erosion of the laurentide region of north america by glacial and glaciofluvial processes collection of seismic reflection data from continental margins and ocean basins surrounding north america makes it possible to estimate the amount of material eroded from the area formerly covered by laurentide ice sheets since major glaciation began in north america a minimum estimate is made of 1 62 106 km3 or an average 120 m of rock physically eroded from the laurentide region this figure is an order of magnitude higher than earlier estimates based on the volume of glacial drift cenozoic marine sediments and modern sediment loads of rivers most of the sediment produced during laurentide glaciation has already been transported to the oceans the importance of continental glaciation as a geomorphic agency in north america may have to be reevaluated evidence from sedimentation rates in ocean basins surrounding greenland and antarctica suggests that sediment production sediment transport and possibly denudation by permanent ice caps may be substantially lower than by periodic ice caps such as the laurentide low rates of sediment survival from the time of the permo carboniferous and precambrian glaciations suggest that predominance of marine deposition during some glacial epochs results in shorter lived sediment because of preferential tectonism and cycling of oceanic crust versus continental crust

the geomorphic impact of glaciers as indicated by tors in north sweden aurivaara 68 n abstract geomorphological investigations carried out on 15 tor like features located on the aurivaara plateau north sweden 68 n provide new insights in the greatly debated age of these landforms erratics and till trapped deep in the tor joints support a pre weichselian age for tor formation moreover the occurrence of various weathering stages in allochtonous material the joint width up to 1 5 m requiring long term weathering and the frequent association of tors with pediment like forms suggest pre quaternary tor formation the juxtaposition of fresh erratics and in situ old weathering features mushroom rocks concentrically weathered well rounded corestones and grus indicates a predominantly cold based regime for the scandinavian ice sheet with erratics carried by the overlying moving ice being repeatedly deposited on tor summits during deglaciation phases the relationships between tors and ice action indicated for the aurivaara plateau result in the proposal of a morphodynamical succession of five tor subtypes ranging from the preservation of well rounded corestones still embedded in grus suggesting negligible glacial erosion to the almost complete removal of tor features by ice scouring a comparison with tors in similar geological and topographical contexts from the unglaciated dartmoor area allows a tentative evaluation of an average overall glacial erosion of 0 10 m on the northern sweden plateaus in sharp contrast with the 190 m overdeepening of the nearby tornetrask basin thus this case study of swedish tors provides additional support to the recent interpretations of relict landscapes in previously glaciated areas and is in accordance with the classical model of glacial selective erosion established in the nordic and arctic mountains

relict non glacial surfaces in formerly glaciated landscapes abstract relict non glacial surfaces occur within many formerly glaciated landscapes and contain important information on past surface processes and long term landscape evolution relict non glacial surfaces are distinguishable from glacial surfaces by large scale morphologies including rounded summits fluvial valleys and cryoplanation terraces and pediments and the presence of tors blockfields and or saprolites preservation during glaciation occurs either through coverage by non erosive cold based ice or as nunataks although surface morphologies and denudation rates indicate a continuous non glacial surface history since preglacial times relict non glacial surfaces are dynamic features that have evolved during the quaternary depending on spatial variables such as lithology slope regolith depth and the abundance of fine matrix and water some surfaces are denuding very slowly while others display more rapid denudation high spatial variability in denudation rates results in changing surface morphologies over time denudation rates also display high temporal variability with much surface evolution having perhaps occurred soon after the initial onset of glaciation or during paraglacial phases while some parts of non glacial landscapes are currently active others may be largely inactive relicts of past higher energy regimes although non glacial surfaces are dynamic much remains to be determined regarding surface denudation rates and the magnitude of morphological changes over time

scotland s denudational history an integrated view of erosion and sedimentation at an uplifted passive margin denudational history is commonly reconstructed from basin sediments derived from the denuded source area and less frequently from the source area itself northern britain is an important source area for the surrounding sedimentary basins and this paper reviews the erosional history of scotland from devonian time to the present using evidence both from onshore geology and geomorphology and from patterns of sedimentation in surrounding basins cover rocks were extensive in scotland during late palaeozoic time but the persistence of sediment source areas within the upland areas of scotland makes it unlikely that basement highs were ever completely buried and depths of post devonian erosion of basement have been correspondingly modest 1 2 km during mesozoic time scotland experienced several major erosional cycles beginning with uplift reactivation of relief and stripping of cover rocks followed by progressive reduction of relief through etchplanation and culminating in extensive marine transgressions in late triassic late jurassic and late cretaceous time mid paleocene pulses of coarse sediment to the moray firth basin coincided with major uplift this uplift was associated with major differential tectonics within the highlands with warping and faulting along the margins of the minch and the inner moray firth basins tectonic activity was renewed on a lesser scale in late oligocene time and continued into late neogene time differential weathering and erosion under the warm to temperature humid climates of neogene time created the major elements of the preglacial relief with formation of valleys basins scarps and inselbergs features often closely adjusted to lithostructural controls and in some cases with precursors that can be traced back to devonian time the history that can be read from the onshore region complements the source area history interpreted from sedimentary basins derived from these areas

middle to late devensian glaciation of north east scotland implications for the north eastern quadrant of the last british irish ice sheet we present a review of over 175 years of research into the middle to late devensian weichselian glaciation of north east scotland based crucially on both its lithostratigraphic and geomorphic records the location of the region and surrounding seabed makes this unusually detailed record significant for deciphering the former interactions and dynamics of scottish and scandinavian ice within the north sea basin nsb which continue to be controversial a twelve stage event stratigraphy is proposed based on a parsimonious interpretation of stratigraphic relationships till lithology and regional clast fabrics striae subglacial bedforms ice marginal features and published geochronometry at critical sites the record of regional glaciation supports converging evidence that the north eastern quadrant of the last british and irish ice sheet biis reached its maximum spatial extent in the late middle to early late devensian and later re expanded following widespread internal glacial reorganisations marine transgression and partial retreat from the central nsb retreat was interrupted by several glacial readvances of limited extent field captured data helps identify important events that are not clearly resolved from remote sensing alone particularly regarding growth phases of the last glaciation

an integrated law of the wall for hydrodynamically transitional flow over plane beds abstract the velocity profile for unidirectional currents is usually determined by two forms of the law of the wall which are valid for smooth and rough boundaries respectively the law of the wall for smooth boundaries outside the viscous sublayer and buffer layer applies where the boundary reynolds number re is less than 5 the velocity in this case depends only on the dimensionless distance y d from the bottom for rough boundaries where re is more than 65 the law of the wall is independent of the boundary reynolds number but the bottom roughness k has to be taken into account the mean or median particle grain size d is commonly substituted for bottom roughness at present there is no generally accepted law of the wall for the transitional flow regime between hydrodynamically smooth and rough conditions in this paper an integrated law of the wall is proposed for the transitional regime yielding velocity profiles that correspond well to observed velocity profiles under these conditions the result is applied to a set of flume data using an improved procedure to determine the bedload transport rate

sources of non glacial loess size quartz silt and the origins of desert loess abstract descriptions of peridesertic loess deposits and possible non glacial origins for silt size quartz are reviewed including direct release from weathered bedrock results from salt and frost weathering experiments are presented and methods by which sufficient energy can be imparted to sand size grains to release silt size fragments are identified these fragments may then be entrained by aeolian or fluvial action results from fluvial comminution experiments indicate that high energy mixed sediment fluvial systems may be very effective in generating quartz silt purely aeolian attrition is also seen as a major source of silt size and finer material results from aeolian abrasion experiments performed using crushed quartz sand are complemented by trials using sands from deeply weathered granite and pannonian sands from hungary field and laboratory observations are therefore seen to argue for the existence of deposits that can be classified as desert loess

theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products publisher elsevier articletitle theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products journaltitle earth science reviews articlelink http dx doi org 10 1016 j earscirev 2016 01 010 content type article copyright copyright 2016 elsevier b v all rights reserved

making loess sized quartz silt data from laboratory simulations and implications for sediment transport pathways and the formation of desert loess deposits associated with the sahara abstract results from a series of laboratory simulations designed to investigate quartz silt generation by a number of geomorphic mechanisms are reviewed these findings are then used to discuss potential sediment pathways involved in the formation of desert loess deposits and a pathway is presented to show a proposed sequence of events involved in the formation of the loess deposits associated with the sahara the results indicate that both fluvial and aeolian systems can be very effective at producing quartz silt it is suggested that fluvial systems within hot desert environments may play a central role in both the production and distribution of loessic silt first the highly energetic but periodic fluvial regimes associated with arid and semi arid environments may be responsible for considerable quantities of particle comminution second the intermittent nature of these systems will help maintain geomorphologically active surfaces from which silt sized material can be easily deflated as well as fluvial comminution a range of weathering mechanisms and aeolian abrasion may also be responsible for the generation of silts in desert loess consequently silt sources and mechanisms of silt generation are likely to be much more diverse for desert loess than those associated with glacial loess

brittle deformation relating to the carboniferous cretaceous evolution of the lambert graben east antarctica a precursor for cenozoic relief development in an intraplate and glaciated region abstract stress inversions of kinematic data measured from brittle deformation structures exposed in the southern prince charles mountains have been carried out to elucidate the drivers of intracontinental deformation responsible for the development of the lambert graben east antarctica four palaeostress fields d 1 d 4 were identified from these data which when coupled with pre existing geochronological and sedimentological evidence provide a clear deformation framework the architecture of the lambert graben is dominantly defined by north south trending d 2 and northeast southwest trending d 3 faults that formed respectively during 1 carboniferous permian intracontinental extrusion and 2 cretaceous transtensional tectonics related to the dispersion of east gondwana interestingly sub ice topography in the southern prince charles mountains and further south towards the gamburtsev mountains is defined by bedrock lineaments that parallel the orientations of the main fault arrays previous work shows that the development of relief in the southern prince charles mountains is tertiary in age and probably not associated with carboniferous permian or cretaceous tectonic events based on a clear spatial link between the orientations of faults and bedrock lineaments we suggest that inherited fault zones were integral in accommodating uplift driven by plate flexure as a response to crustal unloading during glacial incision

