detrital carbonate peaks on the labrador shelf a 13 7 ka template for freshwater forcing from the hudson strait outlet of the laurentide ice sheet into the subpolar gyre abstract the laurentide ice sheet lis was a large dynamic ice sheet in the early holocene the glacial events through hudson strait leading to its eventual demise are recorded in the well dated labrador shelf core md99 2236 from the cartwright saddle we develop a detailed history of the timing of ice sheet discharge events from the hudson strait outlet of the lis during the holocene using high resolution detrital carbonate ice rafted detritus ird 18o and sediment color data eight detrital carbonate peaks dcps associated with ird peaks and light oxygen isotope events punctuate the md99 2236 record between 11 5 and 8 0 ka we use the stratigraphy of the dcps developed from md99 2236 to select the appropriate r to calibrate the ages of recorded glacial events in hudson bay and hudson strait such that they match the dcps in md99 2236 we associate the eight dcps with h0 gold cove advance noble inlet advance initial retreat of the hudson strait ice stream hsis from hudson strait opening of the tyrrell sea and drainage of glacial lakes agassiz and ojibway the opening of foxe channel and retreat of glacial ice from foxe basin are represented by a shoulder in the carbonate data r of 350 years applied to the radiocarbon ages constraining glacial events h0 through the opening of the tyrell sea provided the best match with the md99 2236 dcps r values and ages from the literature are used for the younger events a very close age match was achieved between the 8 2 ka cold event in the greenland ice cores dcp7 8 15 ka bp and the drainage of glacial lakes agassiz and ojibway our stratigraphic comparison between the dcps in md99 2236 and the calibrated ages of hudson strait bay deglacial events shows that the retreat of the hsis the opening of the tyrell sea and the catastrophic drainage of glacial lakes agassiz and ojibway at 8 2 ka are separate events that have been combined in previous estimates of the timing of the 8 2 ka event from marine records sw iceland shelf core md99 2256 documents freshwater entrainment into the subpolar gyre from the hudson strait outlet via the labrador north atlantic and irminger currents the timing of freshwater release from the lis hudson strait outlet in md99 2236 matches evidence for freshwater forcing and lis icebergs carrying foreign minerals to the sw iceland shelf between 11 5 and 8 2 ka the congruency of these records supports the conclusion of the entrainment of freshwater from the retreat of the lis through hudson strait into the subpolar gyre and provides specific time periods when pulses of lis freshwater were present to influence climate

chronology of late wisconsinan glaciation in middle north america abstract we propose a chronology of late wisconsinan glacial fluctuations in middle north america from alberta to wisconsin based on radiocarbon dates derived solely from wood previous chronologies of the southwestern margin of the north american continental ice sheet have depended to a considerable degree on radiocarbon dates from fine grained organic sediment this material is commonly contaminated with older carbon resulting in chronologic confusion by using only dates from wood much of the confusion disappears however because of the scarcity of wood dates only four of the sixteen identified fluctuations are accurately dated an advance into iowa about 14 000 to 13 500 bp an advance into south dakota and iowa about 12 300 bp an advance into the lake michigan basin about 11 700 bp and an advance into the lake superior basin about 9900 bp in addition the beginning of late wisconsinan glaciation before 20 000 bp is fairly well documented none of the fluctuations in the western part of the region are accurately dated

the quaternary lancaster sound trough mouth fan nw baffin bay the development of the lancaster sound trough mouth fan tmf and glacial history in arctic canada were studied using a high resolution seismic profile across the entire fan and two piston cores stacked tills separated by erosion surfaces on the shelf pass seaward through till deltas into thick transparent glacigenic debris flow gdf deposits on the slope separated by thin well stratified glaciomarine layers an age model was built by ties to the ocean drilling program site 645 the deepest gdf on the seismic profile was indicative of the onset of shelf crossing glaciation in the early pleistocene the transition of the growth of lancaster sound tmf from an aggradational sequence unit m to an aggradational progradational sequence unit f occurred at the middle pleistocene transition in glacial cyclicity in the most recent glacial cycle gdf sheets were deposited during heinrich events 4 and 2 according to the correlation of the main detrital carbonate beds in two piston cores the outmost till wedge reflects the maximum advance of the grounding glacier far seaward of previously proposed last glacial maximum ice extent copyright 2011 john wiley sons ltd

11 000 yrs of environmental change in the northwest passage a multiproxy core record from central parry channel canadian high arctic article i nfo piston core 97022 004pc 74 48 0 n 97 05 9 w 267 m water depth represents a rare paleoenvironmental archive from the understudied west central canadian arctic archipelago lithological biogeochemical and microfossil dinoflagellate cysts non pollen palynomorphs benthic and planktonic foraminifera character istics in combination with a chronostratigraphy based on seventeen radiocarbon dates show seven promi nent paleoenvironmental episodes since the end of the last regional glaciation the basal diamict zone i records decoupling of previously grounded glacial ice followed by ice proximal conditions zone iia commencing at 10 8 cal ka bp age depth model extrapolation after an interval of pervasive sea ice zone iib ice distal conditions are established zone iic although sparse microfossils are present in glaciomarine sediments zone ii noticeable biological activity with heightened abundances and diversities across all groups begins in the postglacial zone iii 10 3 10 0 cal ka bp when planktonic foraminifera neogloboquadrina pachyderma appear as planktonics are excluded from the study area today due to shal low inter channel sills this likely signals the inflow of relatively warm and saline atlantic derived arctic intermediate water below 250 m presumably facilitated by glacio isostatically enhanced deglacial water depths the subsequent zone iv 10 0 7 0 cal ka bp characterized by heightened biological productivity in both plankton and benthos and reduced seasonal sea ice cover may correspond to a previously proposed holocene thermal maximum this apparent amelioration ends by the mid holocene zone v 7 0 5 7 cal ka bp when arctic intermediate water is excluded from the study area and water depths approach modern values high arctic conditions with seasonal sea ice cover a circulation dominated by arctic ocean surface water and microfossil assemblages similar to modern are found from 5 7 cal ka bp onwards zones vi vii as only minor environmental fluctuations are apparent during the late holocene shorter term climatic episodes e g little ice age are not recognized in this record

