the instability theory of drumlin formation and its explanation of their varied composition and internal structure despite their importance in understanding glaciological processes and constraining large scale flow patterns in palaeo glaciology there is little consensus as to how drumlins are formed attempts to solve the drumlin problem often fail to address how they are created from an initially flat surface in the absence of obvious cores or obstacles this is a key strength of the instability theory which has been described in a suite of physically based mathematical models and proposes that the coupled flow of ice and till causes spontaneous formation of relief in the till surface encouragingly model predictions of bedform height and length are consistent with observations and furthermore the theory has been applied to a range of subglacial bedforms and not just drumlins however it has yet to confront the myriad observations relating to the composition and internal structure of drumlins and this could be seen as a major deficiency this paper is a first attempt to assess whether the instability theory is compatible with the incredible diversity of sediments and structures found within drumlins we summarise the underlying principles of the theory and then describe and attempt to explain the main types of drumlin composition e g bedrock till glaciofluvial sediments and combinations thereof contrary to a view which suggests that the presence of some sedimentary sequences e g horizontally stratified cores is inconsistent with the theory we suggest that one would actually expect a diverse range of constituents depending on the inheritance of sediments that pre date drumlin formation the duration and variability of ice flow and the balance between erosion and deposition till continuity at the ice bed interface we conclude that the instability theory is compatible with and potentially strengthened by what is known about drumlin composition and as such offers the most complete and promising solution to the drumlin problem to date

bedforms of the keewatin ice sheet canada abstract by compiling glacial bedforms on a map that covers most of one sector of the laurentide ice sheet it is possible to make some suggestions about their genesis based largely on spatial relationships it can be concluded that drumlins and ribbed moraine form at the base of actively flowing ice under similar dynamic conditions for either landform to exist however there must have been enough sediment available in the base of the glacier to leave or form a feature large enough to be recognizable the presence or absence of sufficient load is related to the geology of the glacier bed and has little to do with regionally changing dynamics of the ice water system likewise given sufficient load it is evident that whether drumlins formed or whether ribbed moraine formed in a certain area is a function of the physical nature of the load which is again related to geology of the source outcrops whether the physical characteristics come into play after the sediment has been released from the ice and is being reshaped by basal drag streamlining etc or whether the nature of the load while entrained changes the behaviour of the basal part of the ice is unclear physical characteristics of the basal sediment load have apparently promoted internal thrusting of coherent slabs of entrained debris and ice to form ribbed moraine on melting whereas drumlins may reflect moulding of plastic subglacial debris or erosional streamlining of both the unconsolidated glacial substrate and bedrock the observation that many eskers cross drumlin fields at nearly right angles to their orientation suggests that conditions producing streamlining and those pertaining to subglacial drainage are separated in time and circumstance the general occurrence of drumlins and eskers throughout the sediment rich portions of the keewatin ice sheet from zone 1 to its edge is difficult to reconcile with the restriction and intimate association of these forms with ribbed moraine almost exclusively in zone 2 because such a zonal relationship exists to some extent around other ice divides at least in labrador nouveau quebec and newfoundland it seems that some condition changed or existed in this zone throughout or at some specific time during the existence of the keewatin ice sheet possibly around the last ice divides reactivation of the ice sheet by alimentation associated with climatic deterioration may have promoted flow of thin brittle ice this may have shattered the glacier forming stacked thrust plates of ice and debris with subsequent stagnation the thrust plates may have melted out without substantial deformation

drumlins carved by deforming till streams below the laurentide ice sheet the peterborough drumlin field 5000 km 2 is the largest in central canada and was formed during readvance of the laurentide ice sheet shortly after 13 000 b p morphometric analysis of 998 drumlins combined with study of 6921 borehole logs and downhole geophysical data has established the distribution of drumlin types and their stratigraphy along a 70 km flow line through the drumlin field in the north drumlins are spindle shaped composed of till resting directly on bedrock and were formed by deformation of till into low pressure zones in the lee of bedrock scarps eskers record subglacial drainage by channeled meltwaters flowing on bedrock to the south drift thickness increases and eskers are absent drumlins are larger and more ovoid and are composed of overridden proglacial and glaciolacustrine sediments overlain erosively by deformation till till is thin over drumlin crests and thick in interdrumlin swales and records subglacial cannibalization and mixing of overridden sediments the drumlin form is the product of erosion by streams of deforming till flowing in swales separating cores composed of older sediments systematic down field changes in drumlin shape and size can be related to decreased duration of subglacial deformation toward the limit of the readvance and increased thickness of preexisting sediments available for dissection by subglacial till streams absence of eskers in the central and southern parts of the drumlin field suggests that the ice base was drained by mass flow of the deforming substrate the substantial subsurface data available from the peterborough area indicate that drumlins are predominantly erosional in origin and are the geomorphological expression of a deforming subglacial bed below the margin of the laurentide ice sheet

classification of planetary craters using outline based morphometrics abstract the morphologies of craters on planetary surfaces reveal clues about the geologic mechanisms by which they originate and subsequently evolve as well as the materials and physical variables inherent to the environment in which they formed we carried out a quantitative multivariate analysis of shape descriptors derived from the outlines of craters formed by volcanic processes on mars io and earth and by impact cratering on the moon using elliptic fourier analysis efa and the zahn roskies z r shape function canonical variate analysis cva was used to construct a statistical model of differences between the crater groups to classify craters produced by various volcanic and impact processes the classification model from canonical variate analysis of efa shape descriptors yielded a 90 rate of success for the assignment of group membership among 406 examined craters it correctly classified 138 of 154 90 ionian paterae 154 of 155 99 lunar impact craters 31 of 35 89 terrestrial basaltic shield calderas 32 of 38 84 terrestrial ash flow calderas and 12 of 24 50 martian basaltic shield calderas the classification model from canonical variate analysis of z r shape function descriptors classified 84 of the total population of the examined craters correctly the analysis correctly classified 96 of ionian paterae 100 lunar impact craters 51 terrestrial basaltic shield calderas and 63 martian calderas but only 16 of the terrestrial ash flow calderas were correctly classified canonical variate analysis of efa and z r results shows that the shapes of ash flow calderas and paterae on io differ the least of all groups included in this study and basaltic shield calderas and martian calderas analyzed together also have few differences the z r model successfully classifies more ionian patera and impact craters than the efa classification model but performs poorly at classifying the other crater groups this result shows that the descriptors convey different shape information the z r model is robust in its ability to classify end member differences in complexity while the efa model is robust in its ability to reliably classify among more groups these differences and similarities in shape confirm previously understood commonalities related to the origin and evolution of various types of craters in general basalt shield calderas on earth and mars are morphologically similar and are thought to have similar origins this study confirms that the 2 d shapes of their craters are quantitatively correlated similarities have been noted between terrestrial ash flow calderas and paterae on io principally in their large sizes shallow magma chambers and complex evolution this study confirms their shapes are also similar impact craters and ionian paterae are most dissimilar as are their evolutions this study demonstrates rigorous landform shape analysis can greatly increase our understanding of the diversity in craters and the processes involved in their formation

osl dating of the massive landslide damming event in the jishixia gorge on the upper yellow river ne tibetan plateau frequent natural hazards cause huge damage to human life and society in the mountainous regions along the eastern rim of the tibetan plateau a massive landslide damming event has been reported in

palaeo ice stream onsets examples from the north eastern laurentide ice sheet in this paper we report on observations of glacial landscapes at the head of geomorphologically interpreted palaeo ice streams i e palaeo ice stream onsets our work is based on the results of sy