internal architecture and evolution of bioclastic beach ridges in a megatidal chenier plain field data and wave flume experiment beach ridges in macrotidal environments experience strong multi annual to multi decennial fluctuations of tidal inundation the duration of tide flooding directly controls the duration of sediment reworking by waves and thus the ridge dynamics flume modelling was used to investigate the impact of low frequency tidal cycles on beach ridge evolution and internal architecture the experiment was performed using natural bioclastic sediment constant wave parameters and low frequency variations of the mean water level the morphological response of the beach ridge to water level fluctuations and the preservation of sedimentary structures were monitored by using side view and plan view photographs results were compared with the internal architecture of modern bioclastic beach ridges in a macrotidal chenier plain mont st michel bay france surveyed with ground penetrating radar the experimentally obtained morphologies and internal structures matched those observed in the field and the three ridge development stages identified in ground penetrating radar profiles early transgressive late transgressive and progradational were modelled successfully flume experiments indicate that flat bioclastic shapes play a key role in sediment sorting in the breaker zone and in sediment layering in the beach and washover fans water level controls washover geometry beach ridge evolution and internal structure low water levels allow beach ridge stabilization and sediment accumulation lower on tidal flats during subsequent water level rise accumulated sediment becomes available for deposition of new washover units and for bayward extension of the beach ridges in the field low frequency water level fluctuations are related to the 4 4 year and 18 6 year tidal cycles experimental results suggest that these cycles may represent the underlying factor in the evolution of the macrotidal chenier coast at the multi decadal to centennial time scale

structure and development of shell cheniers in essex southeast england investigated using high frequency ground penetrating radar abstract the stratigraphy and internal structure of contemporary shell cheniers on the southern essex coast england were investigated using a high frequency 900 mhz ground penetrating radar gpr system the cheniers lie on the seaward edge of laterally eroding saltmarshes at the junction with the tidal flats four main types of chenier are recognised each with a characteristic morphology and dynamic status migrated radar reflection profiles obtained from each of the chenier types achieved a sub decimetre vertical resolution ground truthing data demonstrate that the radar profiles accurately delineate the subsurface stratigraphy and sedimentary structure of the cheniers interpretation of the radar stratigraphy derived from the radar reflection profiles allows various deposits to be identified these have resulted from overwashing overtopping sedimentation across the whole of a seaward dipping beachface or berm ridge welding onto the upper beachface each chenier is characterised by a different spatial arrangement of these four basic depositional units the results from the gpr surveys support the hypothesis that each of the chenier types represents a distinct phase in the development of these contemporary coastal landforms however a simple model involving a linear progression through chenier types 1 4 with time appears inappropriate instead two main evolutionary pathways are proposed with the sequence of development being principally controlled by the nature of the cheniers longer term sediment budgets

glaciation erosion and uplift over part of east antarctica isostatic rebound effects after the removal of large glacial loads are well known from several parts of the world but isostatic uplift during glaciation and caused by glacial erosion is less well documented this is because preglacial landscapes which provide a datum against which glacial erosion and related uplift is measured are generally either destroyed by glaciation or not recognized remnants of such a preglacial land surface are exposed in the prince charles mountains of east antarctica we now report that the amount of glacial erosion here is 1 5 km measured by the difference in level between the preglacial land surface and the present mean mostly subglacial rock surface the isostatic uplift caused by this erosion is 0 8 km both amounts are thought to be well above average for east antarctica and glaciated areas in general because the region is traversed by east antarctica s largest outlet glacier system parts of which are believed to surge periodically

critical bed shear stress and threshold of motion of maerl biogenic gravel abstract a determination of the critical bed shear stress of maerl is a prerequisite for quantifying its mobility rate of erosion and deposition in conservation management the critical bed shear stress for incipient motion has been determined for the first time for samples from biogenic free living maerl beds in three contrasting environments open marine intertidal and beach in galway bay west of ireland the bed shear stress was determined using two methods law of the wall and turbulent kinetic energy in a rotating annular flume and in a linear flume the velocity profile of flowing water above a bed of natural maerl grains was measured in four runs of progressively increasing flow velocity until the flow exceeded the critical shear stress of grains on the bed the critical shields parameter and the mobility number are estimated and compared with the equivalent curves for natural quartz sand the critical shields parameters for the maerl particles from all three environments fall below the shields curve along with a previously reported correlation between maerl grain shape and settling velocity these results suggest that the highly irregular shapes also allow maerl grains to be mobilised more easily than quartz grains with the same sieve diameter the intertidal beds with the roughest particles exhibit the greatest critical shear stress because the particle thalli interlock and resist entrainment in samples with a high percentage of maerl and low percentage of siliciclastic sand the lower density lower settling velocity and lower critical bed shear stress of maerl results in its preferential transport over the siliciclastic sediment at velocities 10 cm s 1 higher than the threshold velocity of grain motion rarely documented subaqueous maerl dunes formed in the annular flume

unequal ice sheet erosional impacts across low relief shield terrain in northern fennoscandia abstract much previous work on late cenozoic glacial erosion patterns in bedrock has focussed on mountain areas here we identify varying impacts of ice sheet erosion on the low relief bedrock surface of the fennoscandian shield and examine the geological topographical and glaciological controls on these patterns we combine gis mapping of topographical hydrological and weathering data with field observations we identify and investigate areas with similar geology and general low relief that show different degrees of ice sheet erosional impact despite similar ice cover histories on two transects with a total area of 84 000 km2 across the northern fennoscandian shield we first establish patterns of glacial erosion and then examine why glacially streamlined areas exist adjacent to areas of negligible glacial erosion the northern transect includes two areas of exceptional glacial preservation the parkajoki area in sweden and the so called ice divide zone in finland each of which preserve tors and deep saprolite covers the southern transect overlapping in the northern part with the first transect includes areas of well developed glacial streamlining with bedrock areas stripped of loose material and barely any weathering remnants for both areas we firstly present contrasting indicators for ice sheet erosional impact streamlined and non streamlined inselbergs parallel and dendritic rectangular drainage patterns and the absence and presence of neogene weathering remnants this is followed by an investigation of factors that possibly influence ice sheet erosional impact ice cover history ice cover duration and thickness bedrock type and structure and topography we find that the erosional impact of the fennoscandian ice sheet has varied across the study area distinct zones of ice sheet erosion are identified in which indicators of either low or high erosion coexist in the same parts of the transects no direct impact of rock type on glacial erosion patterns was found but an indirect control appears clear bedrock geology and long term differential weathering and tectonic evolution determined the topography of the pre glacial landscape and these topographic differences subsequently influenced ice sheet dynamics and thereby partly controlled patterns of ice sheet erosion ice cover duration and former ice thickness were not significant controls on glacial erosion patterns extensive preservation of pre glacial relief through low glacial erosion is attributed to the maintenance throughout the pleistocene of divergent flow and frozen bed conditions in the fennoscandian ice sheet in contrast glacial streamlining and strong glacial erosion were caused mainly by acceleration of flow around major obstacles and flow towards major depressions on the ice sheet bed the relatively strong ice sheet erosion towards the gulf of bothnia is the result of a combination of favourable factors bedrock structure and river valleys aligned sub parallel to ice sheet flow and convergent ice flow towards the baltic

shelly cheniers on a modern macrotidal flat mont saint michel bay france internal architecture revealed by ground penetrating radar abstract the relationship between short term morphodynamics and internal structure of shelly chenier ridges is investigated using ground penetrating radar and core x ray analysis complemented with a comparison of aerial photographs these cheniers are located on the uppermost part of a macrotidal flat in mont saint michel bay north western france they actively migrate landward over the salt marshes by washover processes when they are submerged during the coincidence of spring tides and enhanced wave activity the architecture of the cheniers is imaged using high frequency radar antennas 400 mhz to 2 6 ghz three types of cheniers are recognized featuring differences in morphology and internal structure the altitude of the banks on the tidal flat relative to the level of tidal flooding as well as local sediment supply seem to be important forcing parameters in chenier development and stages of evolution on a multi annual time scale evolution of this chenier system is influenced by a combination of the 18 year and the 4 year tidal cycles superimposed on wave activity during minima of the 18 year cycle spring tide level is globally lower on the tidal flat allowing salt marsh progradation and chenier ridge development during peak periods of the tidal cycle former barriers are reworked by wave action and experience onshore migration associated with salt marsh erosion the 4 year tidal cycle coupled with enhanced wave activity is probably responsible for the deposition of large washover units observed in gpr reflection profiles

infilling stratigraphy of macrotidal tide dominated estuaries controlling mechanisms sea level fluctuations bedrock morphology sediment supply and climate changes the examples of the seine estuary and the mont saint michel bay english channel nw france this article presents a synthesis of recent works performed on two macrotidal tide dominated estuaries located along the southern coast of the english channel the seine estuary and the mont saint michel bay nw france on the basis of very high resolution seismic data combined with sediment cores and ams dating these works allow reconstructing the holocene in ll of such singular estuaries yet rarely documented in the literature rate in sea level rise bedrock morphology sediment supply are the main controlling factors of the general in ll pattern both in lls are composed of two units the transgressive systems tract tst and highstand systems tract hst the maximum ooding surface mfs is dated around 6500 cal bp when the holocene sea level rise slowed down and corresponds to the main architectural change the tst is poorly preserved compared with the hst that constitutes the main depositional unit making this incised valley ll deviating from the classical models the hst is composed above the axis of the incised valley of a highly wandering estuarine channel belt characterized by deep tidal reworking throughout its building on the margin of this high energy estuarine system the hst is made of wave dominated barriers tidal ats and offshore tidal banks they constitute dominantly aggradational systems the aggradation rate of which is in accordance with the rate of sea level rise from 6500 cal bp onwards consequently the impacts of the rapid climate changesthat characterized mid to late holocene times can be preserved in these marginal successions various sedimentary and geometrical signatures indicate that periodic enhanced storminess episodes probably related to the bond cold events 1500 years periodicity are responsible for the most drastic environmental changes in these macrotidal tide dominated settings the role of long term tidal cycles as well as the impact of human activities are also considered

giant dune morphologies and dynamics in a deep continental shelf environment example of the banc du four western brittany france the aim of this paper is to improve the knowledge of spatio temporal variability of very large to giant dunes in deep tide dominated environment their growth mechanisms and dynamic evolution still remain in debate and constitute a significant scientific challenge in regard to difficulties to achieve accurate measures of migrations this motivated the realization of three recent swath bathymetry surveys across the wide dune field of the banc du four located offshore the western brittany the banc du four field is composed of more than 500 large dunes exhibiting a great diversity of morphologies ranging from 2d to 3d shapes and reaching the largest sizes of such sedimentary structure ever described wavelength over 1000 m and height over 30 m the analysis of the entire ensemble of dune parameters offers the opportunity to discuss the relative influence of forcings and the validity of the equations predicting the bedform geometry in shallow waters our results show a good height spacing power law correlation but do not strike the usual statistical regression observed in previous studies the steeper positive slope of the equation and the outstanding values of the height parameter reveal the complexity of local hydrodynamic regime interacting with bed load transport and sediment supply no clear relationship has been observed between asymmetry and size parameters water depth can not be considered as a major factor controlling the size of dunes otherwise the vertical sedimentary variation suggests the action of storm waves on dune crests and the horizontal migration rates of dunes in the eastern field of the banc du four were found to range from 3 to 20 m yr 1 such velocities were only mentioned before by van landeghem et al 2012 on deep continental shelves 70 m deep and attest of the still present morphodynamical equilibrium of the large dunes furthermore as was previously found for similar dunes in the irish sea no power law relationship between dune migration rates and dune size can be deduced similarly no strong correlation is established between migration rates and the degree of asymmetry nevertheless the dune asymmetry can be used to predict the migration direction as for dimension parameters the evolution asymmetry depends on the study area and migration rates more precisely when the crest lines of dunes form straight lines and their migration rates remain below 8 10 m yr 1 the degree of dune asymmetry increases with the migration rate this trend is reversed for the dunes with higher migration velocities and crests which are sinuous these results relativize the migratory predictions based on punctual geometrical observations