late wisconsinan glaciation and postglacial relative sea level change on western banks island canadian arctic archipelago abstract the study revises the maximum extent of the northwest laurentide ice sheet lis in the western canadian arctic archipelago caa during the last glaciation and documents subsequent ice sheet retreat and glacioisostatic adjustments across western banks island new geomorphological mapping and maximum limiting radiocarbon ages indicate that the northwest lis inundated western banks island after 31 14 c ka bp and reached a terminal ice margin west of the present coastline the onset of deglaciation and the age of the marine limit 22 40 m asl are unresolved ice sheet retreat across western banks island was characterized by the withdrawal of a thin cold based ice margin that reached the central interior of the island by 14 cal ka bp the elevation of the marine limit is greater than previously recognized and consistent with greater glacioisostatic crustal unloading by a more expansive lis these results complement emerging bathymetric observations from the arctic ocean which indicate glacial erosion during the last glacial maximum lgm to depths of up to 450 m

lake agassiz during the younger dryas abstract lake agassiz was ponded on the northward sloping surface of the hudson bay and arctic ocean basins as the laurentide ice sheet retreated the level of lake agassiz abruptly fell 12 9 cal 11 14 c ka bp exposing the lake floor over a large region for 1000 yr the routing of overflow during this moorhead low water period is uncertain and there is evidence on the continent and in ocean basins for both an easterly route through the great lakes st lawrence to the north atlantic and for a northwesterly route through the clearwater athabasca mackenzie system to the arctic ocean the moorhead low water phase coincides with the younger dryas cooling and a cause effect relationship has been proposed by attributing a change in ocean thermohaline circulation to the re routing of lake agassiz freshwaters from the gulf of mexico to more northern oceans paleoclimatic interpretations from ecosystems in lake sediments in the region and a simple calculation of the paleohydrological budget of lake agassiz indicate that the climate remained wet and cool throughout the yd in this region and was not warm nor dry enough to allow evaporative loss to offset the influx of meltwater and precipitation thus the moorhead phase resulted from changes in the outlet that carried overflow

the quaternary geology of the labrador shelf the regional distribution of glacial and postglacial sediments on the labrador shelf was mapped by medium and high resolution seismics and shows a succession of tills that extend to the continental shelf edge at approximately 600 m the uppermost of these tills is less laterally extensive and does not occur on the labrador shelf bank tops in water depths less than 160 m we infer that the last glacial episode was less extensive than the preceding ones glaciomarine stratified silts and postglacial muds are found overlying the upper till in the deep basins that separate the banks the upper till glaciomarine silts and postglacial muds are interpreted to constitute one complete deglacial cycle radiocarbon age determination suggests this deglacial sequence began at approximately 20 000 years bp on the northernmost labrador shelf and hudson strait seismic evidence indicates a late glacial readvance locally deposited a till that overlies the glaciomarine sediments associated with the last glacial advance

late wisconsinan glaciation of amund and ellef ringnes islands nunavut evidence for the configuration dynamics and deglacial chronology of the northwest sector of the innuitian ice sheet geomorphic and chronologic evidence from amund and ellef ringnes islands documents the configuration dynamics and collapse of the northwest sector of the innuitian ice sheet these data record the inundation of the ringnes islands by northwestward flowing ice from divides spanning the alpine and lowland sectors of the innuitian ice sheet ice flow indicators and granite dispersal along eastern amund ringnes island suggest massey sound was filled by an ice stream discharging coalescent alpine and lowland ice from norwegian bay in contrast the interior of amund ringnes island was overridden by predominantly non erosive granite free ice from a divide in the lowland sector of the ice sheet glacial landforms on ellef ringnes island record coverage by largely non erosive ice but it remains uncertain whether these features relate to northward flowing lowland ice or a cold based local ice cap deglaciation of the ringnes islands commenced 10 000 14c years ago deglacial dates between 9 7 and 9 2 ka bp rec

a proposed diachronic revision of late quaternary time stratigraphic classification in the eastern and northern great lakes area abstract a succession of stratigraphic codes 1933 1961 1983 has guided attempts to refine classifications and naming of stratigraphic units for quaternary deposits of the great lakes region the most recent classifications for the late quaternary of the lake michigan lobe 1968 and the eastern great lakes 1972 have been widely used but later work has created the need for revision an attempt has been made to integrate the two previous classifications following the diachronic system of the 1983 code of stratigraphic nomenclature a new nomenclature for the higher more broadly recognized units was presented in 1997 we here present the diachronic nomenclature for finer subdivisions recognized in the eastern and northern great lakes following the interglacial sangamon episode the three parts of the wisconsin episode are further subdivided as follows the ontario subepisode former early wisconsinan comprises the greenwood willowvale and guildwood phases the elgin subepisode former middle wisconsinan comprises the port talbot brimley and farmdale phases and the michigan subepisode former late wisconsinan consists of nissouri erie port bruce mackinaw port huron two creeks onaway gribben marquette abitibi and driftwood phases succeeding interglacial time to the present is the hudson episode

chronology and provenance of last glacial peoria loess in western iowa and paleoclimatic implications geologic archives show that the earth was dustier during the last glacial period one model suggests that increased gustiness stronger more frequent winds enhanced dustiness we tested this at loveland iowa one of the thickest deposits of last glacial age peoria loess in the world based on k rb and ba rb loess was derived not only from glaciogenic sources of the missouri river but also distal loess from non glacial sources in nebraska optically stimulated luminescence osl ages provide the first detailed chronology of peoria loess at loveland deposition began after 27 ka and continued until 17 ka osl ages also indicate that mass accumulation rates mars of loess were not constant mars were highest and grain size was coarsest during the time of middle peoria loess accretion 23 ka when 10 m of loess accumulated in no more than 2000 yr and possibly much less the timing of coarsest grain size and highest mar indicating strongest winds coincides with a summer insolation minimum at high latitudes in north america and the maximum southward extent of the laurentide ice sheet these observations suggest that increased dustiness during the last glacial period was driven largely by enhanced gustiness forced by a steepened meridional temperature gradient

deglaciation basin formation and post glacial climate change from a regional network of sediment core sites in ohio and eastern indiana abstract many paleoclimate and landscape change studies in the american midwest have focused on the late glacial and early holocene time periods 16 11 ka but little work has addressed landscape change in this area between the last glacial maximum and the late glacial 22 16 ka sediment cores were collected from 29 new lake and bog sites in ohio and indiana to address this gap the basal radiocarbon dates from these cores show that initial ice retreat from the maximal last glacial ice extent occurred by 22 ka and numerous sites that are 100 km inside this limit were exposed by 18 9 ka post glacial environmental changes were identified as stratigraphic or biologic changes in select cores the strongest signal occurs between 18 5 and 14 6 ka these midwestern events correspond with evidence to the northeast suggesting that initial deglaciation of the ice sheet and ensuing environmental changes were episodic and rapid significantly these changes predate the onset of the bolling postglacial warming 14 8 ka as recorded by the greenland ice cores thus deglaciation and landscape change around the southern margins of the laurentide ice sheet happened 7 ka before postglacial changes were felt in central greenland