the sensitivity of subglacial bedform size and distribution to substrate lithological control the varied nature of a glacial substrate has often been invoked to account for the varied distribution arrangement and morphological expression of subglacial bedforms the ease and rate of sediment production from the bedrock substrate and the rheological mechanical and hydraulic properties of the resultant subglacial sediment layer have been argued in certain samples or settings to account for the local to regional distribution of bedforms and bedform types and varying morphological expression such as size shape elongation or spatial arrangement from locally coherent observations and relationships it is tempting to extrapolate such patterns to express a more fundamental lithological control on subglacial bedform properties and formation here we use a large systematically compiled dataset 30 000 subglacial bedforms inscribed by the irish ice sheet to explore potential lithological controls upon subglacial bedform distribution and morphological expression we expect any lithological control upon form size or distribution to be expressed through a spatial correlation between bedform properties and those of the bed lithology we extract bedform morphometric and distribution statistics and underlying lithology data from datasets held in a geographic information system for comparative analyses at ice sheet scale i e considering our whole bedform population we find no apparent relationship between the lithology of either bedrock or till substrate and bedform occurrence density of arrangement or size regional to local examples do in contrast exhibit some coincident changes in substrate and bedform expression these are typically manifest as an abrupt shortening or lengthening of drumlins at a lithological boundary superimposed upon more gradual regional trends however not all local variability observed in the bedform population can be attributed to a lithological driver and where it can be inferred lithological control is often in tandem with or subordinate to other drivers such as topographic setting or ice dynamics we propose a hierarchy of controls upon bedform form and distribution strongly coupled to spatial scale lithological differences can modulate local scale form and expression but bedform incidence and properties are primarily governed by ice sheet scale glaciological patterns and drivers only under certain unknown conditions at a local scale is the bedforming process sensitive to the lithological properties of the bed

size and shape characteristics of drumlins derived from a large sample and associated scaling laws abstract ice sheets flowing across a sedimentary bed usually produce a landscape of blister like landforms streamlined in the direction of the ice flow and with each bump of the order of 102 to 103 m in length and 101 m in relief such landforms known as drumlins have mystified investigators for over a hundred years a satisfactory explanation for their formation and thus an appreciation of their glaciological significance has remained elusive a recent advance has been in numerical modelling of the land forming process in anticipation of future modelling endeavours this paper is motivated by the requirement for robust data on drumlin size and shape for model testing from a systematic programme of drumlin mapping from digital elevation models and satellite images of britain and ireland we used a geographic information system to compile a range of statistics on length l width w and elongation ratio e where e l w for a large sample mean l is found to be 629 m n 58 983 mean w is 209 m and mean e is 2 9 n 37 043 most drumlins are between 250 and 1000 metres in length between 120 and 300 metres in width and between 1 7 and 4 1 times as long as they are wide analysis of such data and plots of drumlin width against length reveals some new insights all frequency distributions are unimodal from which we infer that the geomorphological label of drumlin is fair in that this is a true single population of landforms rather than an amalgam of different landform types drumlin size shows a clear minimum bound of around 100 m horizontal maybe drumlins are generated at many scales and this is the minimum or this value may be an indication of the fundamental scale of bump generation proto drumlins prior to them growing and elongating a relationship between drumlin width and length is found with r2 0 48 and that is approximately w 7 l 1 2 when measured in metres a surprising and sharply defined line bounds the data cloud plotted in e w l space and records a scale dependent maximum elongation limit approximated by emax l1 3 when l measured in metres for a given length for some reason as yet unknown drumlins do not exceed the elongation ratio defined by this scaling law we also report and compare our statistics to an amalgamated sample 25 907 drumlins of measures derived from around 50 published investigations any theory must be able to explain the drumlin statistics and fundamental scaling properties reported herein and they thus provide powerful tests for drumlin modelling

the morphological characteristics of ribbed moraine abstract ribbed rogen moraines are large subglacially formed transverse ridges that cover extensive areas of the former laurentide scandinavian and irish ice sheets given their ubiquitous and conspicuous nature it is surprising that their characteristics are poorly understood to date most ribbed moraine studies have been spatially restricted and rely on small sample sizes thus published accounts of their characteristics are most likely unrepresentative of ribbed moraines generally this study addresses this deficit by producing the first representative data set on ribbed moraine size shape pattern and distribution and we describe their spatial characteristics various remote sensing and gis techniques were used to map ribbed moraines over a combined area of 81 000 km 2 in canada ireland and sweden producing a database of just over 33 000 individual landforms in all 25 082 ridges were mapped in two areas in central and northern quebec the lac naococane and river kaniapiskau regions 5637 in the lake rogen area of central sweden and 2500 in north central ireland in comparison against the published accounts of ribbed moraine we demonstrate that some widely held assertions are inaccurate or untrue and we show that ribbed moraine morphological characteristics are more complex than has been hitherto reported our data reveals they exist over a larger scale range than was previously stated this study concludes by presenting a list of ribbed moraine characteristics and argues that formational hypotheses must account for these if they are to remain valid explanations of ribbed moraine genesis

a model for spiral flows in basal ice and the formation of subglacial flutes based on a reiner rivlin rheology for glacial ice 1 flutes are elongated sediment ridges formed at the base of glaciers and ice sheets in this paper we show that flutes can be the product of a corkscrew like spiral flow in basal ice that removes sediment from troughs between flutes and deposits it at their crests as first suggested by shaw and freschauf in order to generate the type of basal ice flow required for this mechanism the viscous rheology of ice must allow for the generation of deviatoric normal stresses transverse to the main flow direction this type of behavior which is commonly observed in real nonlinearly viscous and viscoelastic fluids can be described by a reiner rivlin rheology here we develop a mathematical model that describes the role of these transverse stresses in generating spiral flows in basal ice and investigate how these flows lead to the amplification of initially small basal topography and the eventual formation of assemblies of evenly spaced subglacial flutes

the death mask of the antarctic ice sheet comparison of glacial geomorphic features across the continental shelf abstract multibeam swath bathymetry data seismic lines and shallow cores were collected seaward of all of the major drainage outlets of the antarctic ice sheet from the pennell coast of north victoria land to the northwestern weddell sea the results show that during the last glacial maximum the ice sheet extended onto the outer shelf in all areas to the shelf break in many swath bathymetry and deep tow sidescan sonar data from the continental shelf also record the geomorphology left by the retreating ice sheet glacial troughs occur offshore of all major glacial outlets in each drainage area except for parts of the ross sea the inner shelf is characterized by acoustic basement interpreted as crystalline substrate the geomorphology of these inner shelf areas consists of erosional features such as grooves that show that ice flow tended to follow the structural grain of the bedrock outer shelf areas are floored by sedimentary substrate where the direction of ice flow was more directly offshore and depositional features characterize the seafloor in these areas the signature of the grounded ice consists of till deposits and large scale geomorphic features mega scale glacial lineations drumlins occur within the region of contact between crystalline and sedimentary substrates meltwater channels also have been imaged in both inner and outer shelf settings while the presence of meltwater across the shelf has long been suspected we present here the first multibeam image of a meltwater channel on the outer continental shelf of antarctica at the shelf break gullies are in all areas where we know ice reached the shelf break we note the remarkable similarity in form of mega scale glacial lineations observed on the outer shelf over sedimentary substrate across all areas surveyed typically measuring 200 600 m crest to crest this suggests that the shape of these landforms is governed by the same process in each drainage area and that process was occurring under a large area of the ice sheet the erosional and transitional features show much variability in form and distribution

bed ribbing instability explanation testing a numerical model of ribbed moraine formation arising from coupled flow of ice and subglacial sediment 1 ribbed moraines are large up to 16 km long ridges of sediment produced transverse to ice flow direction that formed widely beneath palaeo ice sheets since ice sheet stability is sensitive to conditions operating at the bed an understanding of ribbed moraine genesis will provide critical information on ice sheet dynamics currently there is no consensus on ribbed moraine formation and various competing hypotheses have been presented to account for their genesis only one of these theories has been developed into a physically based numerical model that quantitatively describes ribbed moraine formation this theory known as the bed ribbing instability explanation brie argues that ribbed moraines are produced by a naturally arising instability in the coupled flow of ice and till brie demonstrates that transverse subglacial ridges i e ribbed moraine spontaneously grow under certain parameter combinations and it predicts their wavelength spacing between ridges the model represents a significant advance because it is the first time a theory of subglacial bedform generation has been developed to make quantitative predictions which can be formally tested this paper discusses the types of tests that are currently possible and reports the results from the first testing of brie this analysis centers on the ability of brie to predict the primary characteristics of ribbed moraine which are patterning and wavelength results show that brie successfully predicts the correct ribbed moraine pattern and appropriate wavelengths the tests fail to falsify the model and it is concluded that brie remains a viable explanation of ribbed moraine formation