relief and saprolites through time on the baltic shield abstract the relief of basement rocks in high latitudes has most often been interpreted as the result of glacial erosion although over large areas its character of exhumed paleoplains has been known for a long time the relief in the precambrian basement of sweden has therefore been analysed in relation to remnants of late proterozoic and phanerozoic cover rocks most of the shield was exposed in the late proterozoic the end result was an extremely flat surface the primary peneplain it was covered by palaeozoic rocks which preserved this surface until re exposure as a sub cambrian surface it makes up the present scenery in large parts of eastern and central sweden the south swedish dome is a key area for the geomorphic analysis of the shield as it has palaeozoic cover rocks on its northern and eastern slopes and mesozoic rocks on its southern and western ones based on the experience from this area classification of the relief in the whole country and experiences from the tropics on relief evolution two theoretical schemes were constructed for a discussion of the relief differentiation the analysis of the relationship between relief saprolite and cover rocks through time led to the conclusion that the most important factor for the present relief differentiation is the time of exposure of the basement surface during the phanerozoic this has important applications for the evaluation of phanerozoic crustal movements as well as implications for the assessment of glacial erosion the approach is thought to be most valuable for comparative studies of shield geomorphology in general and with the laurentian shield in particular

kaolins in gravels and saprolites in north east scotland evidence from stable h and o isotopes for palaeocene miocene deep weathering stable isotope ratios can provide important evidence for estimating groundwater temperatures during the formation of clay minerals in response to chemical weathering of rocks at the landsurface in this paper we investigate weathering kaolins found in buchan ne scotland stable oxygen and hydrogen isotopes for kaolins from weathered clasts in post cretaceous fluvial gravels and from clay rich saprolites developed in dalradian metamorphic and caledonian igneous rocks indicate weathering under warm groundwater temperatures of 23 5 c comparisons with cenozoic palaeotemperatures derived from north sea sediments indicate that weathering took place under humid tropical climates of the palaeocene eocene the 25 m deep weathering profiles in the buchan gravels indicate that these coarse quartzite and flint bearing gravels are older than previously thought and were likely deposited by rivers which also fed thick palaeocene early eocene deep water sand fans in the western north sea basin a later phase of kaolin weathering at lower temperatures of 15 5 c took place in the middle miocene kaolinitic weathering remnants are largely confined to elevations of 100 m in eastern buchan and define a well preserved deeply weathered cenozoic landscape in north east scotland

cenozoic weathering covers in buchan scotland and their significance isolated occurrences of chemically weathered rock or saprolite are common in many formerly glaciated areas around the north atlantic1 6 these pockets of saprolite are widely regarded as remnants of cenozoic deep weathering covers which existed before the first extensive northern hemisphere glaciations at 2 4 myr ref 7 and their apparent survival has long been used as evidence for the local ineffectiveness of glacial erosion2 4 8 9 however the scarcity of correlative deposits means that the age and significance of these fragmented saprolites are uncertain i report here the results of a detailed survey which shows that in buchan scotland a combination of cenozoic tectonic stability and extremely limited pleistocene glacial erosion has allowed the preservation of deep weathering covers of probable miocene to early pleistocene age on a scale not previously reported from any other formerly glaciated area

a relict landscape in the centre of fennoscandian glaciation geomorphological evidence of minimal quaternary glacial erosion abstract the parkajoki area in northeastern sweden is situated near the central area of fennoscandian glaciation despite its location the area is dominated by landforms induced by subaerial weathering and erosion processes such as well developed tors and associated saprolites boulder fields and boulder depressions the glacial geomorphology is dominated by lateral and proglacial meltwater channels subglacial imprints indicative of thawed bed conditions and reshaping by glacier sliding e g fluting drumlins striae are lacking hence most of the landscape still exhibits a preglacial appearance because of its location near the central area of glaciation we attribute preservation to frozen bed conditions of overriding ice sheets the widespread distribution of well developed tors and boulder fields and the degree of chemical weathering of the bedrock indicate that the area has been protected from glacial erosion during all glacial cycles since ice sheet initiation in the late cenozoic unlike most other areas with tors in glaciated regions the parkajoki area is uniquely situated in the lowlands at 150 400 m a s l moreover this relict landscape is surrounded by glacial landscapes including drumlins ribbed moraine and eskers of varying age and at similar elevation hence topographical reasons for this area being persistently cold based cannot be invoked by inference we conclude that strain heat release never managed to cancel the initial subglacial permafrost conditions we attribute this to divergent ice flow towards the convex outline of the ice sheet margin during deglaciations and to the relative roughness of the area compared to its surroundings the implication is that to explain preservation throughout the quaternary all large scale glaciations must have had a similar evolution concerning ice sheet configuration and internal dynamics as the last glaciation for which good constraints on evolution and outline are available

cenozoic landscape evolution of an east antarctic oasis radok lake area northern prince charles mountains and its implications for the glacial and climatic history of antarctica abstract ice free areas antarctica reveal a multi million year history of landscape evolution but most attention up to now has focused on the transantarctic mountains the amery oasis in the northern prince charles mountains borders the lambert glacier amery ice shelf system that drains 1 million km2 of the east antarctic ice sheet and therefore provides a record of fluctuations of both local and regional ice since the ice sheet first formed in early oligocene time this glacial record has been deciphered by i geomorphological mapping from aerial photographs and on the ground ii documenting the relationship between thick well dated uplifted glaciomarine strata and the underlying palaeolandscape iii examining surficial sediment facies and iv surface exposure dating using 10be and 26al the se amery oasis records at least 10 million years of landscape evolution beginning with a pre late miocene phase of glacial erosion followed by deposition of glaciomarine strata of the battye glacier formation pagodroma group in late miocene time a wet based ice sheet next expanded over the se amery oasis following which deposition of the glaciomarine pliocene bardin bluffs formation pagodroma group took place both formations were uplifted by at least 500 and 200 m respectively their tops are characterised by geomorphological surfaces upon which intensive periglacial activity took place higher level bedrock areas were subjected to deep weathering and tor formation early pleistocene time was characterised by expansion of a cold based ice sheet across the whole area but it left little more than patches of sandy gravel and erratic blocks late pleistocene expansion of local ice the battye glacier saw deposition of moraine mound complexes on low ground around radok lake and ice dammed lake phenomena subglacial drainage of the lake escaped to the east exhuming the sediment filled gorges holocene landscape modification has been relatively superficial overall the landscape of the amery oasis evolved primarily under the influence of wet based probably polythermal glaciers in miocene and pliocene times whereas the quaternary period was characterised mainly by cold based glaciers that had comparatively little impact on the landscape

the last ice sheet in north west scotland reconstruction and implications recent models of the last scottish ice sheet suggest that nunataks remained above the ice surface in areas peripheral to the main centres of accumulation this proposition has been investigated on 140 mountains over an area of 10 000 km2 in nw scotland outside the limits of the later loch lomond readvance in this area there is evidence for a single high level weathering limit that separates glacially eroded terrain from higher areas of in situ frost debris this limit occurs at altitudes ranging from 425 to 450 m in the outer hebrides to 950 m on the mainland and is best developed on lithologies that resisted breakdown after ice sheet downwastage interpretation of this weathering limit as a periglacial trimline cut by the last ice sheet at its maximum thickness is supported by 1 joint depth and schmidt hammer measurements that indicate significantly more advanced rock breakdown above the weathering limit 2 a much greater representation of gibbsite a pre late devensian weathering product in the clay fraction of soils above the limit 3 cosmogenic isotope dating of the exposure ages of rock outcrops above and below the limit 4 the sharpness of the limit at some sites and its regular decline along former ice flowlines and 5 shear stress calculations based on the inferred altitude and gradient of the former ice surface reconstruction of the ice surface based on trimline evidence indicates that the mainland ice shed lay near or slightly east of the present watershed and descended northwards from 900 m to ca 550 m at the north coast independent dispersion centres fed broad ice streams that occupied major troughs on skye an ice dome 800 m deflected the northwestwards movement of mainland ice but the mountains of rum were over ridden by mainland ice up to an altitude of ca 700 m the outer hebrides supported an independent ice cap that was confluent with mainland ice in the minches extrapolation of the trimline evidence indicates that most reconstructions of ice extent are too conservative and suggests that low gradient ice streams extended across the hebridean shelf offshore wider implications of this research are 1 that blockfields and other periglacial weathering covers are not all of the same age or significance depending on the resistance of different lithologies to frost weathering 2 that the contrasting degree of glacial modification in the western and eastern highlands of scotland may reflect a former cover of predominantly warm based ice in the former and predominantly cold based ice in the latter and 3 that the approach and techniques developed in this study have potential application for constraining ice sheet models not only in areas peripheral to the main centres of ice accumulation in britain and ireland but also in other mountain areas where nunataks protruded through warm based late pleistocene ice masses

growth and decay of a marine terminating sector of the last british irish ice sheet a geomorphological reconstruction the boundary conditions that govern ice sheet dynamics can change significantly with the development of marine margins this paper uses the glacial landscape in western scotland to reconstruct changes in the british irish ice sheet that accompanied the growth and decay of a marine sector over the malin shelf ice advanced from a restricted mountain ice sheet with tidewater margins after 35 ka bp and reached the continental shelf in 7 ka average rate of 30 m a 1 early ice flow had been directed through north south geologically controlled over deepened fjords that were carved during previous restricted glaciations this flow regime was abandoned with development of the malin shelf ice sheet sector ice flow direction switched by 90 degrees and was drawn westwards towards the shelf edge the marine ice sheet phase saw episodes of west east ice divide migration by up to 60 km over west central scotland possibly linked to ice streaming and calving events at the ice sheet margin however permanent and stationary ice divides and zones of cold based ice associated with subglacial topographic highs also characterised the marine glacial stage over western scotland the north channel ice divide remained a constant though migratory feature while the biis occupied the malin shelf it finally collapsed at the end of the killard point stadial when the irish ice sheet began to rapidly decay 16 5 ka bp this permitted the scottish ice sheet to temporarily advance over north east ireland previously identified as the east antrim coastal readvance before it too retreated at rates in the order of 10 2 m a 1 although the imprint of extensive shelf edge ice sheet glaciation exists in the coastal landscape of western scotland the dominant landscape features relate to a restricted marine proximal mountain ice sheet with markedly different flow configurations similar first order geomorphological features relating to restricted glacial conditions are likely to be preserved in subglacial highlands under interior parts of modern ice sheets

