. crag-and-tail formations, drumlins and glacial lineations) on top of both thresholds suggests fast-flowing ice. A series of smaller transverse ridges is identified on both thresholds and probably records ice-front oscillations during the final deglaciation. The distal parts of the sediments have been remobilized by slides that occurred after glacial retreat from the thresholds. Earthquake activity due to the isostatic rebound following ice retreat from this area was the most likely triggering mechanism for the slides. The location of the ice front on a prominent bedrock threshold indicates that the basin configuration was important in locating the maximum position of the climatically induced re-advance, i.e. a topographic control on the maximum Younger Dryas position in the Ofotfjorden and Tysfjorden area is suggested.     

BRITICE Glacial Map,version 2: a map and GIS database of glacial landforms of the last British–Irish Ice Sheet

During the last glaciation, most of the British Isles and the surrounding continental shelf were covered by the British–Irish Ice Sheet (BIIS). An earlier compilation from the existing literature (BRITICE version 1) assembled the relevant glacial geomorphological evidence into a freely available GIS geodatabase and map (Clark et al. 2004: Boreas 33, 359). New high‐resolution digital elevation models, of the land and seabed, have become available casting the glacial landform record of the British Isles in a new light and highlighting the shortcomings of the V.1 BRITICE compilation. Here we present a wholesale revision of the evidence, onshore and offshore, to produce BRITICE version 2, which now also includes Ireland. All published geomorphological evidence pertinent to the behaviour of the ice sheet is included, up to the census date of December 2015. The revised GIS database contains over 170 000 geospatially referenced and attributed elements – an eightfold increase in information from the previous version. The compiled data include: drumlins, ribbed moraine, crag‐and‐tails, mega‐scale glacial lineations, glacially streamlined bedrock (grooves, roches moutonnées, whalebacks), glacial erratics, eskers, meltwater channels (subglacial, lateral, proglacial and tunnel valleys), moraines, trimlines, cirques, trough‐mouth fans and evidence defining ice‐dammed lakes. The increased volume of features necessitates different map/database products with varying levels of data generalization, namely: (i) an unfiltered GIS database containing all mapping; (ii) a filtered GIS database, resolving data conflicts and with edits to improve geo‐locational accuracy (available as GIS data and PDF maps); and (iii) a cartographically generalized map to provide an overview of the distribution and types of features at the ice‐sheet scale that can be printed at A0 paper size at a 1:1 250 000 scale. All GIS data, the maps (as PDFs) and a bibliography of all published sources are available for download from: https://www.sheffield.ac.uk/geography/staff/clark_chris/britice .     

冰雷达探测研究南极冰盖的进展与展望

南极冰盖是地球上最大的陆缘冰体,其物质收支和稳定性对全球气候变化和海平面升高有重要的影响。冰雷达,或称无线电回波探测,是冰川学家调查南极冰盖冰下特征的主要方法。在过去的50年里,冰雷达被广泛用于测量冰盖厚度、内部构造和冰下地貌,这些参数是计算冰盖体积和物质平衡、重建过去冰雪积累和消融率以及冰盖动力和沉积过程的基础。现在,冰雷达测量覆盖了南极绝大部分区域,极大地提升了人们对南极冰盖和全球系统间相互作用的理解。首先,简要介绍了冰雷达及其技术发展,然后着重评述了冰雷达在探测研究南极冰盖厚度和冰下地形、内部反射层、冰下湖和冰下水系、冰床粗糙度以及冰晶组构上的进展。最后,对未来冰雷达探测研究南极冰盖的前景进行了展望,并给出我国的现状。
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Regional reconstruction of subglacial hydrology and glaciodynamic behaviour along the southern margin of the Cordilleran Ice Sheet in British Columbia,Canada and northern Washington State,USA

Subglacial landsystems in and around Okanagan Valley, British Columbia, Canada are investigated in order to evaluate landscape development, subglacial hydrology and Cordilleran Ice Sheet dynamics along its southern margin. Major landscape elements include drumlin swarms and tunnel valleys. Drumlins are composed of bedrock, diamicton and glaciofluvial sediments; their form truncates the substrate. Tunnel valleys of various scales (km to 100s km length), incised into bedrock and sediment, exhibit convex longitudinal profiles, and truncate drumlin swarms. Okanagan Valley is the largest tunnel valley in the area and is eroded >300 m below sea level. Over 600 m of Late Wisconsin-age sediments, consisting of a fining-up sequence of cobble gravel, sand and silt fill Okanagan Valley. Landform–substrate relationships, landform associations, and sedimentary sequences are incompatible with prevailing explanations of landsystem development centred mainly on deforming beds. They are best explained by meltwater erosion and deposition during ice sheet underbursts.During the Late-Wisconsin glaciation, Okanagan Valley functioned as part of a subglacial lake spanning multiple connected valleys (few 100s km) of southern British Columbia. Subglacial lake development started either as glaciers advanced over a pre-existing sub-aerial lake (catch lake) or by incremental production and storage of basal meltwater. High geothermal heat flux, geothermal springs and/or subglacial volcanic eruptions contributed to ice melt, and may have triggered, along with priming from supraglacial lakes, subglacial lake drainage. During the underburst(s), sheetflows eroded drumlins in corridors and channelized flows eroded tunnel valleys. Progressive flow channelization focused flows toward major bedrock valleys. In Okanagan Valley, most of the pre-glacial and early-glacial sediment fill was removed. A fining-up sequence of boulder gravel and sand was deposited during waning stages of the underburst(s) and bedrock drumlins in Okanagan Valley were enhanced or wholly formed by this underburst(s).Subglacial lake development and drainage had an impact on ice sheet geometry and ice volumes. The prevailing conceptual model for growth and decay of the CIS suggests significantly thicker ice in valleys compared to plateaus. Subglacial lake development created a reversal of this ice sheet geometry where grounded ice on plateaus thickened while floating valley ice remained thinner (due to melting and enhanced sliding, with significant transfer of ice toward the ice sheet margin). Subglacial lake drainage may have hastened deglaciation by melting ice, lowering ice-surface elevations, and causing lid fracture. This paper highlights the importance of ice sheet hydrology: its control on ice flow dynamics, distribution and volume in continental ice masses.     