glacier stress patterns and sediment transfer associated with the formation of superimposed flutes abstract glacial flutes superimposed on megaflutes in the foreland of austre okstindbreen north norway provide information suitable for the identification of the areas of erosion and deposition during the formation of the superimposed flutes and the calculation of the volumes of sediment involved in this process detailed analysis of the geometry particle size and clast fabric of the sediments involved in these landforms indicate that material was transferred to an incremental zone from an adjacent erosional trough along with additional material derived directly from the glacier sediment transfer is restricted to an upper till with a thickness of about 0 40 m but deformation imposed during the formation of the superimposed flutes can be traced to a depth of around 0 75 m in a lower sheared silty sand stress patterns revealed by clast fabric analysis in the superimposed flutes and adjacent parts of the megaflute indicate compressive flow at the proximal end of the bedform in the lee of an apparently obstructing boulder convergent extending flow in the central part of the bedform with a characteristic herringbone pattern and divergent extending flow at the terminal zone of the feature the convergent divergent pattern is considered to be a response to ice flow cells caused by high pressure over the troughs between the flutes and low pressure over the flute crestline these ice flow cells are considered to be initiated by factors such as upstanding boulders and bedrock knobs clusters of boulders within the diamicton and lateral and longitudinal oscillations developed in response to identifiable perturbations first order estimates of the rates of sediment transfer involved in the formation of the superimposed flutes vary between 0 0077 m3 m 2 a 1 and 0 38 m3 m 2 a 1 these figures have been applied in an exploratory fashion to megaflutes and drumlins from elsewhere in order to estimate the time required for glacier bedforms to develop and test the reliability of the figures derived the most meaningful results suggest that glacier bedforms are created within a period of between 4 and 400 years irrespective of their size this suggests that glacier bedforms develop rapidly and are maintained in a condition of steady state that reflects the balance between glacier energy and resisting bed material

holocene colluviation and its implications for tracing human induced soil erosion and redeposition on the piedmont loess lands of the qinling mountains northern china abstract colluvial components incorporated in loess soil sequences have preserved evidence of soil erosion and redeposition in the middle reaches of the yellow river drainage basin holocene loess soil sequences on the gentle sloped lands were studied by measuring magnetic susceptibility particle size distribution caco3 and extraction and identification of the non dust materials the analytical results indicate that in the early holocene soil erosion and redeposition by overland flow occurred even on the loess lands with small ground gradient in response to climatic amelioration linked to global climatic change no obvious soil erosion and redeposition events were found to correspond with the climatic fluctuations recorded in the loess soil sequence during the holocene climatic optimum the primary neolithic farming did not cause apparent erosion and redeposition because the loess landscape was mantled by well developed ustic luvisols and probably by forest vegetation both provided adequate resistance to sheet erosion by overland flow enhanced soil erosion and redeposition are traceable back to ca 4000 a b p it is attributed to intensified land use of arable farming and human settlement from the early bronze age onward a major event of sheet erosion and redeposition by overland flow occurred in the loess landscape during 4000 3600 a b p in the xia dynasty soil degradation and reduced resistance caused by climatic aridity and intensified dust accumulation may have played a secondary role in promoting soil erosion since 3100 a b p these results provide new insights into the history of soil erosion in the semi arid and semi humid loess regions and into the evaluation of human induced accelerated soil erosion against the background of the holocene natural erosion forced by monsoonal climatic fluctuation

rapid erosion drumlin formation and changing hydrology beneath an antarctic ice stream what happens beneath a glacier affects the way it flows and the landforms left behind when it retreats direct observations from beneath glaciers are however rare and the subglacial environment remains poorly understood we present new repeat observations from west antarctica that show active processes beneath a modern glacier which can normally only be postulated from the geological record we interpret erosion at a rate of 1 m a 1 beneath a fast flowing ice stream followed by cessation of erosion and the formation of a drumlin from mobilized sediment we also interpret both mobilization and increased compaction of basal sediment with associated hydrological changes within the glacier bed all these changes occurred on time scales of a few years or less this variability suggests that an ice stream can reorganize its bed rapidly and that present models of ice dynamics may not simulate all the relevant subglacial processes

late holocene drought responsible for the collapse of old world civilizations is recorded in an italian cave flowstone a severe drought in parts of low latitude northeastern africa and southwestern asia 4200 yr ago caused major disruption to ancient civilizations stable isotope trace element and organic fluorescence data from a calcite flowstone collected from the well watered alpi apuane karst of central western italy indicate that the climatic event responsible for this drought was also recorded in mid latitude europe although the timing of this event coincides with an episode of increased ice rafted debris to the subpolar north atlantic the regional ocean atmosphere response seems atypical of similar holocene ice rafting events furthermore comparison of the flowstone data with other regional proxies suggests that the most extreme part of the dry spell occurred toward the end of a longer term climate anomaly

palaeoenvironmental implications of the prehistorical catastrophes in relation to the lajia ruins within the guanting basin along the upper yellow river china archaeological excavations have exposed a shocking picture of the prehistorical catastrophes in the lajia ruins in the upper yellow river the grouped skeletons resting on the dwelling floors show

the composition and internal structure of drumlins complexity commonality and implications for a unifying theory of their formation investigation of drumlins is significant to both glaciology and palaeoglaciology but the sheer diversity of their composition and internal structure is often cited as a major obstacle towards a satisfactory unifying explanation of their formation this paper presents the first systematic survey of the vast literature on this subject with the aim of concisely summarising observations and identifying any emergent patterns or commonality that theories of drumlin formation should be able to explain results confirm that investigations are often limited by availability of suitable sediment exposures 40 of studies report data from 5 drumlins and 44 do not specify sample size although borehole data and geophysical techniques can alleviate this problem however it is clear that the constituents of drumlins are incredibly diverse in terms of their composition e g a range of lithologies clast shapes sizes and fabrics structure e g sediments that are sorted homogeneous surface conformable unconformable and evidence of deformation e g ranging from pervasive to non pervasive limited to absent despite this diversity our review leads us to suggest that drumlin composition can be simplified to five basic types i mainly bedrock ii part bedrock part till iii mainly till iv part till part sorted sediments and v mainly sorted sediments this is a potentially significant step in that it reduces the oft cited complexity of drumlin composition and provides a more realistic goal for theories or numerical models of drumlin formation to target the various types can occur within the same drumlin field which leaves us with two possible implications for drumlin formation 1 different types of drumlin are formed seeded by different processes despite being morphologically similar equifinality investigation of drumlin composition may therefore reveal diagnostic processes explanations for these different types of drumlin and we argue that bedrock drumlins are an example 2 a single process occurs across large parts of the ice bed interface to create drumlinised terrain in a variety of sediments investigation of drumlin composition may in this case simply reflect pre existing sediments but importantly the way in which the drumlin forming mechanism modifies is modified by them we argue that the latter simpler explanation applies to all other types of drumlin excluding purely bedrock forms and conclude that the diversity in drumlin composition is no obstacle to a single unifying theory

fresh arguments against the shaw megaflood hypothesis a reply to comments by david sharpe on paleohydraulics of the last outburst flood from glacial lake agassiz and the 8200 bp cold event