hydrodynamic behaviour of coarse bioclastic sand from shelly cheniers cheniers from mont saint michel bay france are coarse shelly sand ridges migrating on the mudflat up to the salt marshes where they accumulate and merge in a littoral barrier in this macrotidal setting and low wave forcing the cheniers are rarely submerged however they are found to move up to several metres during coincidence of spring tide and wave activity their processes of migration morphology and internal structure composition of the beddings grain size sorting and grain arrangement are thought to be closely related to the hydrodynamic behaviour of the coarse and shelly sediment this paper focuses on the hydrodynamic behaviour of bioclastic sand sampled from the cheniers settling velocities of the shell fragments were measured using a 2 m long sedimentation tube thresholds of motion under unidirectional current velocity and turbulence vertical profiles were characterized in a small recirculating flume using laser doppler anemometry lda the flat shaped bioclastic particles feature low settling velocities and reveal a good resistance to the re suspension effect of the flow when imbricated in a sediment bed the shear stress in the bottom boundary layer has been measured in the viscous and log sub layers nikuradse roughness heights ks for shell debris beds of different sizes have been quantified it is found that ks 2 56d50 this value is close to the ones used for classic rounded sand grains despite their major differences of shape the dual behaviour of the shell fragments low settling velocity good resistance to unidirectional flow should be considered as a key to understanding how this coarse material is transported across the tidal flat and finally accumulated as cheniers further flume experiments including wave activity and tidal fluctuations are necessary to better quantify these complex processes copyright 2010 john wiley sons ltd

settling velocities and entrainment thresholds of biogenic sands shell fragments under unidirectional flow settling velocities and entrainment thresholds of biogenic sedimentary particles under unidirectional flow conditions are derived on the basis of settling tower and laboratory flume experiments material consisting predominantly of equant blocks shell fragments of cerastoderma edule density s 2800 kg m 3 or of mica like flakes and elongate rods mytilus edulis fragments s 2720 kg m 3 are used in separate series of experiments differences in the measured settling and threshold properties are related primarily to particle shape the selection of a characteristic length scale for non spherical grains is investigated by comparing two approaches used to define the grain size d of the sediment samples grain settling and sieve analysis that are used to derive data for the threshold criteria in terms of the shields and movability diagrams the empirical curves effectively predict the threshold conditions for any grain shape provided that grain size is defined in terms of grain settling velocity however a functional distinction is made between the characteristic hydraulic grain size defined by grain settling for grain transport applications and the actual physical grain size defined by sieve analysis

late pliocene glaciation and landscape evolution of vestfirðir northwest iceland abstract vestfir ir northwest iceland has been extensively modified by glacial activity but the timing and landscape impact of pre last glacial maximum lgm glacial activity is not well known we present new geochemical and chronological data from two tephras o oli and hjallanes which constrain the age of high altitude ice dammed lake sediments near the plateau surface of skagafjall located on the outer westernmost fjords of vestfir ir our findings provide the first dated evidence of late pliocene glaciation on vestfir ir the 2 26 0 11 ma fission track age of the airfall rhyolitic o oli tephra is compatible with reversed palaeomagnetic polarity data and suggests an ice dammed lake existed on skagafjall at broadly the same time as a major and well documented late pliocene downturn in global climate we assess the likely impact of late pliocene and subsequent glacial activity on vestfir ir landscapes and propose that 1 geomorphological features and sedimentary deposits on or near the skagafjall plateau surface could have survived beneath relatively low basal velocity areas and or relatively non erosive areas of glacial maxima ice sheets on vestfir ir at glacial maxima and 2 the heavily incised fjord network the principal macroscale landscape feature of vestfir ir existed in the late pliocene

evidence of rapid cenozoic uplift of the shoulder escarpment of the cenozoic west antarctic rift system and a speculation on possible climate forcing the cenzoic west antarctic rift system characterized by cenozoic bimodal alkalic volcanic rocks extends over a largely ice covered area from the ross sea nearly to the bellingshausen sea it is bounded on one side by a spectacular 4 to 5 km high rift shoulder scarp maximum bedrock relief 5 to 7 km from northern victoria land queen maud mountains to the ellsworth whitmore horlick mountains jurassic tholeiites crop out with the late cenozoic volcanic rocks along the section of the transantarctic mountains from northern victoria land to the horlick mountains the cenozoic rift shoulder diverges here from the jurassic tholeiite trend and the tholeiites are exposed discontinuously along the lower elevation 1 2 km section of the transantarctic mountains to the weddell sea various lines of evidence no one of which is independently conclusive lead the authors as others have also suggested to interpret the following the transantarctic mountains part of the rift shoulder and probably the entire shoulder has been rising since about 60 ma at episodic rates of approximately 1 km m y most recently since mid pliocene time rather than continuously at the mean rate of 100m m y uplift rates vary along the scarp which is cut by transverse faults the authors speculate that this uplift may have climatically forcedmore the advance of the antarctic ice sheet since the most recent warm period they suggest a possible synergistic relation between episodic tectonism mountain uplift and volcanism in the cenozoic west antarctic rift system and waxing and waning of the antarctic ice sheet beginning about earliest oligocene time less

glacial geomorphology modeling processes and landforms abstract the primary goal of glacial geomorphology is to provide physically based explanations of the past present and future impacts of glaciers and ice sheets on landform and landscape development to achieve this requires the integration of studies of landform with studies of the processes responsible for form development over a wide range of spatial and temporal scales during the twentieth century significant improvements in approaches to recognizing and describing glacial landforms have been matched by impressive advances in understanding and modeling ice flow and glacial erosion and deposition processes at present process models are being tested explicitly in terms of predicting the development of known forms which also provides new insight into the controls on form development evaluations of the implications of deformable beds for process and form development are also being attempted finally we are reassessing long held beliefs about the significance of glacial action in landform development and sediment production as we head towards the twenty first century glacial geomorphology will advance through the use of three dimensional numerical models that include ice flow basal sliding with explicit consideration of deformable beds erosion and deposition processes and underlying material characteristics these models will be used to address form evolution and test process models and will include both the temporal and spatial aspects of form development space based landform recognition as well as the challenges presented by the concerns of global change will place heightened emphasis on large scale problems which will include reexamination of the basic significance of glacial versus nonglacial action in landform development and sediment production

scales of boundary resistance in coarse grained channels turbulent velocity profiles and implications gravel bed surfaces are characterized by morphological features occurring at different roughness scales the total shear stress generated by the flow above such surfaces is balanced by the sum of friction drag grain stress and form drag components created by bed forms to facilitate a better understanding of total resistance and bed load transport processes there is a need to mathematically separate shear stress into its component parts one way to do so is to examine the properties of vertical velocity profiles above such surfaces these profiles are characterized by an inner layer that reflects grain resistance and an outer layer that reflects total resistance a flume based project was conducted to address these concerns through systematically comparing different roughness scales to ascertain how increased roughness affects the properties of vertical velocity profiles great care was taken to create natural roughness features and to obtain flow data at a high spatial and temporal resolution using an acoustic doppler velocimeter average vertical velocity profiles above each roughness scale were clearly segmented the vertical extent of the inner flow region was directly related to the scale of roughness present on the bed and independent of flow depth increasing with increased roughness on a rough but rather uniform plane bed made of heterogeneous coarse sediments with no bed forms the shape of the velocity profile was clearly dominated by the local variations in grain characteristics when pebble clusters were superimposed the average shear stress in the outer flow region increased by 100 from the plane bed conditions the ratio of inner grain shear stress to outer total shear stress for this pebble cluster experiment was 0 18 under shallow flow conditions and 0 3 under deep flow conditions the grain stress component that should be used in bed load transport equations therefore appears to vary in these experiments between 15 and 30 of the total channel stress increasing with decreased resistance roughness height ks d50 values at the grain scale for the plane bed and pebble cluster experiments were 0 73 and 0 63 respectively these are values that should be used in flow resistance equations to predict grain resistance and grain stress for bed load transport modeling

extent of the last ice sheet in northern scotland tested with cosmogenic 10be exposure ages the extent of the last british irish ice sheet biis in northern scotland is disputed a restricted ice sheet model holds that at the global last glacial maximum lgm ca 23 19 ka the biis terminated on land in northern scotland leaving buchan caithness and the orkney islands ice free an alternative model implies that these three areas were ice covered at the lgm with the biis extending offshore onto the adjacent shelves we test the two models using cosmogenic 10be surface exposure dating of erratic boulders and glacially eroded bedrock from the three areas our results indicate that the last biis covered all of northern scotland during the lgm but that widespread deglaciation of caithness and orkney occurred prior to rapid warming at ca 14 5 ka copyright 2008 john wiley sons ltd

a new approach to the relief of great britain i the machine readable database abstract we describe a simple if fairly large database of relief and geology for great britain and the surrounding shelf based on the kilometre squares of the national grid the area covered is 560 000 km 2 319 490 km 2 classified as sea and 240 510 km 2 as land squares which included the coastline were classed as land the database affords a new approach to establishing the importance of rock resistance to erosion as a factor affecting the form of the present land surface the legacy of older tectonic movements and the role of neotectonics it can be used to test existing hypotheses or propose new ones which can then be tested against other evidence initial data on land shelf contrasts are presented the current coastal zone is found to be a sector of relatively abrupt transition from shelf depths between 60 and 70 m to land elevations exceeding an average of over 100 m even within a few kilometres of the coast this transition zone is attributed to the isostatic buoyancy of a land area steadily unloaded by denudation and attacked by the transgressing sea as water levels rise after each glacial stage the database is a novel and powerful way to tackle some fundamental aspects of the evolution of the relief of great britain and further papers will explore issues such as relative rock resistance and evidence for neotectonic movement

