the late devensian 22 000 bp irish sea basin the sedimentary record of a collapsed ice sheet margin the late devensian 20 ka bp glacial geology of the irish sea basin 4000 km2 is an event stratigraphy recording the entry of marine waters into a glacio isostatically depressed basin and the rapid retreat of the irish sea glacier as a tidewater ice margin marine limits occur up to 140 m o d across much of the central basin the ice margin was uncoupled from its bed exposing a subglacially scoured topography to glaciomarine processes the irish sea glacier was a major drainage conduit of the last british ice sheet calving of the marine ice margin resulted in fast flow surging of ice streams recorded by drumlin fields around the northern basin margin and tunnel valleys rapid evacuation of the basin may have stranded large areas of dead ice in peripheral zones e g cheshire shropshire lowlands and initiated the collapse of the ice sheet thick wedges of ice contact glaciomarine sediments were deposited during ice retreat as morainal bank complexes by successive tidewater ice margins stabilized at pinning points around the irish sea coast where morainal banks occur on the seaward side of drumlin swarms there is a clear sequential relationship between rapid ice loss from calving ice margins the development of fast flowing ice streams drumlinization and the pumping of subglacial sediment to tidewater raised delta complexes are locally associated with marine limits along the high relief coastal margins of wales east central ireland and the lake district associated valley infill complexes record downslope resedimentation of heterogenous sediments into the marine environment during ice retreat co eval offshore deposits are represented by well stratified glaciomarine complexes that infill a subglacially scoured topography that shows networks of tunnel valleys glaciomarine mud drapes occur well to the south of the maximum limit of grounded ice in the basin e g north devon scilly islands southern ireland the age of these distal sediments previously mapped as pre devensian tills is constrained by amino acid ratios basin rebound following deglaciation was rapid with over 100 m recovery in 3 ka and was followed by a low marine still stand peat accumulating in offshore areas now as much as 55 m below sea level has been drowned by the postglacial eustatic rise in sea level the glacio sedimentary model identified in this paper involving rapid ice retreat and related sedimentation triggered by rising relative sea level suggests that isotatic downwarping is an important mechanism for deglaciating continental shelves

re advance of scottish ice into the solway lowlands cumbria uk during the main late devensian deglaciation the complex glacial geomorphology and stratigraphy of the solway lowlands cumbria uk reflects dynamic ice flow during the main late devensian glaciation with numerous ice flow shifts and re advances now recognised the final incursion of scottish ice into the region scottish re advance during a late stage of deglaciation has provoked widespread debate and even scepticism arising from ill defined marginal limits and a paucity of landforms sediments and dates in an attempt to resolve some of these issues this paper has applied both geomorphological and stratigraphic techniques to critically review evidence pertaining to the scottish re advance a major deltaic landform sediment assemblage at holme st cuthbert a thin patchy till sheet and eskers at thursby and sowerby wood verify that scottish ice re advanced into the solway lowlands at a late stage of deglaciation the thin patchy till sheet associated with the re advance is indicative of a short lived pulse with ice thought to have flowed rapidly across water saturated sediment and into proglacial lakes dammed up against higher ground to the east the holme st cuthbert delta which evolved from a subaqueous outwash fan marks a major still stand and development of a large ice contact lake in the vicinity of wigton the extent of the glaciation is difficult to elucidate with the till sheet disparately exposed as far east as lannercost although identification of the blackhall wood re advance till member probably limits its se extent to just south of carlisle coeval re advance of lake district ice is possible given that a final stage lobate movement of ice out of the vale of eden and into the solway lowlands is now recognised however there is no direct stratigraphic correlation or dating control

effects of the canyon topography on ground motions at the feitsui damsite abstract a numerical scheme based on the boundary integral equation formulation is applied to a two dimensional sh model of the feitsui canyon to study the effects of the canyon topography on strong ground motion the medium responses of this model for plane sh waves incident at various angles were calculated in a frequency band ranging from 0 to 6 0 hz it appears that the shielding and scattering effects of canyon topography can greatly modify the ground motions based on these medium responses the recorded sh ground motion at one station is considered as the input to simulate the corresponding component at the second station the observed and the simulated ground motions at the second station show satisfactory agreement in ground displacement and velocity the simulated accelerations corresponding to the high frequency part of ground motions did not fit the observations well this discrepancy shows the limitation of our simplified model which does not include the effects of the dam and the variation of topography in the third dimension in order to improve the simulation of ground accelerations it is necessary to have the 3 d modeling

new evidence for a grounded irish sea glaciation of the isles of scilly uk abstract new geomorphological and sedimentological data support previously published reconstructions of an irish sea glacier advance as far south as the northern shores of the isles of scilly in the celtic sea 49 58 n offshore boulder covered bars with cores of diamicton are interpreted as moraines which together with onshore moraine ridges and trimlines mark the maximal extent of the irish sea glacier at bread and cheese cove st martin s a heterogeneous sediment unit wedged between periglacial gelifluctates is interpreted as a mixture of glacitectonized and cannibalized glacilacustrine and marine deposits as well as primary subglacial tills macrofabric signatures reflect a subglacial origin but the whole unit has been subject to down slope mass movement micromorphological analysis indicates highly variable response to deformation and glacitectonism with most features relating to ductile behaviour and wet deformation the characteristics suggest deposition during a short lived advance of the irish sea glacier over pre existing marine and contemporaneous proglacial lacustrine sediments once blocks of stratified sediment and diamicton had been driven onshore they were subject to paraglacial remobilization resulting at most locations in complete disaggregation the distribution of re worked glacigenic sediments coincides with the glacial limits inferred from the geomorphological evidence

clastic dykes in over consolidated tills evidence for subglacial hydrofracturing at killiney bay eastern ireland abstract a swarm of vertical gravel filled dykes up to 6 m high and several decimetres wide cut through an over consolidated till at killiney bay the dykes are rooted in a gravel layer and many display plumes of clastic debris ejected into the overlying sediments burst out structures such features have not previously been described these clastic dykes are interpreted as the infillings of hydrofractures which formed when water pressures in the basal gravel layer exceeded the overburden pressure and tensile shear strength of the capping till the burst out structures extend up to 7 m from the tops of the dykes and provide strong evidence for forceful upward flow evidence suggests that the hydrofractures formed subglacially probably during a minor re advance their presence in late devensian 26 13 ka bp tills with an irish sea provenance irish sea till may have important implications for the subglacial hydrology of the last irish sea ice sheet these hydrofractures cannot form in unconsolidated glacimarine sediment and their presence precludes a glacimarine origin for these deposits they greatly affect the geotechnical properties of irish sea tills in particular providing very permeable routes through otherwise impermeable layers with important consequences in situations elsewhere in the irish sea basin where they have been used as aquicludes in landfill and low level nuclear waste disposal sites

reconstructing the last irish ice sheet 2 a geomorphologically driven model of ice sheet growth retreat and dynamics the ice sheet that once covered ireland has a long history of investigation much prior work focussed on localised evidence based reconstructions and ice marginal dynamics and chronologies with le

depositional model in subglacial cavities killiney bay ireland interactions between sedimentation deformation and glacial dynamics subglacial meltwater drainage and sedimentary processes play a major role in ice sheet dynamic but there is a lack of study of subglacial environment because modern ice sheet beds remain inaccessible previous authors already intended to provide diagnostic criterion and recent investigations suggest that fluid pressure variations are a key factor in subglacial environment this paper investigated the late devensian sedimentary record in order to describe subglacial sedimentological facies associations and deformation features related to fluid overpressures we used an integrated approach based on stratigraphy sedimentology and deformations styles to demonstrate a subglacial depositional model the studied interval is composed of stratified gravel and sand interbedded with diamicton and boulder pavement deposited in depressions formed by irregularity of the upper surface of diamicton deformation structures include convolutes dykes and normal micro faulting dykes show different dip directions from vertical oblique to subhorizontal from which both directions of shortening and extension can be determined vertical dykes are formed under pure shear strain related to ice weight only oblique dykes imply both ice bed coupling and simple shear strain between ice and substrate induced by flowing ice related to progressively increasing meltwater drainage intensity horizontal dykes are formed when minimum strain is vertical and therefore the overpressure exceeds the weight of overburden they are associated with high meltwater drainage intensity and ice bed uncoupling and refer to hydrofracturing overall depositional and deformation sequence also illustrates the increasing intensity of meltwater drainage in small cavity as high energy channelised deposits and in large cavities where subaqueous fan are deposited under hydraulic jump conditions moreover large cavities represent lee side cavities formed by fast flowing ice over an obstacle hydrofracturing is likely to occur when a dense fluid potentially associated with catastrophic drainage of an upstream cavity enters the low pressure confined environment of a downstream cavity and is injected under pressure in the soft substrate the studied glacial sequence represents a regional pattern probably related to an allocyclic control on sedimentation linked to long term glacial dynamics rather than local depositional conditions based on these results we provided a synthetic model linking depositional and deformation processes during ice sheet growth and decay but also valid at different timescales from annual to seasonal scale

pore water controlled grain fracturing as an indicator for subglacial shearing in tills genetic classification of glacial deposits still proves to be highly controversial in many cases it remains unclear whether a particular sediment has been sub glacially deformed in the present paper it is suggested that micromorphological research is helpful where current techniques fail to lead to unambiguous interpretations it is argued that the occurrence of fractured grains in glacial sediments is indicative of subglacial shearing and that such grains may be used as diagnostic evidence of glaciotectonism deformational mechanisms associated with the fracturing process are outlined and explained using a series of thin sections from wijnjewoude the netherlands

foam rubber modeling of topographic and dam interaction effects at pacoima dam a study of the topographic and dam interaction effects was made using a 3 d foam rubber model of the actual topography around the pacoima dam accelerograph which recorded over 1 g high frequency horizontal ground accelerations during the 1971 san fernando earthquake scaling of frequency from the model to the earth depends on the average value of shear wave velocity in the upper few hundred meters assuming e 2 km sec for vertically incident sh waves the spectral ratio of the ground acceleration on the ridge to the free field flat surface indicates an amplification of about 60 per cent around 6 5 hz on the n76 w component topography has little effect upon the motion recorded on the s14 w component motion on the ridge is lower than the free field motion on both horizontal components for frequencies above 9 hz amplification peaks shift to higher or lower frequencies depending on the assumed shear wave velocity in the upper few hundred meters results from nonvertically incident sh waves show that the topographic effect is dependent on the direction of approach of the seismic energy the effect is either de amplification in part by shadowing or amplification relative to the case where no topography is present depending on whether the canyon is on the ray path or not the fourier spectrum of the ground motion at the dam crest shows peak frequencies at about 5 hz and 10 hz resonance which correspond to the normal modes of the dam a study of dynamic interaction between the pacoima dam and the ridge shows that the coupling is less than 2 per cent at about 10 hz and less than 12 percent at about 5 hz