pressure dependent viscosity and interfacial instability in coupled ice sediment flow we study an interfacial instability in the coupled flow of ice and subglacial sediment both modelled as viscous media unlike other interfacial instabilities in coupled viscous flows at zero reynolds number the mechanism considered here does not rely on buoyancy or the effect of an upper free surface but on the pressure dependence of the sediment viscosity specifically the instability relies on sediment rheology being such that when sediment flows in simple shear sediment flux increases with compressive normal stress at the ice sediment interface when the velocity of the interface is kept constant when ice moves over a shallow bump in the interface it generates a higher compressive stress on the bump s upstream side than in its lee if in addition the effective sediment viscosity is low compared with that of ice interfacial velocity remains approximately constant and this then implies that more sediment flows into the bump than out of it causing it to grow modelling ice as a newtonian material we show that this mechanism works for a wide range of sediment rheologies including the highly nonlinear shear thinning ones typically thought most appropriate for the description of nearly plastic sediment the instabilities predicted are essentially two dimensional with infinite transverse wavelength and a nonlinear model shows that growth is unbounded until cavitation occurs in the lee of evolving bumps on the interface the instability mechanism does not seem to predict the formation of common glacial landforms but may explain the formation of water filled cavities on deformable glacier beds

formation of mega scale glacial lineations on the dubawnt lake ice stream bed 2 sedimentology and stratigraphy mega scale glacial lineations msgls are highly elongate subglacial landforms produced beneath zones of fast flowing ice while qualitative data on their morphology have existed for several decades studies of their composition and sedimentology are comparatively rare sediment exposures along the course of the finnie river in nunavut northern canada provide a window into the internal stratigraphy and sedimentology of msgls formed by the dubawnt lake palaeo ice stream during regional deglaciation of the laurentide ice sheet stratigraphic sections record evidence for an initial advance of ice into the study area followed by ice sheet recession and deposition of glacifluvial and glacilacustrine outwash subsequently the dubawnt lake palaeo ice stream overrode and reworked this outwash subglacially forming an msgl till this till comprises a sandy red diamicton facies forming the core of the msgl ridges and containing variably deformed lenses stringers and rafts of outwash the sedimentology of this diamicton is consistent with an origin as a glacitectonite and hybrid till formed by a combination of non pervasive subglacial sediment deformation and lodgement facies variations from stratified to massive diamicton reflect in turn variations in strain and subglacial transport distance the occurrence of stratified glacifluvial sediments within these ridges and the well preserved nature of many of the sandy inclusions within the diamicton imply relatively short transport distances and incomplete mixing msgls under the dubawnt lake palaeo ice stream formed through a combination of subglacial erosion and deposition this included non pervasive subglacial sediment deformation and the reworking of pre existing sediment depocentres during streaming flow these results highlight the importance of sediment supply to msgl formation with the presence of abundant pre existing sediments which were subsequently overridden being critical to lineation formation

the planar shape of drumlins abstract the asymmetry of the planar shape of drumlins is an established paradigm in the literature and characterizes drumlins as resembling tear drops with a blunt bullet shaped stoss end and a tapering pointed lee end it is widely cited and never been seriously questioned in this paper the planar shape of 44 500 drumlins mapped in various regional settings from drumlin fields in north america and northern europe were objectively analysed by means of geographic information system tools two parameters were considered the first denoted here as as pl focuses on the relative position of the point of intersection between the axes of the maximum length and the maximum width it is defined as the distance between the upstream i e beginning of the drumlin and the intersection point measured along the longitudinal axis divided by the entire length of the long axis results indicate that the intersection point of the majority of drumlins 64 is very close to the longitudinal midpoint 0 33 as pl as pl a is defined as the ratio between the area of the upstream half of the drumlin to that of the entire drumlin results show that for most drumlins 81 the upper half area is almost as large as the down half 0 45 as pl a

on the significance of normal stress effects in the flow of glaciers mctigue and others 1985 identified a possible problem in the type of constitutive equation usually used for modeling the creep behaviour of polycrystalline ice they pointed out that glen s flow law necessarily excludes the consideration of normal stress effects which are of great significance in other disciplines that consider non newtonian fluids using the second order fluid with material parameters evaluated from laboratory data as a tentative model for ice they reached the conclusion that normal stress effects may be discernible in natural glacier flow but as noted by mctigue and others the second order fluid fails to represent the non linear rate dependence of ice in shear therefore it is in fact not a suitable constitutive model for glacier ice in shearing flow in this note parallel to what mctigue and others did for the second order fluid we present a similar analysis for i the modified second order fluid and ii the power law fluid of grade 2 both of which are constitutive models recently proposed by man as a tentative generalization of glen s flow law both models i and ii can represent normal stress effects and both agree with glen s flow law in the prediction of the depth profile of velocity in the steady laminar flow of glaciers for ease of comparison the same creep data of mctigue and others are used in quantifying the material parameters in these two models both models i and ii predict far less pronounced normal stress effects in glaciers than those estimated by mctigue and others whose data analysis in fact suffered from inconsistencies and over parameterization

palaeo earthquake and palaeo mudflow events at the machangyuan ruins in the huangshui river valley northeastern margin of the tibetan plateau palaeo earthquake event recorded by loess rapture fissures n30 40 w and n40 50 e trending and palaeo mudflow event recorded by red clay deposits were identified at the machangyuan ruins in the huangshui river valley at the foot of the northeastern tibetan plateau comparative analysis of grain size distributions and geochemical elements of local outcrops indicates that the red mudflows were sourced from the gullies on the valley side where unconsolidated neogene red clay formation was exposed the palaeo earthquake was associated with regional tectonic structures including a nnw trending left lateral strike slip fault lajishan fault and a ne trending local fault bazhougou fault analysis combining optically stimulated luminescence ages and radiocarbon ages of archaeological record dates the palaeo earthquake and palaeo mudflow events to ca 4 80 ka bp during that stage many neolithic settlements distributed around the machangyuan ruins enhanced human activities of the majiayao culture disturbed

geospatial analysis of controls on subglacial bedform morphometry in the new york drumlin field implications for laurentide ice sheet dynamics the outline and trend of 6566 subglacial bedforms in the new york drumlin field have been digitized from digital elevation data a spatial predictive model has been used to extend values of bedform elongation over an area measuring 200 km 110 km the resulting surface is used in conjunction with depth to bedrock data and an assumed duration of ice residence to test three proposed controls on bedform elongation upon comparison the resulting display of morphometry is best explained by differences in ice velocity across the field of study the existence of multiple zones of fast moving ice located along the southern margin of the laurentide ice sheet is implied by the observed patterns of bedform elongation and orientation we present two interpretations that are consistent with the observations first enhanced basal sliding caused by decreasing effective pressure near a calving margin is suggested as a possible mechanism by which localized fast ice flow is initiated and maintained second topographically controlled ice streams likely occupied the fjord like troughs of the appalachian upland northern rim contrary to previous understanding of the laurentide southern margin in new york state the resulting palaeoglaciological reconstruction illustrates a dynamic mosaic of ice stream and or outlet glacier activity copyright 2009 john wiley sons ltd

subglacial bedforms reveal an exponential size frequency distribution subglacial bedforms preserved in deglaciated landscapes record characteristics of past ice sediment flow regimes providing insight into subglacial processes and ice sheet dynamics individual forms vary considerably but they can often be grouped into coherent fields typically called flow sets that reflect discrete episodes of ice flow within these bedform size frequency distributions predominantly height width and length are currently described by several statistics e g mean median and standard deviation that arguably do not best capture the defining characteristics of these populations this paper seeks to create a better description based upon semi log plots which reveal that the frequency distributions of bedform dimensions drumlin mega scale glacial lineation and ribbed moraine plot as straight lines above the mode this indicates by definition an exponential distribution for which a simple and easily calculated yet statistically rigorous description is designed three descriptive parameters are proposed gradient the exponent characterising bedforms likely least affected by non glacial factors area normalised y intercept 0 quantifying spatial density and the mode below small features are less prevalent due to i measurement data sampling and mapping fidelity ii possible post glacial degradation or iii genesis not being created sub glacially this new description has the benefit of being insensitive to the impact of potentially unmapped or degraded smaller features and better captures properties relating to ice flow importantly using flow sets can now be more usefully compared with each other across all deglaciated regions and with the output of numerical ice sheet models applications may also exist for analogous fluvial and aeolian bedforms identifying the characteristic exponential and that it is typical of emergent subglacial bedforms is a new and potentially powerful constraint on their genesis perhaps indicating that ice sediment interaction is fundamentally stochastic in nature