glaciology of the british isles ice sheet during the last glacial cycle form flow streams and lobes abstract the last ice sheet over the british isles together with other mid latitude pleistocene ice sheets and in contrast to the modern ice sheets of greenland and antarctica had a relatively low profile low summit elevation and extensive elongated lobes at its margin a thermo mechanically coupled numerical ice sheet model driven by a proxy climate has been used to explore the properties that would have permitted these characteristics to develop the approach the key to quantitative palaeoglaciology is to determine the boundary conditions that permit the simulated ice sheet to mimic the evolution of the real ice sheet through the last glacial cycle simulations show how a british ice sheet may have been confluent with a scandinavian ice sheet during some parts of its history and how unforced periodic and asynchronous oscillations could occur in different parts of its margins marginal lobes are a reflection of streaming within the ice sheet such streams can be ephemeral dynamic streams located because of ice sheet properties or fixed streams whose location is determined by bed properties the simulations that best satisfy constraints of extent elevation and relative sea levels are those with major fixed streams that strongly draw down the ice sheet surface in these the core upland areas of the ice sheet were cold based at the last glacial maximum basal streaming velocities were between 500 and 1000 ma 1 compared with surface velocities of 10 50 ma 1 in inter stream zones shear stresses were as low as 15 25 kpa in streams compared with 70 110 kpa in upland areas and 60 84 of the ice flux was delivered to the margin via streams

direct observation of ca na ordering and structure polarity in ca rich intermediate plagioclase feldspar with incommensurate modulated structure ca na ordering and structural polarity of subcells in an intermediate plagioclase with modulated structure have been observed using z contrast imaging with an aberration corrected scanning transmission electron microscope neighboring lamellar domains with i 1 symmetry are related by inversion twin operation instead of anti phase domain boundaries or apbs as in all previously reported structure models the boundaries between lamellar domains have i 1 symmetry instead of c 1 symmetry modulated plagioclase has unique ca na and al si ordering structure that is different from those in end member structures of anorthite and low albite the modulated structures of intermediate plagioclase are not metastable structures formed during phase transition but instead thermodynamically stable structures at low temperature due to ca na ordering within the subcells with i 1 symmetry

bedrock erosion surfaces record former east antarctic ice sheet extent east antarctica hosts large subglacial basins into which the east antarctic ice sheet eais likely retreated during past warmer climates however the extent of retreat remains poorly constrained making quantifying past and predicted future contributions to global sea level rise from these marine basins challenging geomorphological analysis and flexural modeling within the wilkes subglacial basin is used to reconstruct the ice margin during warm intervals of the oligocene miocene flat lying bedrock plateaus are indicative of an ice sheet margin positioned 400 500 km inland of the modern grounding zone for extended periods of the oligocene miocene equivalent to a 2 meter rise in global sea level our findings imply that if major eais retreat occurs in the future isostatic rebound will enable the plateau surfaces to act as seeding points for extensive ice rises thus limiting extensive ice margin retreat of the scale seen during the early eais

glacigenic and associated marine sediments in the weddell sea fjords of spitsbergen and the barents sea a review abstract studies of glacigenic and associated marine sediments in the weddell sea fjords of spitsbergen and barents sea show that during periods of limited glacial coverage interstadials large amounts of material may be brought into the sea mainly by meltwater the major load is deposited relatively close to the icefront 1 at times of maximum glacial extent grounded ice extends onto the shelf with the shelf edge as the outer limit and land and inner shelf areas are eroded main glacial depositional sites are then the outer shelf and shelf edge the sedimentary environment outside an ice shelf is characterized by low clastic supply mainly deposited from icebergs due to low turbidity and a supply of nutrient rich water biogenic matter accumulates leading to a glaciomarine sequence rich in bioclastics sea level lowering results in grounding and seaward expansion of the ice shelf subsequent eustatic rise causes the grounded ice to float deglaciation and sea level rise reduces clastic input to the shelves glacial sediments are reworked and carbonates may accumulate forming tillite carbonate sequence e g late precambrian break up of marine ice sheets such as the barents sea ice sheet seems to have taken place stepwise in response to eustatic rise and climatic changes

re exposed basement landforms in the disko region west greenland disregarded data for estimation of glacial erosion and uplift modelling classifications of large scale landscapes in greenland have traditionally been based on type and intensity of glacial erosion with the general idea that present landforms are mainly the result of

using a gis filtering approach to replicate patterns of glacial erosion jansson k n stroeven a p alm g dahlgren k i t glasser n f goodfellow b w 2011 using a gis filtering approach to replicate patterns of glacial erosion earth surface processes and landforms 36 3 408 418

minimal east antarctic ice sheet retreat onto land during the past eight million years the east antarctic ice sheet eais is the largest potential contributor to sea level rise however efforts to predict the future evolution of the eais are hindered by uncertainty in how it responded to past warm periods for example during the pliocene epoch 5 3 to 2 6 million years ago when atmospheric carbon dioxide concentrations were last higher than 400 parts per million geological evidence indicates that some marine based portions of the eais and the west antarctic ice sheet retreated during parts of the pliocene1 2 but it remains unclear whether ice grounded above sea level also experienced retreat this uncertainty persists because global sea level estimates for the pliocene have large uncertainties and cannot be used to rule out substantial terrestrial ice loss 3 and also because direct geological evidence bearing on past ice retreat on land is lacking here we show that land based sectors of the eais that drain into the ross sea have been stable throughout the past eight million years we base this conclusion on the extremely low concentrations of cosmogenic 10be and 26al isotopes found in quartz sand extracted from a land proximal marine sediment core this sediment had been eroded from the continent and its low levels of cosmogenic nuclides indicate that it experienced only minimal exposure to cosmic radiation suggesting that the sediment source regions were covered in ice these findings indicate that atmospheric warming during the past eight million years was insufficient to cause widespread or long lasting meltback of the eais margin onto land we suggest that variations in antarctic ice volume in response to the range of global temperatures experienced over this period up to 2 3 degrees celsius above preindustrial temperatures 4 corresponding to future scenarios involving carbon dioxide concentrations of between 400 and 500 parts per million were instead driven mostly by the retreat of marine ice margins in agreement with the latest models5 6 analysis of cosmogenic isotopes from a marine sediment core shows that much of the land based east antarctic ice sheet has remained stable for the past eight million years including during the warm pliocene epoch

late cenozoic deep weathering patterns on the fennoscandian shield in northern finland a window on ice sheet bed conditions at the onset of northern hemisphere glaciation abstract the nature of the regolith that existed on the shields of the northern hemisphere at the onset of ice sheet glaciation is poorly constrained in this paper we provide the first detailed account of an exceptionally preserved deeply weathered late neogene landscape in the ice sheet divide zone in northern finland we mine data sets of drilling and pitting records gathered by the geological survey of finland to reconstruct regional preglacial deep weathering patterns within a gis framework using a large geochemical data set we give standardised descriptions of saprolite geochemistry using a variant of the weathering index of parker wip as a proxy to assess the intensity of weathering we also focus on mineral prospects and mines with dense pit and borehole data coverage in order to identify links between geology topography and weathering geology is closely linked to topography on the preglacial shield landscape of northern finland and both factors influence weathering patterns upstanding resistant granulite granite gabbro metabasalt and quartzite rocks were associated with fresh rock outcrops including tors or with thin fines values above 3000 and 4000 beneath valley floors developed along mineralised shear and fracture zones weathering penetrated locally to depths of 50 m and included intensely weathered kaolinitic clays with wip fines values below 1000 late neogene weathering profiles were varied in character tripartite clay gruss saprock profiles occur only in limited areas bipartite gruss saprock profiles were widespread with saprock thicknesses of 10 m weathering profiles included two discontinuities in texture materials and resistance to erosion between saprolite and saprock and between saprock and rock limited core recovery when drilling below the soil base in mixed rocks of the tana belt indicates that weathering locally penetrated deep below upper fresh rock layers such deep seated weathered bands in rock represent a third set of discontinuities incipient weathering and supergene mineralisation also extended to depths of 100 m in mineralised fracture zones the thin weathering crusts found extensively beneath till may represent types of early or middle pleistocene palaeosols we confirm that glacial erosion has been very limited

emergence of the shackleton range from beneath the antarctic ice sheet due to glacial erosion abstract this paper explores the long term evolution of a subglacial fjord landscape in the shackleton range antarctica we propose that prolonged ice sheet erosion across a passive continental margin caused troughs to deepen and lower the surrounding ice sheet surface leaving adjacent mountains exposed geomorphological evidence suggests a change in the direction of regional ice flow accompanied emergence simple calculations suggest that isostatic compensation caused by the deepening of bounding ice stream troughs lowered the ice sheet surface relative to the mountains by 800 m use of multiple cosmogenic isotopes on bedrock and erratics 26al 10be 21ne provides evidence that overriding of the massif and the deepening of the adjacent troughs occurred earlier than the quaternary perhaps this occurred in the mid miocene as elsewhere in east antarctica in the mcmurdo dry valleys and the lambert basin the implication is that glacial erosion instigates feedback that can change ice sheet thickness extent and direction of flow indeed as the subglacial troughs evolve over millions of years they increase topographic relief and this changes the dynamics of the ice sheet

sand grain threshold in relation to bed stress history an experimental study besides particle size density and shape the erodibility of a sediment bed depends also upon the exposure to prethreshold velocities in the overlying flow such flow effectively rearranges the grains at and below the bed surface causing them to become more resistant to subsequent erosion the effects of the stress history leading up to the critical condition for sediment movement are investigated for unidirectional flows generated in a recirculating laboratory flume the sediment beds investigated consisted of cohesionless quartz sand grains with mean grain diameters of 0 194 mm fine sand 0 387 mm medium sand and 0 774 mm coarse sand with narrow particle size distributions the critical threshold shear velocity target value for the three beds was established within 2 5 min of increasing the flow from zero velocity the subsequent experiments were performed under prethreshold velocities at 70 for 5 10 20 40 and 80 min exposure duration 80 for 5 10 20 40 and 80 min exposure duration 90 and 95 for 5 10 20 40 80 and 120 min exposure duration of the target value following exposure to these different prethreshold conditions the flow was increased then to reach actual critical conditions within a period of 2 5 min the critical condition for the initiation of sediment movement was established using visual observation supplemented by video recordings according to the yalin criterion the results show that if the exposure duration to prethreshold velocities remains constant then the critical shear velocity increases with increasing prethreshold velocity likewise if the prethreshold velocity remains constant then the critical shear velocity increases with increasing exposure duration in some circumstances the critical shear velocity was found to increase by as much as 27 an empirical formula is proposed to account for the exposure correction to be applied to the critical shear velocities of sand sized sediment beds this is prior to their inclusion into bedload transport formulae for an improved prediction of the magnitude and nature of transport

initiation of motion of calcareous sand this paper examines the initiation of motion of four natural and five sieved calcareous sand samples in unidirectional flow flume experiments yield the sediment transport rate as a function of bed shear stress up to bed form development reference based criteria are supplemented by visual observations to determine the critical shear stress the results are compared with published data for rounded and irregular particles in terms of the median sieve size and the corresponding nominal and equivalent diameters as functions of particle reynolds number the comparison shows that the critical shear stresses of the irregular particles are higher than the shields curve in the hydraulically smooth flow regime and lower in the rough turbulent flow regime