dynamic cycles ice streams and their impact on the extent chronology and deglaciation of the british irish ice sheet we present results from a suite of forward transient numerical modelling experiments of the british and irish ice sheet biis consisting of scottish welsh and irish accumulation centres spanning the last glacial period from 38 to 10 ka bp the 3d thermomechanical model employed uses higher order physics to solve longitudinal membrane stresses and to reproduce grounding line dynamics surface mass balance is derived using a distributed degree day calculation based on a reference climatology from mean 1961 1990 precipitation and temperature patterns the model is perturbed from this reference state by a scaled ngrip oxygen isotope curve and the specmap sea level reconstruction isostatic response to ice loading is computed using an elastic lithosphere relaxed asthenosphere scheme a suite of 350 simulations were designed to explore the parameter space of model uncertainties and sensitivities to yield a subset of experiments that showed close correspondence to offshore and onshore ice directional indicators broad biis chronology and the relative sea level record three of these simulations are described in further detail and indicate that the separate ice centres of the modelled biis complex are dynamically interdependent during the build up to maximum conditions but remain largely independent throughout much of the simulation the modelled biis is extremely dynamic drained mainly by a number of transient but recurrent ice streams which dynamically switch and fluctuate in extent and intensity on a centennial time scale a series of binge purge advance retreat cycles are identified which correspond to alternating periods of relatively cold based ice associated with a high aspect ratio and net growth and wet based ice with a lower aspect ratio characterised by streaming the timing and dynamics of these events are determined through a combination of basal thermomechanical switching spatially propagated and amplified through longitudinal coupling but are modulated and phase lagged to the oscillations within the ngrip record of climate forcing phases of predominant streaming activity coincide with periods of maximum ice extent and are triggered by abrupt transitions from a cold to relatively warm climate resulting in major iceberg melt discharge events into the north sea and atlantic ocean the broad chronology of the modelled biis indicates a maximum extent at 20 ka with fast flowing ice across its western and northern sectors that extended to the continental shelf edge fast flowing streams also dominate the irish sea and north sea basin sectors and impinge onto sw england and east anglia from 19 ka bp deglaciation is achieved in less than 2000 years discharging the freshwater equivalent of 2 m global sea level rise a much reduced ice sheet centred on scotland undergoes subsequent retrenchment and a series of advance retreat cycles into the north sea basin from 17 ka onwards culminating in a sustained younger dryas event from 13 to 11 5 ka bp modelled ice cover is persistent across the western and central highlands until the last remnant glaciers disappear around 10 5 ka bp

sedimentary evidence for deforming bed conditions associated with a grounded irish sea glacier southern ireland along the south coast of ireland a shelly diamict facies the irish sea till has been variously ascribed to subglacial deposition by a grounded irish sea glacier or to glacimarine sedimentation by suspension settling and iceberg rafting observations are presented here from five sites along the south coast to directly address this question at these sites sedimentary evidence is preserved for the onshore advance of a grounded irish sea glacier which glacitectonically disturbed and eroded pre existing sediments and redeposited them as deformation till recession of this irish sea glacier resulted in the damming of ice marginal lakes in embayments along the south coast into which glacilacustrine sedimentation then took place these lake sediments were subsequently glacitectonised and reworked by overriding glacier ice of inland origin which deposited deformation till on top of the succession there is no evidence for deposition of the irish sea diamicts by glacimarine sedimentation at these sites the widespread development of subglacial deforming bed conditions reflected the abundance of fine grained marine and lacustrine sediments available for subglacial erosion and reworking stratigraphical and chronological data suggest that the advance of a grounded irish sea glacier along the south coast occurred during the last glaciation and this is regionally consistent with marine geological data from the celtic sea these observations demonstrate extension of glacier ice far beyond its traditional limits in the celtic sea and on land in southern ireland during the last glaciation and remove the stratigraphical basis for chronological differentiation of surficial glacial drifts and thus the munsterian glaciation in southern ireland copyright 2001 john wiley sons ltd

glacial isostatic adjustment of the british isles new constraints from gps measurements of crustal motion summary we compared estimates of crustal velocities within great britain based on continuous global positioning system cgps measurements to predictions from a model of glacial isostatic adjustment gia the observed and predicted values for vertical motion are highly correlated indicating that gia is the dominant geodynamic process contributing to this field in contrast motion of the eurasian plate dominates the horizontal motion component a model of plate motion was adopted to remove this signal in order to estimate intraplate horizontal motion associated with gia however a coherent pattern of horizontal motion was not evident in the resulting velocity field we adopted a recently published model of the british irish ice sheet to predict vertical crustal motion for a large number of spherically symmetric earth viscosity models our results show that the adopted ice model is capable of producing a high quality fit to the observations the cgps derived estimates of vertical motion provide a useful constraint on the average value of viscosity within the upper mantle values of model lithospheric thickness and lower mantle viscosity are less well resolved however a suite of predictions based on an alternative ice model indicates that the vertical motion data are relatively insensitive to uncertainties in the ice loading history and so the constraints on upper mantle viscosity are robust

stress reconstruction and lithosphere dynamics along the sumatra subduction margin abstract gravity modeling and stress reconstructions over the sumatra subduction margin between indian australian and asian plates provide insights into the geodynamic setting and kinematics of the area based on ridge trench interaction plate obliquity plate age and seismicity the area was divided into two sectors northern and southern for gravity modeling along two profiles the stress data with magnitude mw 5 0 and focal depth 100 km for the subducting plate were compiled for six zones i vi and were isolated under different time domains for high resolution study the reconstructed gravity models were used to isolate the events for subducting oceanic indian australian and overriding continental asian plates the data sets for the subducting lithosphere were used for stress inversion to determine the predominant stress regimes for pre earthquake and post earthquake phases of the sumatra earthquake of december 2004 the results were interpreted in terms of operative intra plate stress fields and for reconstruction of kinematics and dynamics along the sumatra subduction margin analysis shows both sparse and intense seismic activities identified over different time segments in the six zones under pre and post december 2004 phases with respect to the incidence of mw 9 2 off sumatra mega earthquake of 2004 the stress regime of the 2004 mega event and subsequent release of strain energy through cyclic changes of the stress obliquities in the northern part of the study area account for heterogeneous coupling caused by rough topography of the subducting indian australian plate reduction of compressive stress regime in the deeper segment of the subducting plate in the northern sector triggered twin great strike slip dominated earthquakes in 2012 in the floor of the northeastern indian ocean the weak zones of a spreading ridge subducting below the asian plate possibly suffered strain localization and weakening the less transmission of co and post seismic strain energy from northern to southern part of the margin was possibly prohibited near the zone of ridge trench interaction the trench orthogonal tearing slab window at the edge of the subducting plate in this area cannot be ruled out the slab window caused by the subduction of the young buoyant ridge and the trench ward movement of the upper plate has transformed the central segment into compressional regime

bayesian modelling the retreat of the irish sea ice stream we present an 8000 year history spanning 650km of ice margin retreat for the largest marine terminating ice stream draining the former british irish ice sheet bayesian modelling of the geochronological data shows the isis expanded 34 0 25 3ka accelerating into the celtic sea to reach maximum limits 25 3 24 5ka before a collapse with rapid marginal retreat to the northern irish sea basin isb this retreat was rapid and driven by climatic warming sea level rise mega tidal amplitudes and reactivation of meridional circulation in the north atlantic the retreat though rapid is uneven with the stepped retreat pattern possibly a function of the passage of the ice stream between normal and adverse ice bed gradients and changing ice stream geometry initially wide calving margins and adverse slopes encouraged rapid retreat 550m a 1 that slowed 100m a 1 at the topographic constriction and bathymetric high between southern ireland and wales before rates increased 200m a 1 across adverse bed slopes and wider and deeper basin configuration in the northern isb these data point to the importance of the ice bed slope and lateral extent in predicting the longer term 1000 a patterns and rates of ice marginal retreat during phases of rapid collapse which has implications for the modelling of projected rapid retreat of present day ice streams copyright 2013 john wiley sons ltd

late quaternary evolution and sea level history of a glaciated marine embayment bantry bay sw ireland ireland experienced a spatially complex pattern of relative sea level rsl changes and shoreline development caused by the interplay of isostatic and eustatic ice equivalent sea level processes since the last glacial maximum lgm using a combination of high resolution marine geophysical data vibrocores foraminiferal analysis and 10 ams radiocarbon dates we reconstruct the late quaternary evolution and rsl history of bantry bay a large 40 km long 5 10 km wide embayment in sw ireland the data indicate two infill phases one before and one after the lgm separated by glacial and lowstand sediments the pre lgm history is not dated and the depositional history is inferred a large sediment lobe formed at the outer edge of bantry bay as a lowstand ice proximal glacimarine outwash system as the ice retreated after the lgm at a sea level ca 80 m lower than present iceberg scour immediately west of this location likely relate to the break up of the local kerry cork ice cap long curvilinear ridges seen both offshore and on top of the sediment lobe probably formed as shoreface ridges under stronger than present tidal currents during a period of rsl stability pre 14 6 ka cal bp a subsequent infill phase is characterised by a basin wide erosional ravinement surface and the deposition of inter and sub tidal estuarine sediments although our data support the general trends our stratigraphic and radiocarbon data suggest a higher sea level between 11 and 13 5 ka cal bp than predicted by existing glacial isostatic adjustment models