normal stress effects in the creep of ice une solution analytique exacte du fluage biaxial d un fluide non lineaire est comparee aux donnees de quatre tests sur de la glace polycristalline le modele donne une excellente representation a la fois du fluage primaire et secondaire

the athabasca fluting field alberta canada implications for the formation of large scale fluting erosional lineations abstract current drumlin and fluting hypotheses centred on processes of formation erosion and deposition by subglacial deformation accretion by lodgement glacial erosion and meltwater erosion and deposition rarely receive comprehensive field testing this paper documents the morphology internal structure and landscape associations of the athabasca fluting field alberta and discusses how well these characteristics are explained by postulated fluting forming processes fluting hypotheses are comprehensively tested by this approach and their rejection or acceptance are firmly based on field observation and knowledge of processes using multiple criteria fluting is shown to be erosional and not to be a product of pervasive subglacial deformation or accretion by lodgement morphology landscape associations bed truncation plus glaciofluvial lags and gravel on fluting ridges are inferred to support erosion by meltwater intact primary bedding synsedimentary faults and injection structures very close to the landsurface showing no sign of shear deformation are used to reject erosion by subglacial deformation fluting is shown to be closely associated with tunnel channels and indicates that these channels were overtopped by broad floods that formed the fluting field and eroded anabranching tracts over the length of the alberta plains prior to fluting formation sequences with melt out and lodgement till conduit sediment and boulder lags record net deposition over an extended time period diapirism prior to fluting formation marks a period of extreme pressure gradients which are related to bed separation following fluting formation there was only minimal landscape modification dominated by glaciofluvial and glaciolacustrine deposition not glacial processes this progression is presented as evidence for two very different glacial regimes separated by a short period with diapirism and extensive landscape modification by meltwater erosion we note that the study area was glaciated only once during the late wisconsinian and it follows that glacial sedimentation and landscape modification were confirmed to this single glacial episode

geomorphological criteria for identifying pleistocene ice streams ice streams are critical regulatory mechanisms in contemporary ice sheets it has been inferred that they also had a significant effect on the dynamics of former ice sheets subsequently many people have invoked their widespread occurrence from a variety of formerly glaciated areas hypothesised locations however have often outweighed meaningful evidence this paper addresses the problem using the characteristics of contemporary ice streams as a basis for their identification from former ice sheet beds a convergence of knowledge gained from contemporary ice stream research coupled with theories of glacial geomorphology allows several geomorphological criteria to be identified as suggestive signatures of ice stream activity it is envisaged that the geomorphological criteria developed here will introduce a more objective approach to the study of former ice streams the criteria are used to construct conceptual land system models of the beds of former ice streams and it is hoped such models can provide an observational template upon which hypotheses of former ice streams can be better based

drumlin formation related to inverted melt water erosional marks drumlin forms are described from maps and air photographs of a part of the athabasca plains northern saskatchewan three major forms spindle parabolic and transverse asymmetrical are recognized these forms which may show superimposed minor elements depart from classical descriptions of drumlins but are similar to moulds of erosional marks created by separated fluid flows assemblages of drumlins also show characteristics similar to those of erosional marks the form analogy between drumlins and moulds of erosional marks is carried to a conclusion that drumlins may be formed by the infilling of erosional marks created on the under side of glaciers by separated subglacial melt water flows estimates of specific discharge are obtained by means of an expected range of reynolds number geomorphological evidence is given for large scale erosion by subglacial melt water a discussion of the sedimentology stratigraphy and deformational structure of the interiors of drumlins shows that they may be explained by the erosional mark hypothesis this paper emphasizes the importance of melt water as a geomorphic agent and may have broad implications for ice sheet dynamics and profiles rates of deglaciation and the occurrence of bedrock thrusting by ice

human impact on the environment in the ethiopian and eritrean highlands a state of the art abstract this review analyses the environmental evolution of the ethiopian highlands in the late quaternary the late pleistocene 20 000 12 000 14c years bp was cold and dry with 1 low lake levels in the rift valley 2 large debris fans on the flanks of lake abhe basin and 3 the blue nile transporting coarse bedload then a period with abundant and less seasonal rains existed between 11 500 and 4800 14c years bp as suggested by increased arboreal pollen high river and lake levels low river turbidities and soil formation around 5000 4800 14c years bp there was a shift to more arid conditions and more soil erosion many phenomena that were previously interpreted as climate driven might however well be of anthropic origin thick sediment deposits on pediments as well as an increase of secondary forest scrub and ruderal species in pollen diagrams are witnesses of this human impact one important aspect of the late quaternary palaeoenvironment is unclear changes in nile flow discharges and rift valley lake levels have been linked to changes in precipitation depth most authors do not take into account changes in land use in the highlands nor changes in the seasonality of rain both of which can lead to a change in runoff coefficients tufa and speleothem deposition around 14 000 years ago tend to indicate that at the end of the last glacial maximum lgm conditions might have been wetter than generally accepted the most important present day geomorphic processes are sheet and rill erosion throughout the country gullying in the highlands and wind erosion in the rift valley and the peripheral lowlands based on existing sediment yield data for catchments draining the central and northern ethiopian highlands an equation was developed allowing to assess area specific sediment yields sy 2595a 0 29 n 20 r 2 0 59 where sy area specific sediment yield t km 2 year 1 and a drainage area km2 with respect to recent environmental changes temporal rain patterns apart from the catastrophic impact of dry years on the degraded environment cannot explain the current desertification in the driest parts of the country and the accompanying land degradation elsewhere causes are changing land use and land cover which are expressions of human impact on the environment deforestation over the last 2000 3000 years was probably not a linear process in ethiopia studies on land use and land cover change show however a tendency over the last decades of increasing removal of remnant vegetation which is slowed down or reversed in northern ethiopia by a set aside policy ongoing land degradation requires urgent action at different levels of society soil and water conservation swc structures are now widely implemented local knowledge and farmer s initiatives are integrated with introduced swc techniques at various degrees impact assessments show clear benefits of the soil conservation measures in controlling runoff and soil erosion in high rain areas runoff management requires greater emphasis during the design of soil conservation structures in such areas investment in swc might not be profitable at farm level although benefits for society are positive this pleads in favour of public support the present land degradation in the ethiopian highlands has a particular origin which includes poverty and lack of agricultural intensification causes of these are to be found in the nature of past and present regional social relations as well as in international unequal development this review strengthens our belief that under improved socio economic conditions land husbandry can be made sustainable leading to a reversal of the present desertification and land degradation of the ethiopian highlands

mid late holocene temperature and precipitation variations in the guanting basin upper reaches of the yellow river abstract the reconstruction of prehistoric temperature and precipitation variations in the upper reaches of the yellow river is essential for understanding the cultural evolution of the region but related information is sparse due to the limitations of the available proxies recent studies have shown that microbial glycerol dialkyl glycerol tetraethers gdgts are promising tools for reconstructing mean annual temperature mat and mean annual precipitation map in terrestrial deposits in this study we reconstructed mid late holocene climatic changes using gdgt distributions in a loess paleosol sequence in the lajia ruins of the neolithic qijia culture guanting basin in the southwestern end of the chinese loess plateau our gdgt records show that map decreased from ca 600 mm to 430 mm while mat decreased from 11 9 c to 8 0 c during the past ca 7000 yr and a drastic decline in map 70 mm accompanied by a 0 8 c decline in mat occurred at 3800 3400 yr bp our results provide direct evidence supporting a hypothesis that the flourishing 4200 4000 yr bp and decline 4000 3600 yr bp of the qijia culture mainly based on millets cultivation and subsequent rise of the xindian kayue culture 3600 2600 yr bp based on mixed agriculture of sheep husbandry and millets cultivation were triggered by climate change