revision of the nw laurentide ice sheet implications for paleoclimate the northeast extremity of beringia and arctic ocean sedimentation abstract for the past half century reconstructions of north american ice cover during the last glacial maximum have shown ice free land distal to the laurentide ice sheet primarily on melville and banks islands in the western canadian arctic archipelago both islands reputedly preserve at the surface multiple laurentide till sheets together with associated marine and lacustrine deposits recording as many as three pre late wisconsinan glaciations the northwest corner of banks island was purportedly never glaciated and is trimmed by the oldest and most extensive glaciation banks glaciation considered to be of matuyama age 780 ka bp inside the limit of banks glaciation younger till sheets are ascribed to the thomsen glaciation pre sangamonian and the amundsen glaciation early wisconsinan stade the view that the western canadian arctic archipelago remained largely ice free during the late wisconsinan is reinforced by a recent report of two woolly mammoth fragments collected on banks and melville islands both dated to 22 ka bp these dates imply that these islands constitute the northeast extremity of beringia a fundamental revision of this model is now warranted based on widespread fieldwork across the adjacent coastlines of banks and melville islands including new dating of glacial and marine landforms and sediments on dundas peninsula southern melville island ams 14 c dates on ice transported marine molluscs within the most extensive laurentide till yield ages of 25 49 ka bp these dates require that late wisconsinan ice advanced northwestward from visount melville sound excavating fauna spanning marine isotope stage 3 laurentide ice that crossed dundas peninsula 300 m asl coalesced with melville island ice occupying liddon gulf coalescent laurentide and melville ice continued to advance westward through m clure strait depositing granite erratics at 235 m asl that require grounded ice in m clure strait as do streamlined bedforms on the channel floor deglaciation is recorded by widespread meltwater channels that show both the initial separation of laurentide and melvile ice and the successive retreat of laurentide ice southward across dundas peninsula into viscount melville sound sedimentation from these channels deposited deltas marking deglacial marine limit forty dates on shells collected from associated glaciomarine rhythmites record near synchronous ice retreat from m clure strait and dundas peninsula to north central victoria island 11 5 ka bp along the adjacent coast of banks island deglacial shorelines also record the retreat of laurentide ice both eastward through m clure strait and southward into the island s interior the elevation and age 11 5 ka bp of deglacial marine limit there is fully compatible with the record of ice retreat on melville island the last retreat of ice from mercy bay northern banks island previously assigned to northward retreat into m clure strait during the early wisconsinan is contradicted by geomorphic evidence for southward retreat into the island s interior during the late wisconsinan this revision of the pattern and age of ice retreat across northern banks island results in a significant simplification of the previous quaternary model our observations support the amalgamation of multiple till sheets previously assigned to at least three pre late wisconsinan glaciations into the late wisconsinan this revision also removes their formally named marine transgressions and proglacial lakes for which evidence is lacking erratics were also widely observed armouring meltwater channels originating on the previously proposed never glaciated landscape an extensive late wisconsinan laurentide ice sheet across the western canadian arctic is compatible with similar evidence for extensive laurentide ice entering the richardson mountains yukon farther south and with the innuitian ice sheet to the north widespread late wisconsinan ice in a region previously thought to be too arid to sustain it has important implications for paleoclimate ice sheet modelling arctic ocean ice and sediment delivery and clarifying the northeast limit of beringia

les ecoulements et le transport glaciaires dans la partie septentrionale du nunavik quebec in northern nunavik orientation of ice flow features and dispersal patterns of lithologic indicators suggest three major regional ice flow phases the oldest ice flow phase is related to the buildup of an outflow centre the ungava centre over the high plateaux located north of the povungnituk hills during that event probably of early wisconsinan age ice flows towards the south and southeast carried cape smith belt rock fragments over a maximum distance of 65 km the second phase is characterized by ice flow mainly towards the north from an outflow ice centre located south of the studied area payne centre the most recent ice flow phase is related to the presence of the northern extension of the new quebec ice divide from that northwest trending ice divide extending between ivujivik and lake nantais ice was flowing outward on the peninsula the orientation of most of the ice flow features and dispersal patterns is associated with that last ice flow north of the cape smith belt glacial transport was at least 70 km in hudson strait ice flow and glacial transport data support the existence of an eastward flowing ice stream

early holocene deglaciation of the eastern coast of hudson bay abstract the final stage of deglaciation of hudson bay was a major holocene catastrophic event marked by the drainage of lake agassiz ojibway at 8 47 ka cal bp and the rapid collapse of the laurentide ice sheet previous work undertaken in the nastapoka river area eastern hudson bay demonstrated that during the relative sea level highstand that shortly followed the drainage of the lake the western margin of the quebec labrador ice sector rapidly retreated eastward to reach a stillstand position in a coastal hill range in this study an analysis of landsat 7tm images has allowed a mapping of large scale glacial landforms outwash deposits eskers flutings and de geer and rogen moraines between kuujjuaraapik se hudson bay and puvirnituq ne hudson bay the key results from this mapping are i ice contact outwash deposits mapped along the entire arc shaped coastline of the eastern hudson bay outline a major ice stillstand phase in the coastal hills that extended at least from kuujjuaraapik to inukjuak the presence of these hills allowed a stabilisation of the ice margin that led to the accumulation of thick and extensive ice contact submarine fans ii the position of these deposits on the down ice side west of large sets of flutings indicates an important phase of sediment delivery by a rapid ice flow phase toward a marine based ice margin iii a second system of outwash deposits observed farther inland indicates a subsequent phase of stabilisation of the ice margin during its retreat toward central quebec labrador