seismically induced rock slope failures resulting from topographic amplification of strong ground motions the case of pacoima canyon california abstract the 1994 northridge earthquake mw 6 7 triggered extensive rock slope failures in pacoima canyon immediately north of los angeles california pacoima canyon is a narrow and steep canyon incised in gneissic and granitic rocks peak accelerations of nearly 1 6 g were recorded at a ridge that forms the left abutment of pacoima dam peak accelerations at the bottom of the canyon were less than 0 5 g suggesting the occurrence of topographic amplification topographic effects have been previously suggested to explain similarly high ground motions at the site during the 1971 mw 6 7 san fernando earthquake furthermore high landslide concentrations observed in the area have been attributed to unusually strong ground motions rather than higher susceptibility to sliding compared with nearby zones we conducted field investigations and slope stability back analyses to confirm the impact of topographic amplification on the triggering of landslides during the 1994 earthquake our results suggest that the observed extensive rock sliding and falling would have not been possible under unamplified seismic conditions which would have generated a significantly lower number of areas affected by landslides in contrast modelling slope stability using amplified ground shaking predicts slope failure distributions matching what occurred in 1994 this observation confirms a significant role for topographic amplification on the triggering of landslides at the site and emphasises the need to select carefully the inputs for seismic slope stability analyses

celtic sea linear tidal sand ridges the irish sea ice stream and the fleuve manche palaeotidal modelling of a transitional passive margin depositional system abstract the linear tidal sand ridges ltsr of the celtic sea constitute the largest examples of their bedform type on earth previous sedimentological and seismic stratigraphic interpretation suggests that the ltsr are moribund tidally remobilised sediments representing the transgressive systems tract this interpretation is supported by two dimensional finite difference model reconstructions of the m 2 tide forced using the output from a glacial isostatic adjustment model to derive palaeotopography and a global ocean model to derive the tides on the ocean boundary used here to reconstruct peak bed stress vectors for the celtic sea for timesteps covering the transgression since the last glacial maximum last 21 ka these data are coupled with interpretation of recent published observations on the outer shelf depocentres of the fleuve manche and the irish sea ice stream isis to confirm that the ltsr distribution is consistent with modelled sand transport paths during transgression the main phase of ltsr growth was between 20 cal ka and 12 cal ka ridge axis orientations reflect the final phase of ltsr construction around 12 cal ka with some later growth of the most southerly ltsr as late as 10 cal ka ltsr growth was from the sw across the shelf towards the ne strong tidal pumping of sediments into slope canyon heads on the outer shelf occurred between 20 cal ka and 12 cal ka contributing to turbidite activity and the growth of the celtic and armorican deep sea fans it is proposed that 1 the fleuve manche shelfal estuary delta and the irish sea ice stream isis shelf fan were the main sediment depocentres supplying the growing ltsr 2 the lack of sediments available in the western channel limited ltsr growth in this area and that the easterly termination of the ltsr in this sector results from sediment starvation and 3 the northeasterly termination in the celtic sea sector is a function of declining peak bed stresses rather than sediment starvation the celtic sea margin represents a passive margin depositional system transitional between true glacial ice stream trough mouth fan systems to the north and the fluvial canyon systems characteristic of the margin to the south this interpretation therefore complements studies of macroscale sedimentation linked to glaciation on the continental margins of the north atlantic

observing artificially induced strain implications for subglacial deformation by analysing a series of four successive thin sections from a ceramic clay that was subjected to uniaxial compression we were able to monitor the development of microstructures in a fine grained sediment the artificially induced microstructures such as unidirectional clay reorientations and linear and circular grain arrangements are identical to features that have been observed in thin sections of subglacially deformed tills and therefore may be used as representative analogues we argue that the structures reflecting slip planar shear displacements as well as rotational movements can be explained by assuming a coulomb plastic response to imposed shear we conclude that sediments subjected to subglacial deformation behave as coulomb materials at least during the final stages of the deformation the present study bridges the gap between field studies experimental studies and theoretical modelling the microscopic observations assist in visualising inferred subglacial processes and facilitate up and downscaling between diverse methodological approaches copyright 2003 john wiley sons ltd

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

extent and timing of the last glacial maximum lgm in britain and ireland a review the last glacial maximum lgm of marine isotope stage mis 2 is defined here as the period either of maximum global ice volume or in which major ice sheets reached their maximum extent both definitions lack clarity as there is no reason to suppose that maximum volumes or limits were reached simultaneously at either all ice sheets or around the margins of individual ice sheets here we review the traditional terrestrial limits of the biis british irish ice sheet examine developments in understanding the offshore record and attempt a redefinition of the extent and timing of the mis 2 maximum as a working chronological model the build up of the biis is assigned to after 35 32 ka the maximum to the period 27 21 ka and a more limited readvance after 19 ka as marginal oscillations driven by ice sheet dynamics appear to have characterised much of the advance and especially the retreat of the biis caution should be exercised in assigning climatic significance to the many identified ice limits or still stands while much of the terrestrial limit in britain remains relatively unaltered significant revision is required offshore around britain and ireland to portray an extensive ice advance to the shelf edge and a period when the biis was conjoined with the adjacent fenno scandinavian ice sheet although there appears to be a broad synchrony between the biis and other major ice sheets local controls have produced some variation in the timing of advance to and retreat from maximum limits for particular ice streams or lobes but much of the offshore limit remains very poorly age constrained consequently a key objective in future research should be to improve and refine the chronological control for all portions of the maximum margin of the biis and its subsequent retreat stages copyright 2010 john wiley sons ltd

geomorphology and sedimentology of surging glaciers a land systems approach the identification of surging glaciers and ice streams in glaciated landscapes is of major importance to the understanding of ice sheet dynamics and for reconstructing ice sheets and climate no single landform or diagnostic criterion has yet been found with which to identify surging glaciers a surging glacier land system model is constructed using observations and measurements from contemporary surging glacier snouts in iceland svalbard u s a and canada for differentiating ancient surging margins from other non surging palaeoglaciers this integrates the suite of landforms sediments and stratigraphy produced at surging glacier margins landforms produced during surging include thrust moraines concertina eskers and subglacial crevasse squeeze ridges sedimentary sequences are usually characterized by multiple stacked diamictons and stratified interbeds which display severe glaciotectonic contortion and faulting hummocky moraine comprising interbedded stratified sediments and mass flow diamictons has also been associated with surge margins where large quantities of supraglacial and englacial debris entrained during the surge event have melted out in situ an example of the application of the land system model is presented for east central alberta canada a surging palaeo ice stream is identified within this part of the southwestern laurentide ice sheet where thrust block moraines crevasse squeeze ridges flutings hummocky moraine and glaciotectonized sediments are juxtaposed

site resonance from strong ground motions at lucerne california during the 1992 landers mainshock analog station luc near lucerne california recorded strong motions from the nearby 1 25 km fault rupture during the 28 june 1992 landers mainshock the records illustrate general issues that can arise at near field stations in the area of the station weathered granite regolith with 400 m s s wave velocity overlies intact granite with 3000 m s s wave velocity a strongly reverberating signal persisted for several seconds after near field velocity pulse passed recovery of the vertical record in digital form provided calibration of resonant site properties and qualitative separation of site and source effects the dominant signal on the vertical spectrum arises from vertically reverberating p waves in an 13 5 m thick layer horizontal spectra marginally resolve the analogous resonance for vertical s waves and coupling of vertical p waves into horizontal motion here resonant amplification of a broad high frequency band 5 40 hz by a factor of a few over lower frequencies sufficed to dominate acceleration records and to make velocity records jittery conversely the amplitude damping times of these resonances are much less than 1 s indicating that the time domain decay of the acceleration signal over 8 s is an incident wave effect overall the raw seismograms represent incident incoming high frequency 5 40 hz signals but poorly resolved directional effects preclude straightforward determination of the incident body waves

periglacial trimlines and the extent of the kerry cork ice cap sw ireland abstract periglacial trimlines on the mountains of sw ireland demarcate a contrast between glacially scoured bedrock on lower ground and thick periglacial regolith covers on summits and plateaus and have previously been interpreted as marking the maximum altitude of the kerry cork ice cap kcic glacially eroded bedrock extends to the highest points on the beara peninsula but the highest summits on the iveragh peninsula support thick regolith covers trimline altitudes on the iveragh peninsula decline radially from a former ice divide near the head of kenmare river from 700 m altitude near the divide to 470 m near dingle bay contrasts in the clay mineral content of above and below trimline soils and cosmogenic isotope exposure ages indicate a lgm age for the trimlines assumption that the trimlines represent the maximum thickness of the kcic implies an ice divide altitude of 825 m however modelling of ice surface profiles demonstrates that an ice divide at 825 m is irreconcilable with stratigraphic evidence for extension of the kcic at least as far as garryvoe 115 km east of the divide we infer that the iveragh trimlines represent a former englacial transition between warm based sliding ice on low ground and cold based ice on high ground rather than delimiting nunataks as previously interpreted modelling of ice surface altitudes suggests that during the lgm the kcic probably overtopped all of the mountains in sw ireland the trimlines of sw ireland thus define the minimum rather than maximum altitude of the kcic at the lgm as well as the sites of former cold patches within the kcic

testing an approach to osl dating of late devensian glaciofluvial sediments of the british isles in the british and irish isles the understanding of expansion and retreat of the last ice sheet in marine isotope stage mis 2 has been hindered by a lack of chronological control for the timing of its maximum extent and subsequent retreat stages in this research a suite of lithofacies from a former distal ice marginal sandur at orrisdale isle of man were identified and selected for optically stimulated luminescence dating different size fractions of quartz grains were extracted from selected lithofacies types with the aim of identifying which depositional environments and grain size fractions are best suited for optical dating tests have been undertaken to identify the luminescence properties of this quartz including preheat dose recovery and constant wave optically stimulated luminescence osl decay curve fitting equivalent dose distributions for all samples were wide and positively skewed indicative of heterogeneous bleaching a comprehensive statistically based decision making protocol has been defined in order to find the appropriate statistical model for burial dose estimation ages in the range of 17 14 ka have been obtained which agree with the expected geologically constrained chronology the orrisdale formation is contemporaneous with heinrich event 1 and the killard point stadial identified in eastern ireland ca 16 4 ka the suggested approach for osl dating of glaciofluvial sediments has the potential for much wider application to high latitude or temperate areas where there is an abundance of such sediments and where improved chronological control is much needed copyright 2009 john wiley sons ltd

styles of till deposition at the margin of the last glacial maximum north sea lobe of the british irish ice sheet an assessment based on geochemical properties of glacigenic deposits in eastern england the geochemical composition of last glacial maximum lgm tills and glacitectonites in east yorkshire and lincolnshire was analysed to assess vertical changes in their geochemical signature with respect to the current regional stratigraphic division and generic models of till deposition results fail to precisely differentiate the traditional basement skipsea and withernsea till types instead there is often more variability within these till types than between them the geochemical signatures are also laterally discontinuous and are often repeated vertically at a number of sites suggesting that the till sequences are composed of a number of lithologically distinct rafts which have been tectonically stacked or elevated to higher levels in the sediment pile at dimlington the production of a glacitectonically folded and stacked moraine is proposed as a mechanism to explain the remarkably thick sequence of withernsea till and the repetitions in geochemical composition we argue that the changes in till matrix geochemistry are related to changes in the dominance of ice lobes of different provenance which reworked pre existing quaternary sediments excavated local bedrock and progressively mixed these sediments with more far travelled material from further north the most significant implication of these findings is that the current lgm till stratigraphy of east yorkshire and lincolnshire is too simplified and therefore does not recognise the sedimentary and structural complexity produced by repeated onshore possibly surging flow by a dynamic north sea lobe at the eastern margin of the british irish ice sheet