superimposition of ribbed moraines on a palaeo ice stream bed implications for ice stream dynamics and shutdown the sediments and landforms preserved on palaeo ice stream beds can provide important information about their subglacial conditions and flow mechanisms and the processes accompanying their shutdown in this paper detailed observations of an intriguing subglacial landform assemblage of ribbed moraines superimposed on glacial lineations on the dubawnt lake ice stream bed north west canadian shield are presented including their morphometry internal structure from ground penetrating radar gpr surveys and from glaciogeological analysis and sedimentological characteristics from sediment architecture and lithofacies analysis the observations suggest an abrupt change in ice dynamics that correlates with two phases of glacial landform development this hypothesis is based on evidence from a deformed lodgement till which subsequently underwent brittle deformation and developed prominent thrust shear structures and tension fractures tension fractures are observed in a sediment exposure and thrust structures are observed in gpr surveys where they occur most prominently in the ribbed moraine ridge crests the presence of the fractures and their association with a population of clasts in the till that are orientated with their a axes transverse to the inferred ice flow direction suggests a compressional flow regime it is therefore inferred that the glacial lineations were formed under an extensional flow regime during ice stream activity but that at some point patches of till under the ice stream stiffened through dewatering the subsequent increase in basal shear stress resulted in compressional flow and the development of subglacial thrusting and the building of ribbed moraines we therefore suggest that ribbed moraines may form in areas of compressional flow under ice streams i e sticky spots and or at the transition between slow and fast ice flow along parts of an ice stream the general absence of ribbed moraines on most other palaeo ice stream beds suggests that either these ice streams continued operating during deglaciation or processes other than the development of localized compressional flow sticky spots led to their shutdown e g ice depletion

extraordinary floods of 4100 4000 a bp recorded at the late neolithic ruins in the jinghe river gorges middle reach of the yellow river china abstract holocene palaeohydrological investigations were carried out in the middle reach of the jinghe river that drains the central part of the loess plateau a set of palaeoflood slackwater deposits was found interbedded in the holocene loess soil sequence within the cliff riverbanks both the sedimentary criteria and the analytical results show that this sediment was sourced from a suspended sediment load of floodwater the major cultural layer of a late neolithic settlement 4300 4000 a bp occurring at the same site is blanketed by this flood deposit directly slopewash with pottery shards charcoal and burnt earth occurs in between the palaeoflood slackwater deposit beds it indicates that each of the slackwater deposit beds has recorded one individual flood event and that these extraordinary floods occurred when the late neolithic people occupied the riverbank terrace land this pre historical settlement was abandoned after the floods eventually due presumably to repeated inundation by overbank floodwater the flood events were osl dated to between 4100 and 4000 a bp and checked by archaeological dating of the anthropogenic remains retrieved from the sequences peak discharges of the floods were estimated to between 19 500 and 22 000 m3 s 1 which is several times gauged maximum floods during the 4200 4000 a bp climatic event severe droughts were documented in china s monsoonal regions and great floods were known from an ancient legend this study provides a well dated sediment record of pre historical floods which occurred at the transition from the neolithic to the bronze age in the yellow river basin it shows that extraordinary floods were part of the abrupt climatic variability during the 4200 4000 a bp climatic event the highly variable unstable and catastrophic climate typified by both droughts and floods resulted in settlement abandonment and possibly the decline of the highly developed late neolithic civilizations in china s monsoonal regions

extraordinary floods related to the climatic event at 4200 a bp on the qishuihe river middle reaches of the yellow river china abstract palaeo hydrological study was carried out in the qishuihe river valley in the middle reaches of the yellow river several bedsets of flood slackwater deposit were identified in the holocene loess soil sequences on the riverbanks they were differentiated from aeolian loess and soils by the parallel and waving beddings and the distinctive stratigraphic breaks separating individual palaeoflood events chronology of the flood events was established by osl dating checked by archaeological identification of the anthropogenic remains retrieved from the sequences the results show that successive floods occurred between 4300 and 4000 a bp in association with the abrupt climatic event of 4200 a bp these overbank floods had the riverbank settlement inundated repeatedly another series of extraordinary floods occurred between 3200 and 3000 a bp when monsoonal climate shifted from the mid holocene climatic optimum toward late holocene dry conditions the climatic event of 4200 a bp and the climatic decline at 3100 a bp were believed to be characterized by droughts previously this work provides solid evidence that both severe droughts and extreme floods were parts of the climatic variability during abrupt climatic event and climatic decline in the semi arid to sub humid zones over the world

antarctic palaeo ice streams we review the geomorphological sedimentological and chronological evidence for palaeo ice streams on the continental shelf of antarctica and use this information to investigate basal conditions and processes and to identify factors controlling grounding line retreat a comprehensive circum antarctic inventory of known palaeo ice streams their basal characteristics and minimum ages for their retreat following the last glacial maximum lgm is also provided antarctic palaeo ice streams are identified by a set of diagnostic landforms that nonetheless display considerable spatial variability due to the influence of substrate flow velocity and subglacial processes during the lgm palaeo ice streams extended via bathymetric troughs to the shelf edge of the antarctic peninsula and west antarctica and typically to the mid outer shelf of east antarctica the retreat history of the antarctic ice sheet since the lgm is characterised by considerable asynchroneity with individual ice streams exhibiting different retreat histories this variability allows antarctic palaeo ice streams to be classified into discrete retreat styles and the controls on grounding line retreat to be investigated such analysis highlights the important impact of internal factors on ice stream dynamics such as bed characteristics and slope and drainage basin size whilst grounding line retreat may be triggered and to some extent paced by external atmospheric and oceanic forcing the individual characteristics of each ice stream will modulate the precise timing and rate of retreat through time

ice stream sticky spots a review of their identification and influence beneath contemporary and palaeo ice streams rapidly flowing ice streams are the arterial drainage routes in continental ice sheets and exert a major influence on ice sheet mass balance recent observations have revealed that ice stream flow exhibits considerable variability with relatively rapid changes taking place in speed and direction this spatial and temporal variability is intimately linked to the conditions at the base of the ice streams and the distribution of localised patches of basal friction known as sticky spots in this paper we provide a detailed review of sticky spot observations from both contemporary and palaeo ice stream beds in order to better understand their nature and influence observations and theoretical considerations reveal four primary causes of stickiness i bedrock bumps ii till free areas iii areas of strong well drained till and iv freeze on of subglacial meltwater these may act together in one location or in isolation and a progressive increase in their distribution could lead to ice stream shut down bedrock bumps are influential under active ice streams where they provide form drag and can create thinner ice which increases the likelihood of basal freeze on increased bed roughness may prevent the lateral migration of some ice streams but bedrock bumps are unlikely to cause ice stream shut down because over long time scales ice stream erosion might be expected to reduce their amplitude the influence of till free areas beneath an ice stream will depend critically on the amount of water that might be drawn out of the surrounding till to lubricate such areas they are likely to be most important in ice stream onset zones but their identification has proved difficult beneath active ice streams if an ice stream operates solely by till deformation it is conceivable that a progressive increase in the exposure of till free areas could lead to shut down through a process of sediment exhaustion areas of strong well drained till have been identified beneath both active and ancient ice streams and are most likely to result from the reorganisation of subglacial meltwater the collapse of an inefficient cannalised system to a more efficient channelised system can occur rapidly and this mechanism has been hypothesised as a candidate for ice stream shut down in both contemporary and palaeo settings basal freeze on has also been observed and inferred from beneath modern and palaeo ice streams and a reduction in basal meltwater supply coupled with ice stream drawdown and the advection of cold ice increases the likelihood of switching off an ice stream a paucity of data from ice stream sticky spots limits a better understanding of their nature distribution and evolution beneath ice streams future technological advances are likely to improve the resolution of the data collected from the beds of modern ice streams but well preserved palaeo ice stream beds also hold potential for investigating their influence on ice stream flow and we present simple landsystems models to aid their identification such data will considerably enhance the basal boundary condition in ice stream models which will ultimately refine our predictions of the response of contemporary ice sheets to future changes in climate