subglacial preservation of valley morphology at amundsenisen western dronning maud land antarctica on the high altitude polar plateau of amundsenisen western dronning maud land east antarctica a subglacial valley with a broad horizontal valley floor interpreted as a sediment floodplain or valley delta was studied by radio echo sounding in addition a small probably glacial valley was mapped within the same subglacial massif basal ice temperatures were calculated using field data on precipitation air temperature and ice sheet thickness discoveries of old landforms which have been preserved more or less intact beneath the former fennoscandian and laurentide ice sheets have received increasing attention during the last decade the aim of this study is to investigate whether preservation of landforms occurs under the east antarctic ice sheet and to discuss under that climatological and glaciological circumstances preservation may take place the results show that the ice sheet covering the investigated localities is frozen to bed and therefore has an insignificant erosional capability the observations suggest that a large scale subglacial sediment deposit and a small valley formed by glacial erosion have survived beneath a cold based ice sheet marginal zone for a long time period the process of glacial preservation recognized for bedrock features and tentatively observed for sediment accumulations should act on similar large scale landforms under any cold based ice sheet present or past on the basis of existing studies of the age and stability of the east antarctic ice sheet a middle pliocene age is suggested for the preserved landforms the presence of the presumed sediment filled valley further indicates that no prolonged periods of basal melting have occurred at the amundsenisen study area during the ice sheet history which includes the quaternary glaciation periods finally calculations of basal temperature for localities at different altitudes within the same subglacial massif were used to demonstrate local altitudinal control of glacial preservation 1997 by john wiley sons ltd

minimal erosion of arctic alpine topography during late quaternary glaciation alpine topography in europe has been shaped by recent glaciations cosmogenic dating of summits in svalbard suggest that arctic alpine topography is a million years old and that subsequent glaciations have preserved rather than eroded the landscape the alpine topography observed in many mountainous regions is thought to have formed during repeated glaciations of the quaternary period1 2 before this time landscapes had much less relief1 2 3 however the spatial patterns and rates of quaternary exhumation at high latitudes where cold based glaciers may protect rather than erode landscapes are not fully quantified here we determine the exposure and burial histories of rock samples from eight summits of steep alpine peaks in northwestern svalbard 79 5 n using analyses of 10be and 26al concentrations4 5 we find that the summits have been preserved for at least the past one million years the antiquity of svalbard s alpine landscape is supported by the preservation of sediments older than one million years along a fjord valley6 which suggests that both mountain summits and low elevation landscapes experienced very low erosion rates over the past million years our findings support the establishment of northwestern svalbard s alpine topography during the early quaternary we suggest that as the quaternary ice age progressed glacial erosion in the arctic became inefficient and confined to ice streams and high relief alpine landscapes were preserved by minimally erosive glacier armour

experiments on the entrainment threshold of well sorted and poorly sorted carbonate sands few studies have examined the hydrodynamic behaviour of carbonate sediments the data presented here are the result of preliminary research on entrainment in well and poorly sorted carbonate sands experiments were performed using naturally occurring sediments in a tilting recirculating freshwater flume results indicate that when of similar size shape and density the transport threshold of carbonate sands is similar to that of quartz however owing to their lower density and often platy or irregular shape skeletal sands require a lower shear stress to initiate transport because the density of carbonate particles may increasingly vary with grain size the threshold of motion in coarse carbonate grains may differ more markedly from that of quartz in poorly sorted samples results show that the coarse grained constituents move before the finer grained components grain properties and boundary layer dynamics are believed to explain this phenomenon rollability of the larger grains combined with physical trapping and immersion within a low velocity sublayer are believed to prevent finer particles from moving given the appropriate sediments and flow conditions it may therefore be possible to deposit and preserve fine grained sediments in a flow regime typically thought to transport such materials

modification of peripheral mountain ranges by former ice sheets the brecon beacons southern uk abstract the brecon beacons in southern wales is a large upland area 900 km 2 close to the periphery of the former british irish ice sheet the geomorphology of the brecon beacons highlands and surrounding areas was mapped using satellite imagery and a digital elevation model dem a fragmentary pattern of glacial lineations is inferred to represent ice build up in the mountains a more coherent set of glacial lineations formed by southward ice flow across the area from the welsh ice cap represents recession from the last glacial maximum lgm during this phase the brecon beacons acted as an obstacle deflecting the ice around higher terrain most of the higher terrain in the brecon beacons therefore appears to have escaped significant glacial erosion during both phases of ice flow due to cold based ice or low basal ice velocities beneath a local ice cap landforms previously interpreted as cirques in fact have v shaped profiles and lack evidence of major over deepening they are interpreted to have been formed primarily by processes other than glacial erosion the overall impact of glaciation in the brecon beacons appears to be limited primarily to the time when the area was over run by the welsh ice cap and the influence of local glaciation has been minimal this study has two main implications for landscape evolution in mountain ranges that lie at the periphery of former ice sheets first due to the presence of cold based ice or low basal ice velocities beneath a local ice cap it may be possible to find fragmentary traces of pre lgm ice flow second the pattern of glacial lineations in and around the brecon beacons indicates that peripheral mountain ranges may have deflected ice flow at the lgm resulting in low basal ice flow velocities and possible frozen bed conditions over higher terrain which consequently resulted in ineffective glacial erosion within the mountains themselves

cheniers and chenier plains a general introduction abstract cheniers and chenier plains have been studied in many coastal regions throughout the world an inventory of the investigations currently being carried out is presented in this and accompanying papers cheniers and chenier plains are azonal features their distribution ranges from tropical to the subarctic regions and they show a variety of features progradational shorelines where mud accretion is temporarily interrupted favour the development of these features in addition to the concept of otvos and price who define cheniers and chenier plains as being formed exclusively in relation to littoral progradational environments it is suggested here that chenier formation in deltaic progradational environments should also be included genetically two types of cheniers can be distinguished depending on the origin of the related sediments cheniers developing due to wave winnowing of the muddy sediments and the subsequent concentration of the coarser particles to form a longshore bar or ridge which is driven shoreward and cheniers built by littoral drift in both cases they can be recognised by their sedimentary structures when cheniers arrive onshore they will migrate further landward due to washover processes under extreme hydrodynamical conditions e g spring tides storm surges etc to summarise the information concerning the episodic development of chenier plains in order to produce a or a few universal model s appears to be very difficult owing to the numerous factors that influence their formation these include river mouth shifting sealevel changes climatic changes storm frequency mortality of shellfish and bay geometry owing to this sensitivity to environmental changes during their formation cheniers are considered to be very important geomorphological features they provide a relatively accurate record of sealevel and environmental changes which can be dated with high resolution

settling velocity and grain shape of maerl biogenic gravel abstract the importance of a grain size dependent shape metric convexity for determining the unusual settling velocity characteristics of maerl a variety of unattached coralline algae has been quantified by modeling of settling tube data a modification of the general settling velocity equation of ferguson and church 2004 involving a dependence of the drag coefficient related constant c2 on grain size produces a satisfactory fit to the experimental observations for a given grain size and at reynolds numbers greater than 220 maerl grains experience greater drag than is predicted for natural quartz grains by ferguson and church 2004 because of this grain size dependent roughness subsequent detailed measurements of maerl grain shape using microscopic image analysis confirm a strong positive linear relationship between grain roughness quantified by the reciprocal of convexity and grain size this departure from the ideal settling characteristics of siliciclastic gravel is hypothesized to explain the observed propensity of maerl under suitable hydrodynamic conditions to form beach deposits with a low percentage of sand maerl samples from three different sedimentary environments open marine intertidal and beach exhibit different linear relationships between roughness and grain size probably resulting from different degrees of abrasion due to a combination of different wave climates and transport histories this spatial variability in grain texture suggests that a general equation for maerl settling velocity is not possible however for maerl and other branched sediment types it may only be necessary to measure the convexity of the middle and largest size fractions to estimate the linear variation of c2 with grain size resulting in an accurate estimate of the settling velocity curve in the broader context of physical sedimentology our results indicate that over a range of bottom current conditions between 200 and 250 mm s 1 where the settling curve of maerl is flat and grain size invariant relative to siliciclastic sediment a larger part of the maerl grain size distribution can remain in suspension compared to the siliciclastic sediment this contrast in physical properties may be an effective process for the spatial separation of coarse siliciclastic and biogenic sediment

periglacial trimlines in the scottish highlands abstract two periglacial weathering limits have been identified on mountains in the highlands of scotland the lower has been interpreted as a periglacial trimline marking the upper limits of loch lomond younger dryas stade glaciers and has been employed to reconstruct the dimensions of parts of the main west highland icefield and certain smaller glaciers and icefields interpretation of the upper weathering limit has been more contentious but recent research has demonstrated 1 significant contrasts in the degree of bedrock weathering above and below this limit 2 that gibbsite a pre devensian weathering product is abundant above the limit but rare in soils farther downslope 3 that the isotopic 26al age of bedrock exposures above the limit greatly exceeds that at lower altitudes and 4 that the altitude of the weathering limit descends regularly along former ice sheet flowlines collectively these attributes suggest that the upper weathering limit is a periglacial trimline defining the maximum altitude of the last ice sheet calculated basal shear stress values are consistent with this interpretation which implies that the mainland ice shed lay at ca 900 m in wester ross and descended gently northwards whilst harris and lewis in the outer hebrides supported an independent ice cap that achieved a maximum altitude of ca 700 m wider implications of this interpretation and the future potential for ice sheet reconstruction are explored

settling characteristics of calcareous sand this paper provides quantitative comparisons and relationships for the fall velocities and drag coefficients of 998 calcareous sand grains collected on oahu hawaii laboratory analyses of the grains provide the three orthogonal dimensions fall velocity and density from which the nominal diameter drag coefficient reynolds number and shape factor are evaluated for each grain the fall velocity and drag coefficient expressed respectively as functions of nominal diameter and reynolds number show strong correlation over a wide range of corey shape factors the present relationships are validated with published data based on quartz grains analysis of the data by flow regime shows that particle shape has a stronger influence on the settling characteristics when turbulent boundary layers develop around the grains

the first stages of erosion by ice sheets evidence from central europe abstract 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 moutonnee 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 moutonnees 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