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

ground motion amplitude across ridges abstract local amplification and wave diffraction on an elongated ridge near sourpi in central greece were studied by the analysis of seismic records of local and regional earthquakes data were obtained during field work especially designed for this purpose these data were analyzed in the frequency and time domains in the frequency domain spectral ratios show amplifications of 1 5 to 3 at the ridge top relative to the base of the ridge the horizontal components of motion are more amplified than the vertical component and the observed spectral ratios seem stable for different earthquake locations theoretical spectral ratios calculated by the indirect boundary element method are dependent on earthquake location but are in general agreement with the observed spectral ratios another dataset from mont st eynard in the french alps showed similar characteristics with spectral amplitudes on the top of the ridge up to four times those on the flank these relative amplifications are within the range predicted by numerical simulations the numerical simulations also show that the topographic effect involves the emission of diffracted waves propagating from the top toward the base of the ridge the use of a seven station array on the ridge at sourpi made it possible to identify such waves the analysis was performed with wave separation methods using singular value decomposition and spectral matrix filtering our results show agreement between experimental data and theoretical results supporting the use of numerical simulations for estimation of purely topography induced amplification on ridge tops our results also show that such amplification is moderate for the ridges under study

advance and retreat of the marine terminating irish sea ice stream into the celtic sea during the last glacial timing and maximum extent abstract the dynamics of the british irish ice sheet biis during the last glacial were conditioned by marine based ice streams the largest of which by far was the irish sea ice stream isis which drained southwest across the celtic shelf the maximum extent and timing of the isis have been constrained by onshore evidence from the uk and ireland and by glacigenic sediments encountered in a small suite of vibrocores from the uk irish continental shelf from which a single radiocarbon date is available these data have long supported ice advance to at least the mid shelf while recent results suggest the isis may have extended 150 km farther seaward to the shelf edge the glacigenic sequences have not been placed within a secure seismic stratigraphic context and the relationship between glaciation and the linear sediment megaridges observed on the outer shelf of the celtic sea has remained uncertain here we report results of sedimentological geochemical geochronological and micropalaeontological analyses combined with a seismic stratigraphic investigation of the glacigenic sequences of the celtic sea with the aims of establishing maximum extent depositional context timing and retreat chronology of isis eight lithofacies packages are identified six of which correlate with seismic facies lithofacies lf1 and lf2 correlate to a seafloor seismic facies sf1 that we interpret to record the postglacial and holocene transgressive flooding of the shelf lithofacies lf10 till lf3 lf4 and lf8 glacimarine correlate to different seismic facies that we interpret to be of glacigenic origin based on sedimentological geotechnical and micropalaeontological evidence and their distribution supported by geochemical evidence from lithofacies lf8 and lf10 indicate extension of isis as far as the celtic sea shelf break new radiocarbon ages on calcareous micro and macrofauna constrain this advance to be between 24 and 27 cal ka bp consistent with pre existing geochronological constraints glacimarine lithofacies lf8 is in places glacitectonically contorted and deformed indicating ice readvance but the nature and timing of this readvance is unclear retreat out of the celtic sea was initially rapid and may have been triggered by high relative sea levels driven by significant glacio isostatic depression consistent with greater ice loads over britain and ireland than previously considered

the interpretation and classification of subglacially deformed materials abstract a general classification of subglacially deformed materials is proposed based on sediment properties and their relationship to styles of subglacial strain deformation till is defined as homogenized usually diamictic material formed by glacially induced shear of subsole materials three types are recognized type a formed by pervasive ductile deformation type b formed by brittle shear and comminution till produced by the reduction of void space by in situ crushing and abrasion the term glacitectonite is adopted for materials that have undergone subglacial shear but retain some of the structural characteristics of the parent material original structures may be truncated by glacitectonic fabric elements type a or distorted but not truncated type b the principles of strain and material response particle orientation mechanisms fabric development and drainage conditions are reviewed examples of deformation tills and glacitectonites from breidamerkurjokull iceland slettmarkbreen norway loch lomond scotland and east yorkshire england are used to illustrate the nature of subglacially deformed materials and their relationship to former subglacial strain and drainage conditions the geologic evidence suggests a continuum of bed strengths can be recognized in deforming substrata ranging from high strength low strain type b deformation till and comminution till to low strength high strain type a deformation till

radiocarbon constraints on the age of the maximum advance of the british irish ice sheet in the celtic sea reconstructions of the british irish ice sheet biis during the last glacial maximum lgm in the celtic sea and southern ireland have been hampered by a paucity of well dated stratigraphic records as a result the timing of the last advance of the largest outlet of the biis the irish sea ice stream to its maximum limit in the celtic sea has been variously proposed as being pre last glaciation early devensian and lgm the irish sea till was deposited by the irish sea ice stream during its last advance into the celtic sea we present 26 stratigraphically well constrained new ams radiocarbon dates on glacially transported marine shells from the irish sea till in southern ireland which constrain the maximum age of this advance the youngest of these dates indicate that the biis advanced to its overall maximum limit in the celtic sea after 26 000 20 000 14c yr bp thus during the last glaciation the most extensive phase of biis growth therefore appears to have occurred during the lgm at least along the celtic sea and irish margins these data further demonstrate that the uppermost inland glacial tills from the area of supposed older drift in southern ireland a region previously regarded as having been unglaciated during the lgm also date from the last glaciation thus most of southern ireland was ice covered at the lgm advance of the biis to its maximum southern limit in the celtic sea may have been a short lived glaciodynamic response facilitated by subglacial bed conditions rather than a steady state response to climate forcing alone

glacial sediments and landforms of holderness eastern england a glacial depositional model for the north sea lobe of the british irish ice sheet abstract borehole records and exposures at coastal cliffs and inland quarries around holderness are used in a critical assessment of the glacial depositional environments that operated at the margin of the north sea lobe of the british irish ice sheet at the last glacial maximum four lithofacies associations lfa are recognized in the area lfas 1 and 4 are laminated to massive diamictons traditionally called the skipsea and withernsea tills members and display deformation structures indicative of stress from the ne attenuated rafts of soft bedrock lithologies ice bed separation canal fill features and pseudo stratification and clast macrofabric data predominantly indicate deformation only to low strains they are interpreted as subaqueous deposits subsequently deformed by glacier ice to create end products that lie on the glacitectonite continuum localized increases in strain signature are equated with the development of shear zones between onshore thickening wedges of deformed pre existing lake sediment lfa 2 comprises cross bedded clinoforms locally steepening into foresets at the core of linear hummocks along the central spine of holderness lfa 2a and rhythmically laminated silts and sands with dropstones lfa 2b these sediments are interpreted as coalescent glacilacustrine subaqueous outwash fans fining distally to lake bottom rhythmites and varves ice contact deposition is evident in compressional folding faulting and hydrofracture filling due to ice overriding and extensional faulting associated with localized ice melt out depressions produced by folding or sagging provided the accommodation space for the later deposition of lfa 2 and 3 through syntectonic sedimentation lfa 3 comprises flat lying beds of well to poorly sorted cross stratified sands and gravels interpreted as proximal proglacial braided outwash deposits laid down largely in ribbon sandar defined by the topography of abandoned ice contact glacilacustrine depo centres well developed ice wedge pseudomorphs in lfa 3 record permafrost conditions during ice sheet marginal recession indicating that north sea lobe oscillations may have been non climatic or surge related a depositional model is proposed in which advance and retreat phase tills glacitectonites and associated ice contact lake sediments are the geological imprint of a single glaciation the initial advance of the north sea lobe is recorded by a westerly thinning advance till lfa 1 later lfa 1 and 4 retreat tills are strictly glacitectonites and are thicker depositional units because later ice readvances encroached upon and cannibalized more substantial sequences of deglacial lake sediment and subaqueous ice contact fans lfa 2 similar regional till architectures will be manifest wherever the palaeogeography resulted in the onshore flow of ice and the concomitant production of glacitectonites from glacial lake sediments

structural geology and sedimentology of the heiligenhafen till section northern germany abstract three pleistocene tills can be distinguished in a coastal cliff section near heiligenhafen northern germany on the basis of structural and petrographic characteristics the lower and middle tills had previously been ascribed to the saalian and the upper till to the late weichselian the former two tills are folded and unconformably overlain by the upper till in this paper structural and sedimentological observations are used to investigate whether the lower and middle tills belong to one glacial advance or two separate saalian advances as was suggested in earlier studies based on fine gravel stratigraphy from the contact with local rocks to the top of the mt there is a steady increase in allochtonous components scandinavian rocks and decrease in parautochtonous chalk and flint and autochtonous components local eocene siltstone and meltwater sediments this is paralleled by a trend towards increasing deformation finite strain from the bedrock to the top of the section the most obvious aspect of this latter trend is the massive appearance of the mt which can be interpreted as the result of homogenization by repeated folding and attenuation of sediment lenses which have been incorporated into the till this interpretation is supported by macroscopic and microscopic observations of structures in both tills the structural analysis of the tills is based on the marked contrast in symmetry between sections parallel and perpendicular to the shear direction structures on all scales in the lt as well as in the mt indicate e w dextral shearing except in the western part of the section where this is overprinted by w e sinistral shearing the sediment inclusions in the chalk rich lt are mainly fragments of one or more strongly extended glaciofluvial delta bodies with a depositional direction towards wsw locally these delta sediments rest on eocene siltstone and contain numerous angular fragments of this local bedrock boudins and lenses of sorted sediments are incorporated into the till and occur as islands of low strain in a high strain homogeneous matrix it is concluded that the lt and mt do not belong to two stratigraphically separate saalian advances the section is alternatively interpreted as one subglacial shear zone deformation till with upward increasing strain and allochtonous component content it probably formed during the younger saalian warthe westward advance from the baltic region folding of the two diamicts occurred due to lateral compression near the late saalian ice margin the section was finally overridden by the late weichselian young baltic advance eroding the folded lt and mt and depositing the ut