hydrological studies of the historical and palaeoflood events on the middle yihe river china abstract palaeo hydrological and sedimentary investigations were carried out in the longmenxia gorge of the middle yihe river five bedsets of flood slackwater deposits swd were found interbedded into holocene aeolian loess soil profiles in the river bank at the longmenxia site they were identified as the deposits of the suspended sediment load during the extreme flood events of the holocene the minimum flood peak discharges were estimated to be 12 300 15 300 m 3 s using the slope area method these are about twice the largest gauged record 7180 m 3 s that has occurred since 1937 these flood events occurred at 3100 3000 a b p 1800 1700 a b p 700 550 a b p and 350 250 a b p as dated by optically stimulated luminescence osl in combination with stratigraphic correlation to which may be added the recorded events of the 223 and 1761 ce floods these were associated with holocene monsoonal shifts and abrupt climatic events this research not only provides palaeoflood discharge estimates on the middle yihe river but also provides important data for understanding interactions between regional hydro climatic systems and global climate change in humid and semi humid monsoonal regions such as china

ribbed moraine formation ribbed rogen moraines are conspicuous landforms found in interior parts of formerly glaciated areas two major theories for ribbed moraine formation have been suggested in recent years i the shear and stack theory which explains ribbed moraine formation by shearing and stacking of till slabs or englacially entrained material during compressive flow followed by basal melt out of transverse moraine ridges and ii the fracturing theory according to which ribbed moraines form by fracturing of frozen pre existing till sheets at the transition from cold to warm based conditions under deglaciating ice sheets in this paper we present new data on the distribution of ribbed moraines and their close association with areas of frozen bed conditions under ice sheets in addition we show examples of ribbed moraine ridges that fit together like a jig saw puzzle these observations indicate that fracturing and extension of a pre existing till sheet may be a predominant process in ribbed moraine formation in summary we conclude that all described characteristics of ribbed moraines are compatible with the fracturing theory while the shear and stack theory is hampered by an inability to explain many conspicuous features in the distribution pattern and detailed morphology of ribbed moraines one implication of the fracturing theory is that the distribution of ribbed moraines can be used to reconstruct the extent of areas that underwent a change from frozen bed to thawed bed conditions under former ice sheets

new evidence for the catastrophic demise of a prehistoric settlement the lajia ruins in the guanting basin upper yellow river nw china abstract the lajia ruins in the guanting basin nw china are a product of the prehistoric qijia culture like pompeii they are a rare example of an archaeological site preserved by a natural disaster and are therefore important in archaeology anthropology and geology however the nature of the disaster s responsible for the destruction of the site remains controversial most studies have focused on an earthquake and a red clay layer directly overlying the site and a detailed stratigraphic study of the mid holocene sedimentary strata combined with other intervals of red clay deposition hence possible disasters is lacking we identified a mid holocene paleosol sequence the shanglajia section at the site which contains two layers of red clay dated to 3950 a bp and 3500 a bp intercalated within the mid holocene paleosol s0 subsequent multi proxy analysis indicated that the characteristics of the two red clay layers resemble those of typical tertiary red clay deposits and the modern gully deposit at the foot of the great red hills but are distinctly different from those of the slackwater deposits of the yellow river and the mid holocene paleosol our results suggest that at 3950 a bp and 3500 a bp two large scale rainstorm induced mudflow events originating from the gullies to the north flooded the lajia area on the second terrace of the yellow river devastating and burying the human settlements we infer that the intensified erosion and mass wasting were caused by human activity in addition natural factors such as rainstorms and earthquakes may also have played an important role in triggering catastrophic mudflow events in the tertiary red clay deposits overall our results provide further insights into prehistoric man land relationships in this environmentally sensitive region which may have implications for modern land use in this region of china and elsewhere

the stability of a viscous till sheet coupled with ice flow considered at wavelengths less than the ice thickness a turbation method is used to analyse the stability of a thin till layer overlain by a deep ice layer ice is modelled as a linearly viscous fluid while the till viscosity has power law dependence on stress and effective pressure a linearized set of equations yields descriptions of the coupling of the ice flow with the sediment flow and reveals parameter ranges where the till perturbation amplitude can grow this shect flow instability is an essential part of any theory of drumlin formation and shows that viscous models of till have the ability to explain typical deforming bed features this is of great significance for large scale ice shect modelling

a highly mobile laurentide ice sheet revealed by satellite images of glacial lineations satellite images of the north american continent reveal previously unrecognized patterns of crossing drift lineations reflecting unexpected shifts of centres of mass of the laurentide ice sheet throughout the last glacial cycle such shifts must must have been associated with major global atmospheric circulation changes

a luminescence dating study of the sediment stratigraphy of the lajia ruins in the upper yellow river valley china abstract pedo sedimentological fieldwork were carried out in the lajia ruins within the guanting basin along the upper yellow river valley in the eolian loess soil sections on the second river terrace in the lajia ruins we find that the land of the qijia culture 4 20 3 95 ka bp are fractured by several sets of earthquake fissures a conglomerated red clay covers the ground of the qijia culture and also fills in the earthquake fissures the clay was deposited by enormous mudflows in association with catastrophic earthquakes and rainstorms the aim of this study is to provide a luminescence chronology of the sediment stratigraphy of the lajia ruins eight samples were taken from an eolian loess soil section xialajia section in the ruins for optically stimulated luminescence osl dating the osl ages are in stratigraphic order and range from 31 94 1 99 ka to 0 76 0 02 ka combined osl and 14c ages with additional stratigraphic correlations a chronological framework is established we conclude that 1 the second terrace of the upper part of yellow river formed 35 00 ka ago which was followed by the accumulation of the eolian loess soil section and 2 the eolian loess soil section is composed of the malan loess of the late last glacial mis 2 and holocene loess soil sequences

the origin and longevity of flow stripes on antarctic ice streams flow stripes along the surface of rapidly moving ice streams are shown to be an expected reaction of a viscous medium flowing over an irregular bed whenever the velocity at the bed is large compared to shearing through the thickness the principal features of the process are as follows at high basal speeds the ice acts as a strongly selective band pass filter transmitting basal undulations on spatial scales of a few ice thicknesses very effectively to the surface the decay of the short scale features by diffusion is strongly retarded by horizontal stress gradients consequently localized disturbances at the bed produce topographic effects on the surface that are advected long distances downstream before decaying away this mechanism may explain many of the flow stripes on active ice streams moving by rapid basal motion

reconstruction of palaeo ice sheets the use of geomorphological data the article discusses the nature of the glacial inversion problem which is defined as the extraction of time slice ice sheet flow patterns from the patchy and partly overprinted landform record present in former ice sheet areas a coherent inversion model for derivation of flow patterns and interior ice sheet configuration from geomorphological data is presented glacial landscapes are classified according to the three criteria of internal age gradients presence or absence of meltwater traces aligned to flow traces and basal condition frozen bed thawed bed inferred from morphology the inversion model uses landscapes classified accordingly spatially delineated into fans as input data relative chronologies at fan intersections are used to sort fans in a relative age stack that can be linked to stratigraphic dating information

deep sea floor evidence of past ice streams off the antarctic peninsula the past existence of a giant ice stream off the northern end of the antarctic peninsula is suggested by a convex upward elongated sediment body now at a mean water depth of 1000 m because of its morphological characteristics i e a set of parallel to subparallel ridges and grooves to 100 km long with an overall width of 25 km we call this depositional body a bundle structure we hypothesize that bundle structures form by accumulation of basal deformation till under their parent ice streams from its location size and overall characteristics the bundle structure described here constitutes the best preserved largest deepest and relatively low latitude evidence of giant ice streams that flowed offshore antarctica during glacial maxima bundle structures reveal the very dynamic behavior of ice caps in the northern antarctic peninsula during the last glacial maximum with catchment areas draining rapidly under marine based critical subglacial conditions bundle structures represent a new megascale streamlined glacial landform