calculating quaternary glacial erosion rates in northeast scotland northeast scotland is an area exhibiting selective erosion by quatemary ice sheets in this area both glacial and preglacial landforms exist in close proximity the depths of erosion which this modification represents are calculated on the assumption of various depths of preglacial weathering a depth of erosion of between 34 and 62 m per unit area is indicated calculated rates of erosion are 0 021 nun a for the entire 2 3 m y of the quaternary and between 0 1 and 0 5 mm aa1 on the assumption that glacial conditions existed in this area for 500 000 years and 100 000 years respectively these figures are compared to the offshore sedimentary record in the adjacent west central north sea the volume of sediment deposited offshore is equivalent to a depth of erosion of 195 m per unit area yielding an average erosion rate of 0 085 mm a i over the entire quatemary rates of erosion were low in the preglacial pliocene 0 049 mm a 1 and early quatemary 0 063 mm a the expansion of ice sheets across the area in the middle quatematy was associated with a sharp increase in the rates of erosion 0 13 mm a but the last late devensian ice sheet in the area was less erosive 0 095 mm a the estimated rates of erosion represented by the offshore sedimentary record therefore exceed the estimated rates of glacial erosion from the onsho re geomorphological reconstruction

glacial modification of granite tors in the cairngorms scotland a range of evidence indicates that many granite tors in the cairngorms have been mod ified by the flow of glacier ice during the pleistocene comparisons with sw england and the use of a space time transformation across 38 tor groups in the cairngorms allow a model to be developed for progressive glacial modification tors with deeply etched surfaces and no or limited block removal imply an absence of significant glacial modification the removal of superstructure and blocks locally forming boulder trains and the progressive reduction of tors to stumps and basal slabs repre sent the more advanced stages of modification recognition of some slabs as tor stumps from which glacial erosion has removed all superstructure allows the original distribution of tors to be recon structed for large areas of the cairngorms unmodified tors require covers of non erosive cold based ice during all of the cold stages of the middle and late pleistocene deformation beneath cold based glacier ice is capable of the removal of blocks but advanced glacial modification requires former wet based glacier ice the depth of glacial erosion at former tor sites remains limited largely to the partial or total elimination of the upstanding tor form cosmogenic nuclide exposure ages phillips et al 2006 together with data from weathering pit depths hall and phillips 2006 from the surfaces of tors and large erratic blocks require that the glacial entrainment of blocks from tors occurred in marine isotope stages mis 4 2 6 and probably at least one earlier phase the occurrence of gla cially modified tors on or close to the main summits of the cairngorms requires full ice cover over the mountains during these stages evidence from the cairngorms indicates that tor morphology can be regarded as an important indicator of former ice cover in many formerly glaciated areas particularly where other evidence of ice cover is sparse recognition of the glacial modification of tors is impor tant for debates about the former existence of nunataks and refugia copyright 2006 john wiley sons ltd

experiments on the settling overturning and entrainment of bivalve shells and related models an experimental and theoretical examination has been made of the settling entrainment and overturning of 176 valves representing 16 common northwest european marine bivalve species together with a comparative study of 15 plastic models in the form of segments from cylindrical tubes settling behaviour in both stagnant and moving water depends on particle mass symmetry and concavo convexity separated empty bivalve shells spin and spiral while settling and if sufficiently elongated also pitch at the observed reynolds numbers the shells and models fall concave up the terminal fall velocity increasing as the square root of the unit immersed mass or weight the drag coefficient is independent of reynolds number but increases with surface roughness and particularly particle elongation turbulence slightly lowers the critical elongation for pitching a separation vortex lies captive on the upper side of each descending particle consequently an empty bivalve shell traversing a suspension of sand traps grains on its upper side at a rate proportional to their volume concentration and terminal fall velocity this process increasing the effective shell mass is limited only by the capacity of the shell and grain spillage due to the possible onset of pitching the ratio non dimensional of a quantity proportional to the applied fluid force and the particle unit immersed weight consistently describes the entrainment of concave up and convex up particles and also the immediate overturning of a valve on settling concave up to the bed these thresholds vary in relative magnitude with bed particle friction and particle concavo convexity in general convex up particles are the most stable the concave up entrainment and overturning thresholds are of a substantially lower but similar magnitude the high frequency of concave up bivalve attitudes in turbidites is understandable largely in terms of the ability of a settling valve to increase in effective mass by grain entrapment convex up attitudes in the lower parts of turbidites may record currents stronger than the overturning threshold

the glaciation of dartmoor the southernmost independent pleistocene ice cap in the british isles the granite uplands of dartmoor have traditionally been considered to be relict permafrost and periglacial landscapes that lay beyond the limits of quaternary glaciations but a variety of landform evidence indicates that a plateau icefield existed on the northern part of the moor constituting the southernmost independent ice cap in the british isles overdeepened or weakly u shaped valley segments fringing north dartmoor document an early extensive phase of glaciation but the most convincing landform evidence relates to more recent valley based glacier occupancy a moraine ridge on the slipper stones represents the most unequivocal palaeo glacier on north dartmoor with a palaeo ela of c 460 m above sea level asl although this relates to the youngest and most restricted phase of glaciation a longer term ela is likely to be represented by the corn ridge proto cirque at 370 410 m asl more extensive valley glaciers are recorded in each of the major drainage basins of north dartmoor by arcuate and linear bouldery ridges and hummocky valley floor drift which are interpreted as latero frontal moraines deposited by outlet lobes of a plateau icefield recession of these lobes is marked by inset sequences of such ridges and occasional meltwater channels plateau ice was predominantly thin and protective and snowblow and preferential accumulation in valley heads facilitated the modest glacial erosion and debris transport recorded in the landforms and sediments it is proposed that the highest plateaux have been occupied by ice for the longest cumulative period of time throughout the quaternary average glacial conditions explaining the distribution of different tor types on northern dartmoor this also explains the lack of tors on the most expansive of the highest plateau terrain ice dispersal centres as the product of a average glacial conditions preferentially removing tors or dampening their production rates b the survival of high relief type 1 tors during glaciation if they occupy summits too narrow to develop significant plateau icefields and or ridges that are bypassed by faster moving ice in adjacent deep valleys and c the survival of subdued type 2 tors in areas glaciated less regularly during the quaternary simple ice flow modelling indicates that a plateau icefield type glaciation is required for significant ice flow to occur and confirms thin ice cover in particular on narrow summits thereby supporting the explanation of tor class distribution the modelling allows us to spatially correlate the geomorphological evidence of margin positions into two major stages and further indicates a strong altitude mass balance feedback leading to an ice cap that is not in balance with its climate and with an extent that is limited by the length of the cold phases rather than their severity

study on structure variations of incommensurately modulated labradorite feldspars with different cooling histories the incommensurately modulated structures of three intermediate plagioclase feldspars with compositions of an 51 are determined by single crystal x ray diffraction analyses the samples selected cover a range of different cooling rate from relatively fast to extremely slow the structures show various ordering states that are directly correlated with the cooling histories of the samples the slowest cooled sample shows an e 1 structure with strong density modulation along with nanoscale exsolution lamellae the fastest cooled sample displays an e 2 structure without second order satellite reflections f reflections and density modulation the sample with intermediate cooling rate shows a less ordered e 1 structure with weak density modulation but the modulation period and orientation are the same as in e 2 structure the comparison of the structures with the same composition reveals the ordering process and phase transitions during the cooling of plagioclase within the compositional range of boggild intergrowth new parameters from modulation waves can be used for quantifying the ordering state of plagioclase feldspars proposed phase relationship and t t t diagram for an 51 plagioclase feldspars are illustrated for explaining the relationship among c 1 e 1 and e 2 structures and relative cooling rates of their host rocks

mineralogy chemistry and palaeoenvironmental significance of an early tertiary terra rossa from northern ireland a preliminary review abstract early tertiary basalts of the antrim plateau have preserved palaeosols developed on late cretaceous limestone these palaeosols include dark red clays and clays with flints that are rich in silica and contain a complex clay mineral assemblage of 2 1 lattice clays kaolinite and sepiolite they occur in infilled solution hollows and joints can be associated with a rudimentary tower karst and are similar in appearance and composition to present day mediterranean terra rossa soils elsewhere grey flint rich clay sized material dominated by silica and kaolinite developed on an undulating landscape sometimes now capped by a lignite horizon deposits and the palaeolandscape suggest a humid tropical to sub tropical environment contrary to previous published inferences of arid conditions but corresponding to humid tropical conditions proposed for the formation of overlying inter basaltic soils

evidence of high energy events in the geological record mid holocene evolution of the southwestern donana national park sw spain abstract four phases are distinguished in the mid holocene evolution of the southwestern donana national park sw spain based on the multidisciplinary study of sediments in drill cores in the oldest phase 5500 5400 cal years bp a coastal brackish lagoon occupied the central part of the study area partly enclosed by the donana spit and limited by fluvial levees an evolution from subtidal to intertidal conditions characterized the areas located close to the inner side of the donana spit during the same phase the following phase 5400 5200 cal years bp is characterized by a high energy event tsunami which caused the breakthrough of the donana spit and the creation of new littoral strands in the inner areas in the first period of the third phase 5200 4200 cal years bp estuarine infilling was probably the dominant process with the accumulation of phyllosilicate rich clays in the lagoon bed this was followed by a renewed phase of instability 4200 4100 cal years bp indicated by the presence of fine storm lain deposits and thicker probably tsunami induced shelly deposits the last phase 4100 3700 cal years bp comprises a an infilling period with a diminution of the marine influence and b two new high energy events which caused the erosion of the donana spit and the creation of new cheniers in the last 7000 years the comparison of these results with a revision of both geological and historical records has revealed the occurrence of twenty tsunamis at least during this period in portugal and southern spain evidences of these high energy events are washover fans bioclastic coarse sized layers within salt marsh deposits or the presence of giant boulders and pebbles the determination of the recurrence period is very difficult owing to the very scarce data although our data and other authors do not discard a possible periodicity of 2000 years for a cycle of two events separated by 300 400 years

hydraulic characteristics of bioclastic deposits new possibilities for environmental interpretation using settling velocity fractions this paper examines the hydraulic behaviour of heterogeneous bioclastic sediments using settling velocity fractions bioclastic deposits are divided into fractions by splitting hydraulically sorted samples compositional analysis of fractions shows that grains exhibit marked variation in composition shape size and density which control the hydraulic behaviour of bioclastic deposits despite the heterogeneous grain properties of fractions analogous to sieve fractions sedimentation analysis shows that they possess a narrow range of settling velocity values flume experiments also show that the settling velocity fractions possess a narrow range of threshold velocity values a threshold relationship is identified in which the velocity required to entrain mixed settling fractions increases from 20 5 to 65 0cm s 1 for settling fractions with mean settling values of 5 5 chi 2 2 cm s 1 to 1 4 chi 36 0 cm s 1 respectively heterogeneous settling fractions due to intergrain effects were found to possess larger threshold velocities than individual components showing that the composition of bioclastic deposits controls hydraulic behaviour thus caution must be exercised in using the threshold relationship for homogeneous sediments or deposits of markedly different composition as settling velocity fractions reflect narrow hydraulic settling and threshold properties of sediments the mean settling velocity of fractions is considered a good indicator with which to interpret transport and depositional processes comparison of settling and sieve size distributions shows that size distributions do not reflect the hydraulic behaviour of bioclastic deposits and should not be used to interpret environmental processes the study indicates that examination of settling velocity fractions for which hydraulic settling and threshold properties are known coupled with compositional analysis of these fractions will allow much greater environmental interpretation of deposit forming and energy processes in reef environments