the southernmost quaternary niche glacier system in great britain until recently the scientific consensus has been that the uplands of south west britain remained unglaciated throughout the quaternary with glacial ice sheet limits lying to the north of the south west peninsula however recent work has shown that small glaciers and ice caps existed in the uplands of exmoor and dartmoor during the late quaternary demonstrating that the consensus of an unglaciated south west britain requires considerable revision here we report geomorphological and sedimentary evidence supported by glacier climate modelling for a quaternary niche glacier from west cornwall south west england this niche glacier represents the southernmost such system from mainland great britain and provides evidence for the presence of extra glacial niche glaciers probably during the last glacial maximum of the devensian glaciation and well outside the limits of the main british irish ice sheet

a critical review of the glaciomarine model for irish sea deglaciation evidence from southern britain the celtic shelf and adjacent continental slope in support of their glaciomarine model for the deglaciation of the irish sea basin eyles and mccabe cited the occurrence of distal glaciomarine mud drapes onshore in the isles of scilly and north devon and of arctic beach face gravels and sands around the shores of the celtic sea glacial and sea level data from the southern part of the irish sea in the terminal zone of the ice stream and the adjacent continental slope are reviewed here to test this aspect of the model the suggestion that the glacial sequences of both the isles of scilly and fremington in north devon are glaciomarine mud drapes is rejected an actively calving tidewater margin only occurred early in the deglacial sequence close to the terminal zone in the south central celtic sea relative sea levels were lower and therefore glacio isostatic depression less than envisaged in the glaciomarine model geochronological sedimentological and biostratigraphical data indicate that the raised beach sequences around the shores of the celtic sea and english channel were deposited at or during regression soon after interglacial eustatic highstands evidence for ice rafting at a time of high relative sea levels is restricted to a phase s earlier than the late devensian these data indicate that the raised beach sequences have no bearing on the style of irish sea deglaciation copyright 2001 john wiley sons ltd

bedrock controls on subglacial landform distribution and geomorphological processes evidence from the late devensian irish sea ice stream abstract ice streams play an important role as regulators of the behaviour of modern ice sheets taking the form of corridors of fast flowing ice similar zones of fast moving ice have also been recognised draining the margins of the late devensian british and irish ice sheet although the geomorphological and sedimentary signatures of palaeo ice streams have received significant attention allowing the identification of these former ice streams the influence of bedrock geology on the processes occurring beneath these palaeo ice streams is less well understood even though subglacial geology has been shown to control the location ice streams within the west antarctic ice steam this paper highlights the role played by bedrock geology on landform distribution beneath a much older ice stream the late devensian irish sea ice stream the spatial relationships displayed between subglacial landforms and bedrock geology are described from anglesey northwest wales and the rhins of galloway southwest scotland both sites occur close to the eastern margin of this irish sea ice stream a link has been established between landform morphology and distribution and the disposition of the main tectonostratigraphical units within the bedrock changes in landform morphology are shown to have been locally controlled by large scale faults and or major lithological boundaries with less durable bedrock lithologies controlling the location and lateral extent of relatively faster flowing portions of the ice stream

fabric shape and the interpretation of sedimentary fabric data abstract the concept of fabric shape provides a powerful means of visualizing and analyzing data on sedimentary fabric the shape of a fabric is defined in terms of the ratios between eigenvalues s1 s2 and s3 derived using the orientation tensor method a continuum of all possible orthorhombic fabric shapes can be clearly represented on equilateral ternary diagrams originally developed for the presentation of particle shape data two indices are of particular value in scaling these diagrams fabric isotropy i s3 s1 and fabric elongation e 1 s2 s1 together these indices uniquely define the shape of any orthorhombic fabric and provide a rational and quantitative basis for reconstructing depositional and de ormational processes equilateral ternary diagrams provide a more versatile and powerful basis for analyzing fabric data than other eigenvalue diagrams now in use use of the diagrams in reconstructing processes of sediment transport and deposition is illustrated using fabric data from subglacial tills and a variety of slope deposits

landform and sediment imprints of fast glacier flow in the southwest laurentide ice sheet evidence for former fast glacier flow ice streaming in the southwest laurentide ice sheet is identified on the basis of regional glacial geomorphology and sedimentology highlighting the depositional processes associated with the margin of a terrestrial terminating ice stream preliminary mapping from a digital elevation model of alberta identifies corridors of smoothed topography and corridor parallel streamlined landforms megaflutes to mega lineations that display high levels of spatial coherency ridges that lie transverse to the dominant streamlining patterns are interpreted as a series of minor recessional push moraines b thrust block moraines or composite ridges hill hole pairs constructed during readvances surges and c overridden moraines cupola hills apparently of thrust origin together these landforms demarcate the beds and margins of former fast ice flow trunks or ice streams that terminated as lobate forms localised cross cutting and or misalignment of flow sets indicates temporal separation and the overprinting of ice streams lobes the fast flow tracks are separated by areas of interlobate or inter stream terrain in which moraines have been constructed at the margins of neighbouring competing ice streams outlet glaciers this inter stream terrain was covered by more sluggish non streaming ice during full glacial conditions thin tills at the centres of the fast flow corridors in many places unconformably overlying stratified sediments suggest that widespread till deformation may have been subordinate to basal sliding in driving fast ice flow but the general thickening of tills towards the lobate terminal margins of ice streams outlet glaciers is consistent with subglacial deformation theory in this area of relatively low relief we speculate that fast glacier flow or streaming was highly dynamic and transitory sometimes with fast flowing trunks topographically fixed in their onset zones and with the terminus migrating laterally the occurrence of minor push moraines and flutings and associated landforms because of their similarity to modern active temperate glacial landsystems are interpreted as indicative of ice lobe marginal oscillations possibly in response to seasonal climatic forcing in locations where meltwater was more effectively drained from the glacier bed further north the occurrence of surging glacier landsystems suggests that persistent fast glacier flow gave way to more transitory surging possibly in response to the decreasing size of ice reservoir areas in dispersal centres and also locally facilitated by ice bed decoupling and drawdown initiated by the development of ice dammed lakes copyright 2008 john wiley sons ltd

thermoluminescence dating of late devensian loesses in southern england scattered across southern england are many isolated deposits of loess like material1 a few such as that at pegwell bay in kent are highly calcareous and unweathered but most have been reworked by fluvial or colluvial processes there is good stratigraphical evidence for a few pre devensian loesses also in kent but dating of more recent loess has so far been based on indirect evidence much work has been done on the pegwell bay loess as it is the most extensive truly aeolian loessic deposit in britain kerney2 compared the late devensian deposits in the isle of thanet and at pegwell bay with similar deposits in holland and belgium where radiocarbon dates have been obtained for interstadial deposits correlation of the east kent deposits with these in northern europe indicates that the loesses in kent were formed between 30 000 and 14 000 yr ago i report here dates for six of the more recent deposits in southern britain from the scilly isles to kent the dates have been obtained on the loess itself using a recently developed thermoluminescence tl dating technique3 4 and confirm the ages as being late devensian

anomalous concentrations of seismically triggered rock falls in pacoima canyon are they caused by highly susceptible slopes or local amplification of seismic shaking anomalously high concentrations of rock falls were triggered in pacoima canyon los angeles california during the 1994 northridge earthquake similar concentrations were also documented from the 1971 san fernando earthquake using an engineering rock mass classification that evaluates the susceptibility of rock slopes to seismic failure based on the fracture properties of a rock mass in terms of a numerical q value that describes rock quality the rock slopes in pacoima canyon were compared with rock slopes in surrounding areas where topography and lithology are similar but rock fall concentrations from the earthquakes were much lower a statistical comparison of q values from five sites surrounding pacoima canyon indicates that seismic susceptibilities are similar to those within pacoima canyon differences in the characteristics of rock slopes between these sites are not sufficient to account for the relatively high concentrations of rock falls within pacoima canyon as compared to low concentrations elsewhere by eliminating susceptibility differences as a cause the most likely explanation for the differences in rock fall concentrations is anomalously high shaking levels in pacoima canyon possibly resulting from topographic amplification within the canyon manuscript received 21 may 2001

flow pattern evolution of the last british ice sheet we present a 10 stage reconstruction of the evolution in ice flow patterns of the last british ice sheet from build up to demise derived from geomorphological evidence 100 flowsets identified in the subglacial bedform record drumlins mega scale glacial lineations and ribbed moraine are combined with ancillary evidence erratic transport paths absolute dates and a semi independently reconstructed retreat pattern to define flow patterns ice divides and ice sheet margins during build up maximum glaciation and retreat overprinting and cross cutting of landform assemblages are used to define the relative chronology of flow patterns and a tentative absolute chronology is presented based on a collation of available dates for ice advance and retreat the ice flow configuration of the last british ice sheet was not static some ice divides were remarkably stable persisting through multiple stages of the ice sheet evolution whereas others were transient features existing for a short time and or shifting in position 10s km the 10 reconstructed stages of ice sheet geometry capture two main modes of operation first as an integrated ice sheet with a broadly n s orientated ice divide and second as a multi domed ice sheet orientated parallel with the shelf edge a thick integrated ice sheet developed as ice expanded out of source areas in scotland to envelop southerly ice caps in northern england and wales and connect with the irish ice sheet to the west and the scandinavian ice sheet across the north sea following break up of ice over the north sea ice streaming probably drove mass loss and ice sheet thinning to create a more complex divide structure where ice flow patterns were largely controlled by the form of the underlying topography ice surface lowering occurred before separation of and retreat to multiple ice centres centred over high ground we consider this 10 stage reconstruction of the evolution in ice sheet configuration to be the simplest palaeo glaciological interpretation of the flowsets identified from the geomorphological record and their relative timing this empirically based reconstruction of flow pattern geometry provides a framework for more detailed local and regional studies and numerical modelling to provide robust explanations of the observed changes in ice sheet structure in terms of climate and glacial dynamics as a minimum numerical model outputs should be able to reproduce the identified flowset patterns in space and satisfy their chronological order