Atmospheric trace metals and sulfate in the Greenland Ice Sheet

Chemical analyses of surface snow and dated deep ice core samples from central Greenland suggest that Zn, Pb and sulfate are presently being deposited there at two to three times the natural rates. No recent increases in Cd or V concentrations were observed. Pre-1900 ice shows no measurable effect of the activities of man and represents a good natural aerosol baseline. High enrichment factors relative to average crustal material were observed for Zn, Pb, Cd and sulfate in all samples indicating a natural source other than continental dust is responsible. A high temperature process or vapor phase origin for these enriched elements, possibly volcanism, seems likely.     

The Keiva ice marginal zone on the Kola Peninsula, northwest Russia: a key component for reconstructing the palaeoglaciology of the northeastern Fennoscandian Ice Sheet

One of the key elements in reconstructing the palaeoglaciology of the northeastern sector of the Fennoscandian Ice Sheet is the Keiva ice marginal zone (KIZ) along the southern and eastern coast of Kola Peninsula, including the Keiva I and II moraines. From detailed geomorphological mapping of the KIZ, primarily using aerial photographs and satellite images, combined with fieldwork, we observed the following. (1) The moraines display ice contact features on both the Kola side and the White Sea side along its entire length. (2) The Keiva II moraine is sloping along its length from c. 100 m a.s.l. in the west (Varzuga River) to c. 250 m a.s.l. in the east (Ponoy River). (3) The KIZ was partly overrun and fragmented by erosive White Sea-based ice after formation. From these observations we conclude that the KIZ is not a synchronous feature formed along the lateral side of a White Sea-based ice lobe. If it was, the moraines should have a reversed slope. Rather, we interpret it to be time transgressive, formed at a northeastward-migrating junction between a warm-based Fennoscandian Ice Sheet expanding from the west and southwest into the White Sea depression, and a sluggish cold-based ice mass centred over eastern Kola Peninsula. In contrast to earlier reconstructions, we find it unlikely that an ice expansion of this magnitude was a mere re-advance during the deglaciation. Instead, we propose that the KIZ was formed during a major expansion of a Fennoscandian Ice Sheet at a time pre-dating the Last Glacial Maximum.     

Ice sheet extent and early deglacial history of the southwestern sector of the Greenland Ice Sheet

The offshore and coastal geomorphology of southwest Greenland records evidence for the advance and decay of the Greenland Ice Sheet during the Last Glacial Maximum. Regional ice flow patterns in the vicinity of Sisimiut show an enlarged ice sheet that extended southwestwards on to the shelf, with an ice stream centred over Holsteinsborg dyb. High level periglacial terrain composed of blockfield and tors is dated to between 101 and 142 ka using ²⁶Al and ¹⁰Be cosmogenic exposure ages. These limit the maximum surface elevation of the Last Glacial Maximum ice sheet in this part of southwest Greenland to ca 750–810 m asl, and demonstrate that terrain above this level has been ice free since MIS 6. Last Glacial Maximum ice thickness on the coast of ca 700 m implies that the ice sheet reached the mid to outer continental shelf edge to form the Outer Hellefisk moraines. Exposure dates record ice surface thinning from 21.0 to 9.8 ka, with downwasting rates varying from 0.06 to 0.12 m yr⁻¹. This reflects strong surface ablation associated with increased air temperatures running up to the Bølling Interstadial (GIS1e) at ca 14 ka, and later marine calving under high sea levels. The relatively late retreat of the Itilleq ice stream inland of the present coastline is similar to the pattern observed at Jakobshavn Isbræ, located 250 km north in Disko Bugt, which also retreated from the continental shelf after ca 10 ka. We hypothesise that the ice streams of West Greenland persisted on the inner shelf until the early Holocene because of their considerable ice thickness and greater ice discharge compared with the adjacent ice sheet.