evidence from nipawin bay in frobisher lake saskatchewan for three highstand and three lowstand lake phases between 9 and 10 10 1 and 11 5 cal ka bp abstract this investigation offers new insight into the drainage history of the northwest arm of lake agassiz basin generally recognized as a key discharge route of glacial meltwater to the oceans sediment lithology ams radiocarbon chronology and pollen stratigraphy from the basal 1 7 m of core in nipawin bay frobisher lake saskatchewan provide evidence for three highstand and three lowstand lake phases between 9 and 10 10 2 and 11 5 cal ka bp banded clay at the base of the core represents highstand phase 1 and a tundra pollen signal in the clay suggests deposition during the latter part of the younger dryas yd cold interval overlying the banded clay is stratified fine to medium coarse sand with plant detritus that is ams dated at 10 000 35 11 352 11 603 cal bp pollen and plant macrofossils in the sand indicate spruce woodland vegetation and a climate warmer than the previous yd cold interval the plant detritus rich sand is interpreted as a shallow water lagoonal deposit that accumulated during lowstand phase 1 a thin layer of clay on top of the sand equates with higher water levels highstand phase 2 while a second stratified sand deposit above the clay layer denotes a second low water interval lowstand phase 2 overlying the upper sand unit is a glaciolacustrine sequence of discontinuous laminated grey clay laminated to faintly laminated reddish grey clay and grey clay rhythmites which represents a re occupation of nipawin bay during highstand phase 3 coinciding with this highstand was a reversion to a colder climate the preboreal oscillation bpo which lasted to the end of glaciolacustrine deposition cessation of clay rhythmite deposition and onset of fes deposition when nipawin bay became a separate sedimentary basin for the first time mark a major drawdown of water levels lowstand phase 3 that corresponds with opening of new outlets east to lake superior basin following retreat of the 10 11 5 cal ka bp marquette re advance enhanced warming at this time would have provided optimal conditions for spruce to have taken advantage of the newly exposed lake bottom bordering nipawin bay and quickly migrate into the area tentative correlations are made between the highstands and lowstands and stages of the recently interpreted glacial lake churchill merged proglacial lakes mcmurray and meadow with a regional lake in the churchill river valley including possible northwestward drainage of glacial lake churchill and a possible merged glacial lake churchill lake agassiz

timing of advance and basal condition of the laurentide ice sheet during the last glacial maximum in the richardson mountains nwt abstract this study presents new ages for the northwest section of the laurentide ice sheet lis glacial chronology from material recovered from two retrogressive thaw slumps exposed in the richardson mountains northwest territories canada one study site located at the maximum glacial limit of the lis in the richardson mountains had calcite concretions recovered from aufeis buried by glacial till that were dated by u th disequilibrium to 18 500 cal yr bp the second site located on the peel plateau to the east yielded a fossil horse equus mandible that was radiocarbon dated to ca 19 700 cal yr bp these ages indicate that the peel plateau on the eastern flanks of the richardson mountains was glaciated only after 18 500 cal yr bp which is later than previous models for the global last glacial maximum lgm as the lis retreated the peel plateau around 15 000 cal yr bp following the age of the tutsieta phase we conclude that the presence of the northwestern margin of the lis at its maximum limit was a very short event in the western canadian arctic

the innuitian ice sheet configuration dynamics and chronology abstract portrayal of north american ice cover during the last glacial maximum is dominated by the laurentide ice sheet leaving little detail for the adjacent innuitian ice sheet iis four decades of geological fieldwork across the queen elizabeth islands now warrant specific treatment of the iis including its chronology configuration dynamics and retreat this reconstruction is relevant to the sedimentary history of the arctic ocean and to high latitude climate forcing the iis was composed of both an alpine and lowland sector the advance of the alpine sector occurred as recently as 19 14 c ka bp geological evidence configures outflow from alpine and lowland divides that produced several palaeo ice streams one extending northwestward across the canadian arctic archipelago to the polar continental shelf retreat of the iis commenced along its southwest margin 11 6 14 c ka bp however most of the ice sheet remained on the continental shelf during the younger dryas by 10 14 c ka bp marine based ice experienced widespread calving through the western and central archipelago in response to holocene warming and ongoing eustatic sea level rise the sea penetrated the eastern archipelago by 8 5 14 c ka bp gutting the alpine sector of the iis regional isobases record the glacioisostatic signature of the ice sheet and are congruent with the primary geological evidence the delayed buildup of the iis was out of phase with the growth of the laurentide ice sheet that occasioned climatic and glacio eustatic forcing in the innuitian region recent modelling experiments reinforce the hypothesis that growth of the laurentide ice sheet culminated in a split jet stream that temporarily favoured augmented precipitation and growth of the iis

major end moraines of younger dryas age on wollaston peninsula victoria island canadian arctic implications for paleoclimate and for formation of hummocky moraine some of the most extensive and massive end moraines of younger dryas age 11 10 14c ka bp yet recognized in north america occur on wollaston peninsula of victoria island on the western part of the peninsula numerous closely spaced end moraines formed in the interval starting 11 100 100 radiocarbon years ago and ending about 10 500 10 200 years ago net recession was generally slow throughout and was punctuated by moraine building and at least two readvances recession is mapped with a resolution that is approximately decadal the moraines form an orderly nested succession and are consistently associated with westward shedding of meltwater which formed a sequence of marine limit deltas we lack firm independent proxy climate evidence needed to assess whether these moraines formed because of cold younger dryas climate rather than because of controls such as topographic setting and water depth but climatic control seems probable the moraines evidently retain glacier ice cores as do most similarly

till stratigraphy and sedimentology at the margins of terrestrially terminating ice streams case study of the western canadian prairies and high plains complex glacigenic sediment sequences arranged in large arcuate assemblages and demarcating the lobate termini of fast ice flow corridors in western canada represent the terrestrial equivalents of the trough mouth fans of the marine palaeo ice stream record intensive sedimentological and stratigraphic investigations at a regional scale are employed in the interpretation of thick units or sequences of diamicton and two depositional scenarios are proposed first glacier marginal till thickening and the stacking of till wedges takes place in areas where an ice stream margin was stationary for short periods of time stacks of subglacial traction tills potentially record annual sub marginal incremental thickening at ice stream margins especially in areas where densely spaced recessional push moraines appear to record seasonal climatic forcing second proglacial lake and valley infilling with glacilacustrine rhythmites and mass flow diamictons is associated with the advection of sub marginal till into preglacial bedrock depressions and records ice stream marginal oscillations and debris influx into discontinuous proglacial lakes which act as subaqueous sediment sinks for materials that would otherwise have been used to construct sub marginal till wedges and moraines glacial overriding modifies the deposits produced in both scenarios through deformation and the dislocation and entrainment of bedrock and sediment rafts at the margins of bedrock valley walls resulting in the widespread development of glacitectonites and megablocks the occurrence of the thickest deposits as preglacial valley fills may be significant in groundwater siphoning away from subglacial deforming layers and the promotion of sticky spots