trough geometry was a greater influence than climate ocean forcing in regulating retreat of the marine based irish sea ice stream marine terminating ice streams are a major component of contemporary ice sheets and are likely to have a fundamental influence on their future evolution and concomitant contribution to sea level rise to accurately predict this evolution requires that modern day observations can be placed into a longer term context and that numerical ice sheet models used for making predictions are validated against known evolution of former ice masses new geochronological data document a stepped retreat of the paleo irish sea ice stream from its last glacial maximum limits constraining changes in the time averaged retreat rates between well defined ice marginal positions the timing and pace of this retreat is compatible with the sediment landform record and suggests that ice marginal retreat was primarily conditioned by trough geometry and that its pacing was independent of ocean climate forcing we present and integrate new luminescence and cosmogenic exposure ages in a spatial bayesian sequence model for a north south 173km transect of the largest marine terminating ice stream draining the last british irish ice sheet from the south and east coasts of ireland initial rates of ice margin retreat were as high as 300 600 m a 1 but retreat slowed to 26 m a 1 as the ice stream became topographically constricted within st george s channel a sea channel between ireland to the west and great britain to the east and then stabilized retreating at only 3 m a 1 at the narrowest point of the trough during the climatic warming of greenland interstadial 2 gi 2 23 3 22 9 ka later retreat across a normal bed slope during the cooler conditions of greenland stadial 2 was unexpectedly rapid 152 m a 1 we demonstrate that trough geometry had a profound influence on ice margin retreat and suggest that the final rapid retreat was conditioned by ice sheet drawdown dynamic thinning during stabilization at the trough constriction which was exacerbated by increased calving due to warmer ocean waters during gi 2

new age constraints for the limit of the british irish ice sheet on the isles of scilly the southernmost terrestrial extent of the irish sea ice stream isis which drained a large proportion of the last british irish ice sheet impinged on to the isles of scilly during marine isotope stage 2 however the age of this ice limit has been contested and the interpretation that this occurred during the last glacial maximum lgm remains controversial this study reports new ages using optically stimulated luminescence osl dating of outwash sediments at battery tresco 25 5 1 5 ka and terrestrial cosmogenic nuclide exposure dating of boulders overlying till on scilly rock 25 9 1 6 ka which confirm that the isis reached the isles of scilly during the lgm the ages demonstrate this ice advance on to the northern isles of scilly occurred at 26 ka around the time of increased ice rafted debris in the adjacent marine record from the continental margin which coincided with heinrich event 2 at 24 ka osl dating 19 6 1 5 ka of the post glacial hell bay gravel at battery suggests there was then an 5 ka delay between primary deposition and aeolian reworking of the glacigenic sediment during a time when the isis ice front was oscillating on and around the ll n peninsula 390 km to the north

theory of glacial erosion transport and deposition as a consequence of subglacial sediment deformation a theory of erosion transport and deposition of unlithified sediments by glaciers is presented it predicts the large scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle together with the resultant intensity of large scale lincations drumlins which will be incised in the landscape the theory also predicts the dispersal patterns of subglacial lithologies together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till it predicts major erosional discontinuities within tills and the formation of boulder pavements it suggests that the dominant proportion of the lowland tills produced by pleistocene mid latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine grained the theory is based on an analysis of glacier dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition

sedimentology and micropalaeontology of glacimarine sediments from the central and southwestern celtic sea thin discontinuous glacigenic sediments occur at or close to the sea bed as far south as 49 n in the celtic sea the northern samples facies a are clast rich overconsolidated lodgement tills or proximal glacimarine sediments containing sparse reworked microfaunas the southern samples facies b are distal glacimarine plastic silty clays containing abundant cold water microfaunas both these facies are correlated with the late devensian scilly till enabling a quantitative reconstruction of ice thicknesses grounding line sea level and shoreline elevations in the celtic sea at 19 000 years bp the ice advance terminated in marine waters towards the shelf edge break and is likely to have constituted a thin lobate surge over deformable marine sediments 1990 the geological society

extent and deglacial chronology of the last british irish ice sheet implications of exposure dating using cosmogenic isotopes over 160 10be and 36cl exposure ages pertaining to the extent and chronology of the last british irish ice sheet biis are critically reviewed despite uncertainties inherent in the use of exposure ages this approach has demonstrated that at the last glacial maximum lgm ca 26 21 ka the biis extended over all low ground in scotland and all or almost all of ireland consistent with recent models depicting extension of the last biis to the atlantic shelf edge all exposure ages obtained for high level sites above trimlines on the mountains of northwestern scotland and ireland pre date the lgm this finding confirms that high plateaux in these areas escaped significant glacial erosion probably under a protective cover of cold based glacier ice that remained frozen to the underlying substrate in terms of deglaciation chronology the southern irish sea basin was probably deglaciated by 19 18 ka with extensive deglaciation of low ground in ireland and northeastern scotland prior to ca 14 5 ka ice cover on low ground in northwestern scotland apparently persisted after 14 0 ka suggesting survival of ice in favourable locations during the lateglacial interstade ca 14 5 12 9 ka attempts to date readvances have had mixed success the wester ross readvance in northwestern scotland was initially dated to ca 16 3 ka but more recent research implies a much younger age ca 14 0 13 5 ka exposure ages indicating a prolonged stillstand during ice sheet retreat in the cairngorms at ca 14 5 14 0 ka conflict with ages obtained on postglacial rockslides and basal radiocarbon dates which indicate deglaciation before ca 15 5 15 0 ka exposure dating has been successfully employed to confirm a loch lomond stadial ca 12 9 11 5 ka age for moraines in several mountain areas and to constrain the minimum altitude of the ice cap that extended over the western grampians at that time copyright 2009 john wiley sons ltd

deep seated downslope slip during strong seismic shaking 1 strong seismic waves cause frictional failure within regolith on hillslopes the material responds anelastically to the combined oscillating dynamic and static gravitational stresses gravity drives preferential downslope movement of the transiently weakened material the net effect over many earthquake cycles is a tens of meter thick slow landslide that is a sackung the classical approach to hillside failure involves downslope dynamic accelerations or equivalently downslope dynamic shear tractions rate and state friction quantifies that the starting coefficient of friction is a few tenths higher that the apparent long term coefficient of friction from the angle of repose 0 96 1 25 in the calculations retaining an extra digit furthermore the dynamic coulomb stress ratio for dynamic to lithostatic stress for s waves refracted toward vertical paths is the dynamic acceleration in g s above the scale depth 1 k the inverse of the wave number in more detail a dynamic acceleration of 0 13 0 35 g destabilizes slopes near the angle of repose seismologists expect such acceleration near active major faults compatible with the widespread occurrence of sackungen on steep slopes accelerations of 0 96 1 25 1 g destabilize shallow slopes leading to net downhill movement analogous to the movement of an object down a vibrating ramp however sackungen are rare or even absent on shallow slopes in california indicating the rarity of sustained 1 g accelerations a second mechanism may drive downhill movement on modest 19 slopes as observed in the san gabriel mountains of california near the san andreas fault the near field velocity pulse on strike slip faults produces only a few tenths g acceleration but particle velocities of over 1 m s 1 persisting over 1 s dynamic stresses and strains scale with this velocity and extend all the way to the free surface coulomb failure criteria are exceeded at shallow depths failure greatly weakens the regolith allowing it to move downhill in response to gravity this mechanism appears capable of producing prehistoric 1 m per event slip on modest slopes observed in the san gabriel mountains

deglaciation of the irish sea basin a critique of the glaciomarine hypothesis the glaciomarine model for deglaciation of the irish sea basin suggests that the weight of ice at the last glacial maximum was sufficient to raise relative sea levels far above their present height destabilising the ice margin and causing rapid deglaciation glacigenic deposits throughout the basin have been interpreted as glaciomarine the six main lines of evidence on which the hypothesis rests sedimentology deformation structures delta deposits marine fauna amino acid ratios and radiocarbon dates are reviewed critically the sedimentological interpretation of many sections has been challenged and it is argued that subglacial sediments are common rather than rare and that there is widespread evidence of glaciotectonism density driven deformation associated with waterlain sediments is rare and occurs where water was ponded locally sand and gravel deposits interpreted as gilbert type deltas are similarly the result of local ponding or occur where glaciers from different source areas uncoupled they do not record past sea levels and the ad hoc theory of piano key tectonics is not required to explain the irregular pattern of altitudes the cold water foraminifers interpreted as in situ are regarded as reworked from irish sea sediments that accumulated during much of the late quaternary when the basin was cold and shallow with reduced salinities amino acid age estimates used in support of the glaciomarine model are regarded as unreliable radiocarbon dates from distinctive foraminiferal assemblages in northeast ireland show that glaciomarine sediments do occur above present sea level but they are restricted to low altitudes in the north of the basin and record a rise rather than a fall in sea level it is suggested here that the oldest dates around 17 000 yr bp record the first late devensian weichselian marine inundation above present sea level this accords with the pattern but not the detail of recent models of sea level change copyright 2001 john wiley sons ltd

deglacial laminated facies on the nw european continental margin the hydrographic significance of british irish ice sheet deglaciation and fleuve manche paleoriver discharges we have compiled results obtained from four high sedimentation rate hemipelagic sequences from the celtic sector of the nw european margin ne atlantic to investigate the paleoceanographic and paleoclimatic evolution of the area over the last few climatic cycles we focus on periods characteristic of deglacial transitions we adopt a multiproxy sedimentological geochemical and micropaleontological approach applying a sampling resolution down to ten microns for specific intervals the investigation demonstrates the relationships between the bay of biscay hydrography and the glacial deglacial history of both the proximal british irish ice sheet biis and the western european continent we identify recurrent phases of laminae deposition concurrent with major biis deglacial episodes in all the studied cores evidence for abrupt freshwater discharges into the open ocean highlights the influence of such events at a regional scale we discuss their impact at a global scale considering the present and past key location of the bay of biscay versus the atlantic meridional overturning circulation amoc