landscape evolution of the dry valleys transantarctic mountains tectonic implications there are different views about the amount and timing of surface uplift in the transantarctic mountains and the geophysical mechanisms involved our new interpretation of the landscape evolution and tectonic history of the dry valleys area of the transantarctic mountains is based on geomorphic mapping of an area of 10 000 km2 the landforms are dated mainly by their association with volcanic ashes and glaciomarine deposits and this permits a reconstruction of the stages and timing of landscape evolution following a lowering of base level about 55 m y ago there was a phase of rapid denudation associated with planation and escarpment retreat probably under semiarid conditions eventually downcutting by rivers aided in places by glaciers graded valleys to near present sea level the main valleys were flooded by the sea in the miocene during a phase of subsidence before experiencing a final stage of modest upwarping near the coast there has been remarkably little landform change under the stable cold polar conditions of the last 15 m y it is difficult to explain the sirius group deposits which occur at high elevations in the area if they are pliocene in age overall denudation may have removed a wedge of rock with a thickness of over 4 km at the coast declining to 1 km at a point 75 km inland which is in good agreement with the results of existing apatite fission track analyses it is suggested that denudation reflects the differences in base level caused by high elevation at the time of extension due to underplating and the subsequent role of thermal uplift and flexural isostasy most crustal uplift 2 4 km is inferred to have occurred in the early cenozoic with 400 m of subsidence in the miocene followed by 300 m of uplift in the pliocene

cosmogenic radionuclides from fiord landscapes support differential erosion by overriding ice sheets the interpretation of differentially weathered mountainous areas along the fringes of pleistocene ice sheets is fundamental for determining ice sheet behavior and thickness during the last glaciation two existing interpretations are either that highly weathered uplands remained as nunataks while freshly eroded troughs held outlet glaciers during the last glaciation or that uplands and lowlands were equally covered by ice but that it was differentially erosive as a function of its spatially variable basal thermal regime cosmogenic radionuclide measurements from 33 bedrock samples and 27 upland erratics from differentially weathered fi ord landscapes on northeastern baffi n island shed light on laurentide ice sheet lis dynamics and thickness tors on weathered upland surfaces have minimum 10 be ages between ca 50 and ca 170 ka n 12 whereas the majority of erratics perched in the uplands range from ca 10 to ca 13 ka n 14 indicating that the whole landscape was glaciated during the last glacial maximum lgm glacially sculpted bedrock n 7 near sea level refl ects the age of deglaciation and increasing amounts of isotopic inheritance in highelevation sculpted bedrock n 3 higher elevation intermediately weathered bedrock n 11 and highest elevation intensely weathered bedrock refl ect the weakening of erosive power as the ice sheet transitioned from fi ords to interfi ord plateaus cold based glaciation of uplands was contemporaneous with warm based glaciation in fi ords suggesting the presence of ice streams 26 al and 10 be concentrations measured in 10 bedrock samples indicate that tors have experienced a complex history of alternating periods of exposure burial and limited glacial erosion the paired isotope data reveal a minimum duration of burial of 80 500 k y presumably by cold based ice and indicate that the tors have an age of at least 150 580 ka these data together with other reconstructions of ice streams in the eastern canadian arctic suggest that ice streams were a large infl uence on northeastern lis dynamics throughout the late quaternary thus the northeastern lis was sensitively tied with north atlantic thermohaline circulation and abrupt climate change

chemical weathering studies in relation to geomorphological research in southeastern canada abstract although chemical weathering provides fundamental information relevant to geomorphology the subject has been overlooked during the 20th century in canada this paper provides an overview of the current state of canadian research on chemical weathering in southeastern canada and takes into account three spatial and temporal contexts 1 the formation of bedrock morphology by chemical weathering 2 occurrences characteristics and age of saprolites and 3 contemporary chemical denudation rates long term geomorphological evolution of southeastern canadian landscapes shows that chemical weathering has played an important role an example is taken from the laurentide region of the canadian shield north of montreal quebec the present topography resulted from the stripping of the former weathering mantle and from the probable subsequent modification of the weathering front first by the action of hillslope processes and rivers and then by glaciers before and during plio pleistocene times the present landscape reflects the timing of the formation of erosion surfaces and of the stripping of the paleozoic cover rocks and exposure of the shield since the late seventies several isolated occurrences of saprolite soil profiles have been discovered in eastern canada and prompted a renewal of the study of these materials about the cenozoic evolution of these regions one of the problems in this field of research is the dating of saprolites and their inclusion in a chronostratigraphic framework along with the other cenozoic surficial deposits and landforms because of the multiple factors involved in the development of secondary minerals in saprolites during the course of weathering it is preferable to distinguish the dating of saprolites from the study of their mineralogical and geochemical evolution fortunately several new techniques are becoming available for the absolute dating of surficial deposits and saprolites including the use of cosmogenic radionuclides saprolites provide a strong potential field of research for our understanding of the geological evolution of eastern canada during the cenozoic contemporary weathering and erosion rates are fields of research that have gained increasing interest recently since modeling landscape geochemical response can be applied to various environmental stress situations such as acidification by rain and forest harvesting rock type may be the main factor explaining the large differences between watersheds in fact variability of cation removal in the temperate zone is probably most closely related to flow paths of water investigations at different scales from entire watersheds to slopes to individual pedons highlight the problem in the catamaran brook watershed new brunswick water chemistry is explained by a mix of groundwater and soil solution from the horizons at the base of the floodplain soils geochemical mass balances based on net outputs give little information on the weathering reactions of primary minerals the weathering products or on the nature of the weathering processes that provide the dissolved load of streams mineralogic and petrographic analysis of selected soil pedons are necessary to determine weathering reactions and their role as sources or sinks for bases silica aluminum and iron in the various compartments through which water percolates before it reaches the stream

limited modification of mid latitude landscapes by ice sheets the case of northeast scotland the paper uses a case study in scotland to examine the amount and processes of landscape modification by quaternary ice sheets there is an inverse correlation between the distribution of landforms of glacial erosion and pre glacial landscape remnants in northeast scotland the implication is that in places ice sheets can preserve a pre glacial landscape unscathed while elsewhere they remove the pre glacial weathered rock the location of glacial protection or erosion is strongly influenced by the topography and its influence on former ice sheet flow and basal thermal regime the classic glacially eroded landscape of areal scouring can be produced by the removal of only 10 50 m of weathered rock furthermore rock basins often regarded as the hallmark of glacial erosion may be directly inherited from the pre glacial pattern of deep weathering

landscape preservation under fennoscandian ice sheets determined from in situ produced 10be and 26al some areas within ice sheet boundaries retain pre existing landforms and thus either remained as ice free islands nunataks during glaciation or were preserved under ice differentiating between these alternatives has significant implications for paleoenvironment ice sheet surface elevation and ice volume reconstructions in the northern swedish mountains in situ cosmogenic 10be and 26al concentrations from glacial erratics on relict surfaces as well as glacially eroded bedrock adjacent to these surfaces provide consistent last deglaciation exposure ages 8 13 kyr confirming ice sheet overriding as opposed to ice free conditions however these ages contrast with exposure ages of 34 61 kyr on bedrock surfaces in these same relict areas demonstrating that relict areas were preserved with little erosion through multiple glacial cycles based on the difference in radioactive decay between 26al and 10be the measured nuclide concentration in one of these bedrock surfaces suggests that it remained largely unmodified for a minimum period of 845 418 461 kyr these results indicate that relict areas need to be accounted for as frozen bed patches in basal boundary conditions for ice sheet models and in landscape development models subglacial preservation also implies that source areas for glacial sediments in ocean cores are considerably smaller than the total area covered by ice sheets these relict areas also have significance as potential long term subglacial biologic refugia

deciphering a non glacial glacial landscape mosaic in the northern swedish mountains relict surfaces contain information on past surface processes and long term landscape evolution a detailed investigation of relict non glacial surfaces in a formerly glaciated mountain landscape of northern sweden was completed based on interpretation of colour infrared aerial photographs analysis in a gis and fieldwork working backwards from landscape to process surfaces were classified according to large and small scale morphologies that result from the operation of non glacial processes the degree of weathering regolith characteristics and the style of glacial modification surfaces were also compared in the gis according to elevation slope angle and bedrock lithology the study revealed five types of relict non glacial surfaces but also two types of extensively weathered glacial surfaces that were transitional to relict non glacial surfaces illustrating spatially variable processes and rates of non glacial and glacial landscape evolution rather than being static preglacial remnants relict non glacial surfaces are dynamic features that have continued to evolve during the quaternary the classification provides hypotheses for landscape evolution that can be field tested through for example terrestrial cosmogenic nuclide studies and geochemical analyses of fine matrix materials the classification may be applicable to relict non glacial surfaces in other formerly glaciated landscapes

solved the enigma of labradorite feldspar with incommensurately modulated structure 1 intermediate plagioclase feldspars are the most abundant minerals in the earth s crust their incommensurately modulated structure has puzzled geologists and crystallographers for decades since the phenomenon in a labradorite was reported in 1940 solving the structure is a necessary step toward mapping the complex subsolidus phase relations of plagioclase solid solution the structure of a homogeneous labradorite an51 single crystal from a metamorphic rock is solved and refined from single crystal x ray diffraction the result structure can be simplified as alternating i 1 like lamellae domains related by inversion twins the inversion boundary shows an anorthite like structure with formula 2 symmetry and is richer in ca than the neighboring domains with opposite polarity no albite like subunits appear in the e plagioclase structure the modulated structure displays a unique al si ordering pattern a density modulation with a variation of 17 mol in composition is also observed and can be properly described only by applying second order harmonic waves for the atomic modulation functions the modulated structure reveals details that cannot be observed from refinement with only main reflections and may be used to assess the ordering state and cooling rate of its host rock the homogeneity of the crystal indicates the closure of the solvus for boggild intergrowth at low temperature the highly ordered modulation supports the thermodynamic stability of e plagioclase both al si ordering and ca na ordering are the driving force for formation of the incommensurately modulated structure 1 embed graphic 1 gif 2 embed mml math 1 gif