glacial terrain zone analysis of a fragmented paleoglaciologic record southeast keewatin sector of the laurentide ice sheet abstract a highly fragmented subglacial landscape is recognized at the regional to sub regional scales in northeastern manitoba canada in the southeast keewatin sector a core region of the laurentide ice sheet new field based ice flow indicator measurements mapping of subglacial landforms remote sensing and aerial photograph and a re examination of previously published data from an 8100 km2 area in northeastern manitoba show that the preserved subglacial record reflects a complex and potentially long glacial history five streamlined landform flowsets are mapped a much higher degree of inheritance in the field based ice flow indicator data than previously reported allows for recognition of multiple ice flow phases analysis of the characteristics of the subglacial landscape combined with a relative age chronology established with field based indicators led to the recognition of disjoint zones with internally consistent glacial histories termed glacial terrain zones gtz these gtz were then classified as 1 relict glacial 2 palimpsest or 3 deglacial in nature our data suggest that while the southern keewatin sector was affected by regional ice divide translocation this alone cannot explain the fragmented high inheritance landscape we suggest that the subglacial landscape was continually evolving and subject to spatio temporal variations in intensity of erosion transportation and or deposition throughout multiple glacial events subglacial bed mosaic preservation of relict and palimpsest terrain likely occurred under large sticky low erosion regions these regions could have formed by at least two different mechanisms heterogeneous switch from warm based to cold based ice or within a warm based subglacial environment from wet to stiff dewatered till establishment of the regionally extensive 700 km wide by at least 500 km long dendritic esker channel system may have caused rapid spatially variable dewatering of the substrate far back under the ice sheet the gtz approach integrates all available data e g flowsets and other landform data striations to advance our interpretation of the spatio temporal evolution of subglacial dynamics in areas where the degree of landscape inheritance and overprinting is spatially highly variable this mosaic may be a characteristic net effect of landscape evolution beneath the core regions of ice sheets

optical ages indicate the southwestern margin of the green bay lobe in wisconsin usa was at its maximum extent until about 18 500 years ago abstract samples for optical dating were collected to estimate the time of sediment deposition in small ice marginal lakes in the baraboo hills of wisconsin these lakes formed high in the baraboo hills when drainage was blocked by the green bay lobe when it was at or very near its maximum extent therefore these optical ages provide control for the timing of the thinning and recession of the green bay lobe from its maximum position sediment that accumulated in four small ice marginal lakes was sampled and dated difficulties with field sampling and estimating dose rates made the interpretation of optical ages derived from samples from two of the lake basins problematic samples from the other two lake basins south bluff and feltz basins responded well during laboratory analysis and showed reasonably good agreement between the multiple ages produced at each site these ages averaged 18 2 ka n 6 and 18 6 ka n 6 respectively the optical ages from these two lake basins where we could carefully select sediment samples provide firm evidence that the green bay lobe stood at or very near its maximum extent until about 18 5 ka the persistence of ice marginal lakes in these basins high in the baraboo hills indicates that the ice of the green bay lobe had not experienced significant thinning near its margin prior to about 18 5 ka these ages are the first to directly constrain the timing of the maximum extent of the green bay lobe and the onset of deglaciation in the area for which the wisconsin glaciation was named

sakami moraine quebec a 500 km long moraine without climatic control the laurentide ice sheet in eastern canada disintegrated step by step as evidenced by several morainic complexes although commonly interpreted as reflecting climatic events it seems probable that the disintegration simply related to changes in the dynamics of the ice margin without climatic control from the example of the late glacial sakami moraine in quebec formed during the drainage of lake ojibway into the tyrrell sea the concept of a re equilibration moraine already briefly proposed is developed the construction of such a moraine implies a stabilization of the ice front when the glacier margin previously floating or calving in a lacustrine or marine basin is suddenly grounded because of topographic blocking or abrupt drop of water level due to drainage the glacial retreat then pauses until the equilibrium profile of the ice is restored and a morainic accumulation can form during the halt

rapid early holocene ice retreat in west greenland abstract the possible demise of the greenland ice sheet and its effect on global sea level rank among the most serious climate threats to society to improve our knowledge about the future behaviour of the ice margin we studied the ice sheet s response to early holocene warming in west greenland using 47 cosmogenic 10 be exposure ages 26 optically stimulated luminescence ages as well as 15 new and 28 previously published radiocarbon ages paired bedrock and boulder ages show that the entire area was covered by warm based ice during the last glacial maximum lgm although glacial erosion was insufficient to completely remove the upper rock surface containing 10 be inherited from a previous period of exposure in bedrock samples above an elevation of 800 m our compilation of 10 be and 14 c ages demonstrates that the ice sheet retreated from the outer coast to the present ice margin between c 11 4 and 10 4 cal ka bp in the godthabsfjord system and between 10 7 0 6 and 10 1 0 4 ka ago in buksefjord whereas the coast at sermilik became ice free at c 10 5 cal ka bp we find no significant changes in the retreat rates between the deep godthabsfjord system and the buksefjord sermilik region which is characterized by only a few narrow and shallow fjords however deglaciation was initiated c 700 900 years earlier in the godthabsfjord system indicating that the deep fjords probably triggered land based deglaciation by dynamic ice loss leading to an overall rapid early holocene ice retreat and drawdown of the ice sheet in west greenland these results demonstrate that even if there was a topographic control on the onset of deglaciation fast ice retreat is not restricted to deep fjord systems but may occur independently of the topographic setting

rapid last deglacial thinning and retreat of the marine terminating southwestern greenland ice sheet abstract marine terminating outlet glaciers are a major source of modern ice loss from the greenland ice sheet gris but their role in gris retreat during the last deglaciation is not well constrained here we develop deglacial outlet glacier retreat chronologies for four regions in southwest and south greenland to improve understanding of spatial variations in centennial to millennial scale ice loss under a warming climate we calculate 10be surface exposure ages of boulders located in fjords near the towns of qaqortoq paamiut nuuk and sisimiut our northernmost study site sisimiut deglaciated earliest at 18 ka to 15 ka with an average thinning rate of 0 1 0 3 m yr 1 inland retreat from sisimiut to the modern ice margin took 7 ka at an average retreat rate of 15 20 m yr 1 a 10be dated moraine 25 km from the modern gris margin deposited at 8 ka suggests a possible ice margin still stand but this does not change overall retreat rates after retreat from the small coastal sisimiut fjords the gris margin was mainly land terminating in this region in contrast earliest exposure occurred at 12 ka near qaqortoq and 11 10 ka near nuuk and paamiut with ice thinning at rates of 0 2 0 3 m yr 1 to instantaneous within measurement uncertainty ice retreat inland through the extensive nuuk paamiut and qaqortoq fjord systems to near modern ice margins occurred in