strong seismic shaking of randomly prestressed brittle rocks rock damage and nonlinear attenuation strong seismic waves produce frequent episodes of transient oscillating dynamic stress in the shallow subsurface within seismically active areas dynamic stress drives slip on a network of preexisting small fracture planes and leaves residual stresses once shaking ceases the residual stresses act as prestresses during the next strong earthquake local failure occurs on fractures at low dynamic stresses where the prestresses are favorably aligned while most of the bulk rock remains intact hence nonlinear attenuation of strong seismic waves commences at dynamic stresses well below those for pervasive coulomb failure consideration of the fractal distribution of prestresses indicates that nonlinear attenuation increases rapidly with dynamic stress vertically propagating shear waves provide simple scaling relationships for quick application 1 nonlinear attenuation is concentrated near the quarter wavelength depth 2 the coulomb stress ratio dynamic lithostatic stress above the quarter wavelength depth scales with the sustained dynamic acceleration in g s rate and state friction thus provides estimates of the maximum sustained acceleration from coulomb stress ratio at failure 1 for regolith that has repeatedly failed 1 5 for shallow intact sandstone and 2 for shallow intact granite a potentially observable effect occurs with shallow brittle regolith on hillslopes where gravity causes net downslope creep during strong shaking analogous crack network behavior with prestress occurs in the damage zone surrounding the rupture zone of major faults pulverized rock forms at shallow depths when rock is repeatedly deformed in strong shaking to just beyond the elastic limit but strain does not localize into a crack network

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

strong ground motion from the uttarkashi himalaya india earthquake comparison of observations with synthetics using the composite source model the uttarkashi earthquake of 19 october 1991 ms 7 0 occurred in the greater himalayan region north of the main central thrust at an estimated depth of 12 km the fault plane solution indicates a low angle thrust mechanism striking northwest consistent with the tectonic pattern of thrusting in the region aftershocks define a belt parallel to and north of the surface trace of the main central thrust roughly 10 km wide and 30 km long the mainshock is located at the northeast edge of this zone the earthquake was recorded on 13 strong motion accelerographs at distances ranging from 25 to 150 km from the epicenter one station at bhatwari peak horizontal acceleration of 272 cm sec 2 is above the aftershock zone the maximum peak horizontal acceleration was about 313 cm sec 2 at uttarkashi at an epicentral distance of 36 km the amplitudes and frequency content of the strong ground motions are more or less consistent with expectations for an earthquake of this magnitude in california synthetics generated using the composite source model and synthetic green s functions zeng et al 1994a b are successful in producing acceleration velocity and displacement with a realistic appearance and the correct statistical properties of the two accelerograms recorded nearest the fault bhatwari and uttarkashi to produce these we introduced trial and error modifications of the layered medium velocity model within uncertainties at more distant stations we first used the velocity structure that worked for the two nearest stations differences emphasize the large potential role of unknown site and wave propagation effects for the station at tehri we explored different velocity models and found one there that was also quite successful we then used these two velocity models to predict strong ground motions at bhatwari and tehri from a potential magnitude 8 5 earthquake filling part of the seismic gap along the himalayan frontal faults the synthetics show peak accelerations that are only somewhat larger than those in the uttarkashi event but much longer durations and increased amplitudes of response spectra at long periods

deformation styles as a key for interpreting glacial depositional environments lithostratigraphical and lithofacies approaches used to interpret glacial sediments often ignore deformation structures that can provide the key to environment of formation we propose a classification of deformation styles based on the geometry of structures rather than inferred environment of formation five styles are recognised pure shear p simple shear s compressional c vertical v and undeformed u these dictate the first letter of the codes the remaining letters conveying the evidence this information can be used to reconstruct palaeostress fields and to infer physical properties of sediments when they deformed individual structures are not diagnostic of particular environments but the suite of structures their relative scale stratigraphical relationships and orientation relative to palaeoslopes and to palaeoice flow directions can be used to infer the environment in which they formed this scheme is applied at five sites in west wales the typical succession is interpreted as subglacial sediments overlain by meltout tills flow tills and sediment flows paraglacial redistribution of glacial sediments is widespread large scale compressional deformation is restricted to sites where glaciers readvanced large scale vertical deformation occurs where water was locally ponded near the ice margin there is no evidence for glaciomarine conditions copyright 2003 john wiley sons ltd

only four major 100 ka glaciations during the brunhes chron early attempts at land sea correlations between continental glaciations and the marine 18o record were premature without a high resolution continental stratigraphy existing schemes of continental subdivision and correlation based facies floras and facies faunas do not provide the necessary resolution this paper reports on a new high resolution subdivision and correlation of the pleistocene of the british isles subdivision and correlation is accomplished primarily by aminostratigraphy based on aminozones which is calibrated by other geochronological means u series tl osl esr 14c palaeomagnetic and 36cl rock exposure dating for correlation with oxygen isotope stratigraphy these data are then used to test the hypotheses of shackleton 1987 and raymo 1997 that only four major glaciations occurred in the brunhes chron during oxygen isotope stages 16 12 6 and 2 their hypotheses are confirmed except for stage 2 when at least part of the british ice sheet was at its maximum somewhat earlier during stages 4 and 3

postglacial relative sea level observations from ireland and their role in glacial rebound modelling the british isles have been the focus of a number of recent modelling studies owing to the existence of a high quality sea level dataset for this region and the suitability of these data for constraining shallow earth viscosity structure local to regional ice sheet histories and the magnitude timing of global meltwater signals until recently the paucity of both glaciological and relative sea level rsl data from ireland has meant that the majority of these glacial isostatic adjustment gia modelling studies of the british isles region have tended to concentrate on reconstructing ice cover over britain however the recent development of a sea level database for ireland along with emergence of new glaciological data on the spatial extent thickness and deglacial chronology of the irish ice sheet means it is now possible to revisit this region of the british isles here we employ these new data to constrain the evolution of the irish ice sheet we find that in order to reconcile differences between model predictions and rsl evidence a thick spatially extensive ice sheet of 600 700 m over much of north and central ireland is required at the lgm with very rapid deglaciation after 21 k cal yr bp copyright 2007 john wiley sons ltd

paired 26al and 10be exposure ages from lundy new evidence for the extent and timing of devensian glaciation in the southern british isles lundy lies in a strategic geographical position for understanding the glacial history of the british isles the island bears evidence of glaciation largely in the form of ice moulded bedrock and glaciallytransported boulders e an unusual occurrence this far south in the british isles irish sea ice penetrated the western bristol channel overriding lundy from the northwest during the last phase of glaciation in this area the results of paired terrestrial cosmogenic nuclide analyses 26al 10be constrain the timing of this extensive glaciation and provide for the first time an age for the exposure of lundy granite following deglaciation the results from nine paired samples yield 26al 10be exposure ages of 31 4e48 8 ka 10be and 31 7e60 0 ka 26al this challenges the view that any glaciation this far south must belong to middle pleistocene glaciations such as the anglian stage c 480e420 ka and a devensian age for the last glaciation is consistent with findings from the isles of scilly further south however the findings suggest early mid devensian marine isotope stage mis 4e3 glaciation of lundy it also implies that the island was exposed or covered for a short time by non erosive cold based ice at the global last glacial maximum lgm during mis 2 26e21 ka the potential exposure of the island throughout mis 2 contrasts with the evidence from the isles of scilly and the celtic sea which were glaciated at the lgm

deforming bed conditions associated with a major ice stream of the last british ice sheet the irish sea ice stream was the largest ice stream to drain the last british ice sheet and around the margins of the celtic sea it deposited a shelly diamict facies that has been variously interpreted as subglacial till or distal glacial marine mud we present new observations on the sedimentology of the shelly diamict and overlying sediments from the south coast of ireland and demonstrate that the shelly diamict facies is a subglacial deformation till deposited during onshore movement of a grounded irish sea ice stream stratigraphic and chronological data indicate that this occurred during the last glaciation and this is regionally consistent with marine geological evidence from the celtic sea we propose that the deforming bed tills along the south coast of ireland are the onshore record of a rapid advance of an unstable irish sea ice stream into the celtic sea this advance was facilitated at least in part by a saturated substrate of readily deformable fine grained marine sediment this implies an unstable and dynamic irish sea ice stream during oxygen isotope stage 2 and therefore extension of the last british ice sheet into the celtic sea far south of its traditionally interpreted limits

active tectonic studies in the united states 1987 1990 the techniques and instrumentation used in active tectonic studies are discussed and recent results are reviewed it is suggested that a critical mass of data on several particular regions has been accumulated making possible critical debates and attempts to assess earthquake hazards particular attention is given to studies of the pacific northwest region basin and range deformation studies and distributed deformation and hidden earthquake sources also included is a comprehensive bibliography for the period

microscopic evidence of subglacial deformation abstract subglacial deformation is by now a well established process however recognition of this process in pleistocene tills is not always clear studies of thin sections of pleistocene tills show that there are a variety of microscopic features caused by deformation this paper gives a review of microscopic evidence for subglacial deformation and starts with a case where deformation could be established macroscopically then traces the microscopic features through planar and rotational movements which are described and separated accordingly attention is also given to features caused by crushing of grains and sediment dewatering these studies indicate that almost all basal tills show evidence of subglacial deformation rather than sedimentary derivation this implies that if we want to understand subglacial processes such tills should be described in tectonic rather than sedimentary terminology