an updated radiocarbon based ice margin chronology for the last deglaciation of the north american ice sheet complex abstract the north american ice sheet complex naisc consisting of the laurentide cordilleran and innuitian ice sheets was the largest ice mass to repeatedly grow and decay in the northern hemisphere during the quaternary understanding its pattern of retreat following the last glacial maximum is critical for studying many facets of the late quaternary including ice sheet behaviour the evolution of holocene landscapes sea level atmospheric circulation and the peopling of the americas currently the most up to date and authoritative margin chronology for the entire ice sheet complex is featured in two publications geological survey of canada open file 1574 dyke et al 2003 quaternary glaciations extent and chronology part ii dyke 2004 these often cited datasets track ice margin recession in 36 time slices spanning 18 ka to 1 ka all ages in uncalibrated radiocarbon years using a combination of geomorphology stratigraphy and radiocarbon dating however by virtue of being over 15 years old the ice margin chronology requires updating to reflect new work and important revisions this paper updates the aforementioned 36 ice margin maps to reflect new data from regional studies we also update the original radiocarbon dataset from the 2003 2004 papers with 1541 new ages to reflect work up to and including 2018 a major revision is made to the 18 ka ice margin where banks and eglinton islands once considered to be glacial refugia are now shown to be fully glaciated our updated 18 ka ice sheet increased in areal extent from 17 81 to 18 37 million km2 which is an increase of 3 1 in spatial coverage of the naisc at that time elsewhere we also summarize region by region significant changes to the deglaciation sequence this paper integrates new information provided by regional experts and radiocarbon data into the deglaciation sequence while maintaining consistency with the original ice margin positions of dyke et al 2003 and dyke 2004 where new information is lacking this is a pragmatic solution to satisfy the needs of a quaternary research community that requires up to date knowledge of the pattern of ice margin recession of what was once the world s largest ice mass the 36 updated isochrones are available in pdf and shapefile format together with a spreadsheet of the expanded radiocarbon dataset n 5195 ages and estimates of uncertainty for each interval

late quaternary sedimentation on the southwestern scotian slope eastern canada relationship to glaciationgeological survey of canada contribution 20070176 the sedimentary record on continental slopes has the potential to preserve a record of glacial retreat on the adjacent continental shelf the glacial history of the southwestern part of the scotian shelf is poorly known air gun and high resolution sparker profiles and numerous sediment cores up to 10 m long have been used to determine the character of sedimentation on the southwestern scotian slope since the last glacial maximum lgm seismic reflection profiles show that glacial till was deposited at shallow depths on the upper continental slope and correlation to dated piston cores farther downslope show that this till dates from the lgm slope sedimentation at this time was dominated by local ice and deposited as plume fallout and turbidites progressively increasing importance of red brown sediment derived from glacial supply to laurentian channel indicates retreat of ice from the shelf edge and diminishing supply of proglacial sediment from the calving embayment in the mid scotian shelf with the

stable isotope evidence for glacial lake drainage through the st lawrence estuary eastern canada 13 1 12 9 ka abstract postglacial varved and rhythmically laminated clays deposited during the transition from glacial lake vermont lv to the champlain sea cs record hydrological changes in the champlain st lawrence valley cslv at the onset of the younger dryas 13 1 12 9 ka linked to glacial lake drainage events oxygen isotope 18o records of three species of benthic foraminifera cassidulina reniforme haynesina orbiculare islandiella helenae from six sediment cores and the freshwater ostracode candona from one core were studied results show six large isotope excursions 0 5 to 2 in c reniforme 18o values five excursions in h orbiculare

glacial geomorphology of terrestrial terminating fast flow lobes ice stream margins in the southwest laurentide ice sheet glacial geomorphological mapping of southern alberta canada reveals landform assemblages that are diagnostic of terrestrial terminating ice streams fast flowing outlet glaciers with lobate snouts spatial variability in features that comprise the landform assemblages reflects changes in a palaeo ice stream activity switch on off and b snout basal thermal regimes associated with climate sensitive steady state flow palaeo ice stream tracks reveal distinct inset sequences of fan shaped flowsets indicative of receding lobate ice stream margins former ice lobe margins are demarcated by a major often glacially overridden transverse moraine ridges commonly comprising glacitectonically thrust bedrock and b minor closely spaced recessional push moraines and hummocky moraine arcs details of these landform types are well exhibited around the former southern margins of the central alberta ice stream where larger scale more intensive mapping identifies a complex glacial geomorphology comprising minor transverse ridges mtr types 1 3 hummocky terrain ht types 1 3 flutings and meltwater channels spillways the mtr type 1 constitute the summit corrugation patterns of glacitectonic thrust moraines or major transverse ridges and have been glacially overrun and moderately streamlined the mtr type 2 sequences are recessional push moraines similar to those developing at modern active temperate glacier snouts the mtr type 3 document moraine construction by incremental stagnation because they occur in association with hummocky terrain the close association of hummocky terrain with push moraine assemblages indicates that they are the products of supraglacial controlled deposition on a polythermal ice sheet margin where the ht type 3 hummocks represent former ice walled lake plains the ice sheet marginal thermal regime switches indicated by the spatially variable landform assemblages in southern alberta are consistent with palaeoglaciological reconstructions proposed for other ice stream fast flow lobes of the southern laurentide ice sheet where alternate cold polythermal and temperate marginal conditions associated with climate sensitive steady state flow sequentially gave way to more dynamic streaming and surging activity

brachiopod δ18o values do reflect ambient oceanography lacepede shelf southern australia although commonly used as proxies for attributes of ancient ocean waters the 18 o values of brachiopods from modern seas are little studied to evaluate the utility of brachiopods as recorders of regional oceanography modern shells from the lacepede shelf 25 000 km 2 of southern australia were analyzed for 18 o and the results were compared to the values of ambient seawater southern ocean waters cover this area of extensive cool water carbonate deposition but there are distinct sectors of seasonal upwelling and lesser fluvial outflow 18 o values of brachiopods across the environmental spectrum from 40 to 300 m water depth are in general isotopic equilibrium with surrounding seawater nevertheless 18 o values from individual sample sites vary as much as 0 60 the area of cold water upwelling in particular is clearly delimited by a group of high 18 o values the range of values across this one shelf on the order of 2 5 is similar to the range of values postulated on the basis of similar results for secular changes in many ancient oceans