sedimentary and structural evolution of a relict subglacial to subaerial drainage system and its hydrogeological implications an example from anglesey north wales uk subglacial drainage systems exert a major control on basal sliding rates and glacier dynamics however comparatively few studies have examined the sedimentary record of subglacial drainage this is due to the paucity of modern analogues the limited recognition and preservation of upper flow regime deposits within the geological record and the difficulty of distinguishing subglacial meltwater deposits from other meltwater sediments e g glacier outburst flood deposits within this study the sedimentological and structural evolution of a subglacial to subaerial ice marginal proglacial drainage system is examined particular emphasis is placed upon the genetic development and preservation of upper flow regime bedforms and specifically recognising them within a subglacial meltwater context facies are attributed to subglacial meltwater activity and record sedimentation within a confined but progressively enlargening subglacial channel system produced under dune to upper flow regime conditions bedforms include rare large scale sinusoidal bedding with syn depositional deformation produced by current induced traction and shearing within the channel margins subglacial sedimentation culminated with the abrupt change to a more ephemeral drainage regime indicating channel abandonment or a seasonal drainage regime retreat of the ice margin led to the establishment of subaerial drainage with phases of sheet flow punctuated by channel incision and anastomosing channel development under diurnal ablation related seasonal discharge the presence of extensive hydrofracture networks demonstrate that proglacial groundwater levels fluctuated markedly and this may have influenced later overriding of the site by an ice stream

the origin of heinrich layers evidence from h2 for european precursor events recent well dated isotopic sr nd fingerprinting of heinrich layer ice rafted detritus ird on the european margin indicates supply from european ice sheets as precursors to laurentide ice sheet lis supply f e grousset et al geology 28 2000 123 126 h snoeckx et al mar geol 158 1999 197 208 these precursorevents lead lis input by up to 1 5 ka f e grousset et al geology 28 2000 123 126 and have been interpreted to indicate lis collapse during heinrichevents stimulated by events originating on the european side of the atlantic f e grousset et al geology 28 2000 123 126 such phasing of ird supply from different sources within heinrich layers therefore has implications for the origin and mechanics of heinrichevents we present evidence here that the ird comprising heinrich layer 2 h2 20 21 14c ka bp on the european continental margin contains detrital campanian upper chalk deriving from bedrock sources eroded on the celtic shelf by the british ice sheet bis in addition to lithic material sourced from the lis high resolution radiocarbon chronology indicates chalk grain deposition as discrete pulses both before and coincident with supply of lis sourced detritus the specificity of the chalk fingerprint to the bis enables a 700 1000 yr lag between the bis and lis events to be identified this phasing indicates a more rapid response of the outlet lobes draining the smaller bis than those draining the lis and implicates external climatic forcing of heinrichevents it is unlikely that this precursorevent represents ird event 18 the recently identified 1 2 ka ird cycle event which immediately precedes h2 because the lag between precursor and main event is here less than 1 5 ka and because such pervasive periodicity is not apparent in european continental margin ird records the later synchroneity between the bis and lis input in h2 identifies glacio eustatic sea level rise associated with lis discharges as a possible feedback mechanism causing destabilisation of ice streams elsewhere during heinrichevents

relative sea level changes glacial isostatic modelling and ice sheet reconstructions from the british isles since the last glacial maximum while contributing 1 m equivalent eustatic sea level rise the british isles ice sheet produced glacio isostatic rebound in northern britain of similar magnitude to eustatic sea level change or global meltwater influx over the last 18 000 years the resulting spatially variable relative sea level changes combine with observations from far field locations to produce a rigorous test for quantitative models of glacial isostatic adjustment local ice sheet history and global meltwater influx after a review of the attributes of relative sea level observations significant for constraining large scale models of the isostatic adjustment process we summarise long records of relative sea level change from the british isles and far field locations we give an overview of different global theoretical models of the isostatic adjustment process before presenting intercomparisons of observed and predicted relative sea levels at sites in the british isles and far field for a range of earth and ice model parameters in order to demonstrate model sensitivity and the resolving power available from using evidence from the british isles for the first time we show a good degree of fit between relative sea level observations and predictions that are based upon global earth and ice model parameters independently derived from analysis of far field data with a terrain corrected model of the british isles ice sheet that includes extensive glaciation of the north sea and western continental shelf that does not assume isostatic equilibrium at the last glacial maximum and keeps to trimline constraints of ice surface elevation we do not attempt to identify a unique solution for the model lithosphere thickness parameter or the local scale detail of the ice model in order to provide a fit for all sites but argue that the next stage should be to incorporate an ice sheet model that is based on quantitative glaciological model simulations we hope that this paper will stimulate this debate and help to integrate research in glacial geomorphology glaciology sea level change earth rheology and quantitative modelling copyright 2006 john wiley sons ltd

activity of the turbidite levees of the celtic armorican margin bay of biscay during the last 30 000 years imprints of the last european deglaciation and heinrich events abstract high resolution sedimentological and micropaleontological studies of several deep sea cores retrieved from the levees of the celtic and armorican turbidite systems bay of biscay north atlantic ocean allow the detection of the major oscillations of the british irish ice sheet biis and fleuve manche palaeoriver discharges over the last 30 000 years which were mainly triggered by climate changes between 30 and 20 cal ka the turbiditic activity on the celtic armorican margin was weak contrasting with previous stratigraphic models which predicted a substantial increase of sediment supply during low sea level stands this low turbidite deposit frequency was most likely the result of a weak activity of the fleuve manche palaeoriver and or of a reduced seaward transfer of sediments from the shelf to the margin however two episodes of turbiditic activity increase were detected in the celtic armorican margin during heinrich events he 3 and 2 this strengthening of the turbiditic activity was triggered by the meltwater releases from european ice sheets and glaciers favouring the seaward transfer of subglacial material at least via fleuve manche palaeoriver at around 20 cal ka a significant increase of turbidite deposit frequency occurred as a response to the onset of the last deglaciation the retreat of the european ice sheets and glaciers induced a substantial increase of the fleuve manche palaeoriver discharges and seaward transfer of continentally derived material into the armorican turbidite system the intensification of the turbiditic activity on the celtic system was directly sustained by the widespread transport of subglacial sediments from the british irish ice sheet biis to the celtic sea via the irish sea basin a sudden reduction of turbiditic activity in the armorican system between ca 19 and 18 3 cal ka could have been triggered by the first well known abrupt sea level rise meltwater pulse at around 19 cal ka favouring the trapping of sediment in the fleuve manche palaeoriver valleys and the decrease of the seaward transfer of continentally derived material the maximum of turbiditic activity strengthening in the celtic armorican margin between ca 18 3 and 17 cal ka was induced by the decay of european ice sheets and glaciers producing the most extreme episode of the fleuve manche palaeoriver runoff and a great seaward transfer of subglacial material into the bay of biscay between ca 17 5 and 16 cal ka the turbiditic activity significantly decreased in both celtic and armorican turbidite systems in response to a global re advance of glaciers and ice sheets in europe the last episode of ice sheet retreat between ca 16 and 14 cal ka is well expressed in the celtic system by a new increase of the turbiditic activity the major episode of sea level rise at around 14 cal ka meltwater pulse 1a precluding the seaward transfer of sediments induced the end of turbiditic activity in both the celtic and the armorican system although two main phases of global sea level rise seem to have had an effect on the celtic armorican margin this work proposes the biis retreat and associated riverine discharges as the main trigger mechanisms of the turbiditic activity in this region during the last 30 000 years

late pleistocene chronostratigraphy and ice sheet limits southern ireland abstract the morpho stratigraphic subdivision of the surficial glacial drifts of ireland into older drift and younger drift is a long standing convention in irish quaternary studies across southern ireland a broad swath of terrain has traditionally been interpreted as munsterian penultimate glaciation in age and large end moraine complexes bordering this zone of older drift such as the south ireland end moraine have long been regarded as marking the limit of the late midlandian last glaciation ice sheet sedimentary sequences exposed along the south coast of ireland provide a window into the stratigraphy of the older drift and have been studied for over a century the present paper supports a fundamental revision of the traditional interpretation of ice sheet limits in southern ireland and argues for an extensive last irish ice sheet which covered much of the area of the older drift at the last glacial maximum the basis for this revision is threefold firstly nineteen new optically stimulated luminescence osl dates on the courtmacsherry raised beach and overlying shallow marine sands demonstrate that on the south coast the beach and shallow marine sands formed during marine isotope stages 4 3 secondly ams radiocarbon dates on reworked shells from the irish sea till demonstrate that this till and overlying inland tills from central and sw ireland were formed after 24 cal ka bp thirdly new osl dates of 24 21 ka bp from deglacial outwash overlying the irish sea till collectively these data are consistent with a last glaciation age for the glacigenic sequence along the south coast of ireland thus supporting a fundamental revision of the age of the older munsterian drifts of southern ireland most of southern ireland was glaciated during the lgm and the moraine belts which have traditionally been interpreted as marking the last glacial limit such as the south ireland end moraine are reinterpreted here as recessional features formed during ice sheet retreat

the palaeoglaciology of the central sector of the british and irish ice sheet reconciling glacial geomorphology and preliminary ice sheet modelling digital elevation models of the area around the solway lowlands reveal complex subglacial bedform imprints relating the central sector of the lgm british and irish ice sheet drumlin and lineation mapping in four case studies show that glacier flow directions switched significantly through time these are summarised in four major flow phases in the region phase i flow was from a dominant scottish dispersal centre which transported criffel granite erratics to the eden valley and forced lake district ice eastwards over the pennines at stainmore phase ii involved easterly flow of lake district and scottish ice through the tyne gap and stainmore gap with an ice divide located over the solway firth phase iii was a dominant westerly flow from upland dispersal centres into the solway lowlands and along the solway firth due to draw down of ice into the irish sea basin phase iv was characterised by unconstrained advance of scottish ice across the solway firth forcing of a numerical model of ice sheet inception and decay by the greenland ice core record facilitates an assessment of the potential for rapid ice flow directional switching during one glacial cycle the model indicates that after fluctuations of smaller radially flowing ice caps prior to 30 ka bp the ice sheet grows to produce an elongate triangular shaped dome over nw england and sw scotland at the lgm at 19 5 ka bp recession after 18 5 ka bp displays a complex pattern of significant ice flow directional switches over relatively short timescales complementing the geomorphologically based assessments of palaeo ice dynamics the palaeoglaciological implications of this combined geomorphic and modelling approach are that a the central sector of the biis was as a major dispersal centre for only ca 2 5 ka after the lgm b the ice sheet had no real steady state and comprised constantly migrating dispersal centres and ice divides c subglacial streamlining of flow sets was completed over short phases of fast flow activity with some flow reversals taking place in less than 300 years