the late quaternary stratigraphic record northwest of montreal regional ice sheet dynamics ice stream activity and early deglacial events the quaternary sediments of previously unstudied buried valleys and sections near montreal are analyzed and other sites are revisited to further develop the stratigraphic framework of the st lawrence lowland and to establish regional glacial and deglacial models the southwest trending buried valleys were investigated by stratigraphic drilling and high resolution seismic profiling the quaternary succession consists from base to top of proximal glaciolacustrine sediments two superposed till sheets argenteuil and oka tills of inferred late wisconsinan age and champlain sea sediments the glacial sediments of this sequence record an ice advance toward south argenteuil till followed by an abrupt ice flow shift toward the southwest oka till compositional and geomorphic data indicate that oka till is ubiquitous and is associated with a regional set of glacial landforms the analysis of a regional digital elevation model in combination with published ice flow indicators shows convergent flow pattern

new marine evidence for a late wisconsinan ice stream in amundsen gulf arctic canada abstract amundsen gulf and adjoining dolphin and union strait and coronation gulf form the southwestern end of the northwest passage adjacent to the beaufort sea extensive high resolution multibeam sonar imagery and sub bottom profiles of the seabed have been acquired primarily in amundsen gulf by arcticnet and the ocean mapping group at the university of new brunswick these data reveal a variety of seabed landforms including mega scale glacial ridge and groove lineations drumlins moraines iceberg scours bedrock outcrops and discontinuous sediment deposits of variable thickness the lineations are widespread especially in southeastern amundsen gulf they resemble modern and paleo bedforms reported from antarctica svalbard greenland and other canadian arctic channels where they have been ascribed to ice streams the glacial sole marks on the seabed in amundsen gulf and regional data from the adjacent mainland and islands outline the configuration of a glacial ice stream from the laurentide ice sheet that occupied amundsen gulf and adjoining waterways during the late wisconsinan part of the northwestward flowing ice stream was deflected around the colville mountains on victoria island and rejoined the main ice stream in amundsen gulf by way of prince albert sound the grounded amundsen gulf ice stream extended northwestward to the outer slope in the beaufort sea where it was buttressed by arctic shelf ice maximum ice stream extent is inferred to have been coincident with the late glacial maximum multi sequence ice contact sediments and stratigraphic relations with glaciomarine sediments indicate that several ice advances and retreats occurred in the northwestern part of the gulf final retreat from the maximum position began prior to 13 000 cal yr bp and terrestrial dates indicate that the retreating ice front had reached dolphin and union strait by about 12 5 cal ka bp

paleoglaciological insights from the age and morphology of the jesse moraine belt western canadian arctic abstract the jesse moraine belt 350 km long was deposited by the northwest laurentide ice sheet on banks and victoria islands in the western canadian arctic archipelago during the last deglaciation the moraine belt is composed of ice cored controlled moraines that contrast markedly with adjacent glacial sediments deposited during preceding ice sheet retreat more than 80 radiocarbon ages from fossil molluscs illustrate that deposition of the moraine belt was brief occurring between 13 75 and 12 75 cal ka bp the chronology demonstrates that moraine deposition was coeval with ice sheet drawdown and ice streaming on adjacent victoria island detailed geomorphic mapping demonstrates that the moraine belt is the product of a regional change in ice marginal processes this involved a predominantly cold based ice margin giving way to polythermal bed conditions which were conducive to widespread deposition of controlled moraines and ice stream bedforms the expansion of warm based thermal regimes in the northwest laurentide ice sheet followed widespread ice sheet withdrawal from m clure strait and western amundsen gulf this suggests a re equilibration of regional ice divides in response to rapidly changing ice sheet margins and surface gradients the reconstructed dynamical behavior of the northwest laurentide ice sheet provides new insights regarding ice sheet responses to late glacial climate and relative sea level change among other variables

the nastapoka drift belt eastern hudson bay implications of a stillstand of the quebec labrador ice margin in the tyrrell sea at 8 ka bp during deglaciation of eastern hudson bay the western margin of the quebec labrador sector of the laurentide ice sheet came to a stillstand about 8 14c ka bp along the nastapoka hills a series of topographic highs along the bay these hills are the northward continuation of the eastern hudson bay cuesta system it left a drift belt consisting of ice contact submarine fans along the western slopes of the hills small frontal moraines on hilltops and grounding line deposits on sills between the hills geomorphological sedimentary and radiometric evidence suggest that the stillstand responsible for deposition of the nastapoka drift belt was either entirely or partly synchronous with the deposition of the sakami moraine farther south there was a period when these two morainic systems marked a continuous ice margin these stillstands occurred due to reduction of ablation at the ice margin in the nastapoka hills ablation slowed down when the ice margin was anchored on higher relief and stood at a region

late pleistocene stratigraphy and chronology of lower chehalis river valley southwestern british columbia evidence for a restricted coquitlam stade sediments in lower chehalis valley span middle wisconsin olympia nonglacial interval to holocene time sediments are divided into six units with chronological control provided by 14 new radiocarbon ages fluvial gravel spans the transition from the late olympia nonglacial interval to the early fraser glaciation glaciolacustrine sedimentation represents the first definitive glacial activity in the valley and indicates that vashon ice in the fraser lowland blocked the mouth of the chehalis valley at ca 18 17 ka bp ice then flowed down the chehalis valley the chehalis valley deglaciated while ice persisted in the fraser lowland forming another lake after this lake drained terraces and fans formed this style of glaciation deglaciation is typical of many watersheds peripheral to the fraser lowland in that local valley ice was slightly out of phase with ice in the lowland this resulted in glacial lakes forming during both advance and retreat phases however in contrast to watersheds in the northwestern fraser lowland no definitive evidence of a coquitlam ice advance was found within the chehalis valley although glaciers in the area were likely active and advancing data from the chehalis valley indicates that they were not as extensive as previously thought since ice source areas in the northeastern fraser lowland are in the leeward area of the coast mountains it is suggested that lower precipitation resulted in limited glacier activity there during the coquitlam stade

a series of large late wisconsinan meltwater floods through the champlain and hudson valleys new york state usa abstract the champlain valley in northeastern new york lies at the junction of two important late wisconsinan proglacial outflow routes the hudson valley to the south and the st lawrence valley to the northeast freshwater outflow from proglacial lakes in the hudson champlain valleys glacial lake albany vermont and the combined ontario and st lawrence valleys glacial lake iroquois may have affected ocean circulation and thereby altered climate during the last deglaciation we have estimated steady state and three flood pulse discharges from these large meltwater reservoirs into the north atlantic using channel geometry and a high resolution digital elevation model of the late wisconsinan paleogeography of the region we estimate the steady state meltwater discharge into the north atlantic to be 0 3 0 6 sv the first flood event was a combined iroquois vermont outflow at around 10 900 14 c yr bp that released 700 km 3 of meltwater into the north atlantic through the hudson valley with an estimated discharge of 1 1 sv a second outflow event released 2500 km 3 through the hudson valley shortly after the first event finally approximately 1500 km 3 was released to the north atlantic through the gulf of st lawrence at the incursion of the champlain sea about 150 300 years later

arctic ocean glacial history while there are numerous hypotheses concerning glacial interglacial environmental and climatic regime shifts in the arctic ocean a holistic view on the northern hemisphere s late quaternary ice sh

laurentide ice sheet dynamics in the bay of fundy canada revealed through multibeam sonar mapping of glacial landsystems abstract recent multibeam sonar data collected in the bay of fundy canada interpreted in conjunction with geophysical profiling and sediment sampling reveal in unprecedented detail a suite of glacial landforms associated with the southwest margin of the laurentide ice sheet these landforms constitute four glacial landsystems 1 subglacial landsystem i in southwestern bay of fundy the elongated grand manan basin contains ice contact sediments of possible mid wisconsinan age overlain by late wisconsinan ice contact sediments strongly imprinted by iceberg furrows and pits in places possible mid wisconsinan glaciomarine sediments have been eroded by late wisconsinan ice creating streamlined landforms eroded bedrock and megafluted ice contact sediment on the flanks of grand manan basin indicate the southwest direction of topographically steered ice 2 subglacial landsystem ii along the southern margin of the bay of fundy an array of drumlins with superimposed esker complexes was formed by glacial ice that emanated northwest from the interior of nova scotia and was deflected to the southwest by the ice flowing out of the bay of fundy to the gulf of maine the esker complexes formed later when the nova scotia ice sheet stagnated and meltwater escaped northwest via topographic gaps 3 ice marginal landsystem i in northern bay of fundy both small de geer moraines and larger basin bounding moraines were created when retreating late wisconsinan ice became grounded in relatively shallow water new radiocarbon ages show that the owen basin moraine in this landsystem was abandoned prior to c 14 600 14 c yr bp cal bp 17 015 17 270 0 7 17 286 17 405 0 3 4 ice marginal landsystem ii this distinctive landsystem consists of numerous arcuate moraines commonly superimposed on one another this landsystem was formed by thin 170 m lightly grounded ice that retreated northeast into the bay of fundy the splayed pattern of the ice margin was a response to a large accommodation space and velocity variations within the ice the isle haute moraine is a large lobate moraine its age is poorly constrained by shells from glaciomarine sediments 13 750 60 14 c yr bp cal bp 15 970 15 996 0 03 16 047 16 665 0 97 but the moraine may be coeval with the onset of a terrestrial ice margin established at the head of the bay of fundy by c 14 300 14 c yr bp cal bp 16 824 17 013

on the dynamical basis for the asian summer monsoon rainfall el nino relationship abstract the dynamical basis for the asian summer monsoon rainfall el nino linkage is explored through diagnostic calculations with a linear steady state multilayer primitive equation model the contrasting monsoon circulation during recent el nino 1987 and la nina 1988 years is first simulated using orography and the residually diagnosed heating from the thermodynamic equation and the uninitialized but mass balanced ecmwf analysts as forcings and then analyzed to provide insight into the importance of various regional forcings such as the el nino related heating anomalies over the tropical indian and pacific oceans the striking simulation of the june august 1987 1988 near surface and upper air tropical circulation anomalies indicates that tropical anomaly dynamics during northern summer is essentially linear even at the 150 mb level the vertical structure of the residually diagnosed heating anomaly that contributes to this striking simulation differs significantly from the specified canonic

postglacial emergence of ungava peninsula and its relationship to glacial history a series of 178 radiocarbon dates of late glacial and postglacial age from raised marine terraces on the hudson strait hudson bay and ungava bay coasts permit a new synthesis of deglaciation h

geomorphological reconstruction of the labrador sector of the laurentide ice sheet abstract the labrador sector formed one of three major domes of the laurentide ice sheet during the last wisconsinan glaciation reconstruction of its evolution is important because it modulated atmospheric and oceanic circulation over 1 100 ka time scales this paper reports new geomorphological evidence of ice flow which is used to reconstruct glimpses of the behaviour of the labrador sector during its evolution flow evidence indicates a southern position 51 n of the labrador ice divide presumed to be the usual location when the ice sheet was close to its maximum extent a previously inferred dramatic surge of labrador ice across hudson strait and onto baffin island at 9 9 9 4 ka is verified seemingly contradictory evidence regarding deglaciation in northern quebec has been analysed and a reconstruction developed that solves this problem deglacial retreat of the labrador ice sheet was highly asymmetric with rapid retreat of the southern margin northwards migration of the centre of mass by 500 km and slow retreat along the northern margin as it shrank its thermal regime changed from cold centred to entirely cold based and it finally fragmented into many residual ice caps and glaciers this dynamic and asymmetric behaviour may be due to destabilisation of the labrador ice sheet triggered by heinrich event ice streaming through hudson strait

laurentide glacial landscapes the role of ice streams glacial landforms of the north american prairie can be divided into two suites that result from different styles of ice flow 1 a lowland suite of level to streamlined till consistent with formation beneath ice streams and 2 an upland and lobe margin suite of thick hummocky till and glacial thrust blocks consistent with formation at ice stream and ice lobe margins southern laurentide ice lobes hypothetically functioned as outlets of ice streams broad branching lowlands bounded by escarpments mark the stable positions of the ice streams that fed the lobes if the lobes and ice streams were similar to modern ice streams their fast flow was facilitated by high subglacial water pressure favorable geology and topography in the midcontinent encouraged nonuniform ice flow and controlled the location of ice streams and outlet lobes