a ploughing model for the origin of weak tills beneath ice streams a qualitative treatment abstract glaciological studies of west antarctic ice streams have shown that weak sub ice stream tills provide the basal lubrication that makes fast ice streaming possible under low driving stresses given the significant current interest in time dependent ice stream behavior there is a clear need for a conceptual model of weak sub ice stream tills that treats in a simple but physically correct way the coupling between evolution of till properties and ice stream dynamics as a possible alternative to the previous viscous bed model we propose a ploughing model that is consistent with the experimentally determined coulomb plastic rheology of sub ice stream till in the ploughing model the till is a several meters thick layer of sedimentary material that is disturbed and transported by ploughing that occurs during sliding of a bumpy ice base the thickness of the till layer is determined in the ploughing model by the amplitude of the largest roughness elements ice keels there is no direct proof for the existence of ice bumps and ice keels beneath the modern west antarctic ice streams but bedforms e g megalineations and bundle structures left behind by pleistocene ice streams strongly support our assumption that an ice stream base is irregular generation of new till material occurs when ice keels protrude through the existing till layer and erode the top of the sub till preglacial sediments based on a single tethered stake measurement of engelhardt and kamb j glaciol 44 1998 223 made at the upb camp ice stream b west antarctica fig 1 we estimate that the till flux due to sliding with ploughing is there 3 yr 1 per meter width to balance the estimated till flux in the upb area substrata erosion by ice keels would have to take place at a high but not unreasonable non dimensional rate of 4 assuming 1 contact area in the case of the west antarctic ice streams erosion of sub till materials by ice keels may be particularly fast and unimpeded because these ice streams are overriding unlithified preglacial tertiary sediments the most significant implication of the proposed ploughing model is that it permits treating basal resistance to ice motion as being velocity independent plastic till rheology while allowing subglacial transport of till as in the viscous bed model models of ice streams with a plastic bed exhibit a greater potential for unstable behavior than models of ice streams with viscous beds

megagrooves and streamlined bedrock in nw scotland the role of ice streams in landscape evolution abstract new multibeam bathymetry data onshore high resolution elevation data nextmap and fieldwork in the ullapool area of nw scotland reveal large scale megagrooves and streamlined bedrock forms in a well defined 20 km wide zone the landsystem is typical of a coherent flow corridor within a grounded ice sheet on bedrock dominated terrain we describe the morphology of the large scale features discuss their likely formation and consider the wider implications for ice sheet dynamics based on the strongly convergent bedform distribution the presence of megagrooves and highly elongate bedrock forms we interpret the erosional landscape to be the signature of a fast flowing tributary that once fed the the minch palaeo ice stream a major artery of the last british irish ice sheet the exact genesis of bedrock megagrooves remains uncertain although focused subglacial abrasion is likely to have carved most of the shallow strongly parallel features whilst glacial meltwater may have carved or modified others bedform morphometry is used to discriminate zones reflecting the degree of glacial streamlining elongation ratios 5 1 we interpret these zones to represent the transition from potentially cold based slow ice sheet flow to warm based fast flow based on these results and the presence of ribbed moraines we suggest a bedform continuum model for onset zones of palaeo ice streams on rigid beds rapid spatial bedform evolution is suggested to reflect an increase in subglacial erosive power that may be diagnostic of palaeo ice sheet thermal boundaries i e from cold to warm based and is also consistent with the expected downstream increase in ice velocity within an ice stream onset zone finally this study speculates on the role played by basal meltwater in ice stream initiation and the role of ice streams and their tributaries in landscape evolution

a meltwater origin for antarctic shelf bedforms with special attention to megalineations the geomorphology of troughs crossing the antarctic shelf is described and interpreted in terms of ice stream hydrology the scale of tunnel channels on the inner shelf and the absence of sediment at their mouths are taken to infer catastrophic drainage drumlins on the inner and outer shelves with pronounced crescentic and hairpin scours are also interpreted as products of catastrophic flow gullies and channels on the continental slope and turbidites on the rise and abyssal plain point to abundant meltwater discharge across the shelf attempts to explain this morphology and sedimentology in terms of release or discharge of meltwater by pressure melting strain heating darcian flow or advection in deforming till are shown to be unrealistic we suggest that meltwater flow across the middle and outer shelves might have been in broad turbulent floods which raises the possibility that megascale glacial lineations msgl on the shelf might originate by erosion in turbulent flow this possibility is explored by use of analogs for msgl from flood and eolian landscapes and marine environments an extended discussion reflects on objections that stand in the way of the flood hypothesis

climate change at the 4 2 ka bp termination of the indus valley civilization and holocene south asian monsoon variability 1 planktonic oxygen isotope ratios off the indus delta reveal climate changes with a multi centennial pacing during the last 6 ka with the most prominent change recorded at 4 2 ka bp opposing isotopic trends across the northern arabian sea surface at that time indicate a reduction in indus river discharge and suggest that later cycles also reflect variations in total annual rainfall over south asia the 4 2 ka event is coherent with the termination of urban harappan civilization in the indus valley thus drought may have initiated southeastward habitat tracking within the harappan cultural domain the late holocene drought cycles following the 4 2 ka bp event vary between 200 and 800 years and are coherent with the evolution of cosmogenic 14c production rates this suggests that solar variability is one fundamental cause behind holocene rainfall changes over south asia

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

formation of mega scale glacial lineations on the dubawnt lake ice stream bed 1 size shape and spacing from a large remote sensing dataset abstract mega scale glacial lineations msgls are the largest flow parallel bedforms produced by ice sheets and are formed beneath rapidly flowing ice streams knowledge of their characteristics and genesis is likely to result in an improved understanding of the rate at which ice and sediment are discharged by ice sheets but there is little consensus as to how they are formed and there are few quantitative datasets of their characteristics with which to formulate or test hypotheses this paper presents the results of a remote sensing survey of 46 000 bedforms on the dubawnt lake palaeo ice stream bed focussing on a central transect of 17 038 that includes highly elongate bedforms previously described as msgls within this transect lineations exceed 10 km in length max 20 km and 23 have elongation ratios 10 1 max 149 1 highly elongate features are interspersed with much shorter drumlin like features but longer bedforms are typically narrower suggesting that their length develops more quickly than or at the expense of their width bedforms are broadly symmetrical in plan form and have a preferred lateral spacing of 50 250 m which implies a regular rather than random pattern of corrugations comparison with drumlins reveals that the more attenuated msgls simply extend the tail of the distribution of data rather than plotting as a separate population taken together this supports the idea of a subglacial bedform continuum primarily controlled by ice velocity but existing hypotheses of msgl formation are either not supported or are insufficiently developed to explain our observations rather we conclude that under conditions of rapid ice flow msgls attain their great length relatively quickly decades through a probable combination of subglacial deformation which attenuates ridges and erosional processes that removes material from between them

climate change and the collapse of the akkadian empire evidence from the deep sea the akkadian empire ruled mesopotamia from the headwaters of the tigris euphrates rivers to the persian gulf during the late third millennium b c archeological evidence has shown that this highly developed civilization collapsed abruptly near 4170 150 calendar yr b p perhaps related to a shift to more arid conditions detailed paleoclimate records to test this assertion from mesopotamia are rare but changes in regional aridity are preserved in adjacent ocean basins we document holocene changes in regional aridity using mineralogic and geochemical analyses of a marine sediment core from the gulf of oman which is directly downwind of mesopotamian dust source areas and archeological sites our results document a very abrupt increase in eolian dust and mesopotamian aridity accelerator mass spectrometer radiocarbon dated to 4025 125 calendar yr b p which persisted for 300 yr radiogenic nd and sr isotope analyses confirm that the observed increase in mineral dust was derived from mesopotamian source areas geochemical correlation of volcanic ash shards between the archeological site and marine sediment record establishes a direct temporal link between mesopotamian aridification and social collapse implicating a sudden shift to more arid conditions as a key factor contributing to the collapse of the akkadian empire

sedimentary records of palaeofloods in the wubu reach along the jin shaan gorges of the middle yellow river china abstract the yellow river is known for the heavy suspended sediment load resulting from intensive soil erosion and human disturbance in the drainage basin palaeohydrological investigations were carried out along the jin shaan gorges in the middle reaches of the yellow river a set of palaeoflood slackwater deposits swds were identified interbedded in the holocene loess soil sequence within the cliff riverbanks analytical results of magnetic susceptibility and particle size distribution indicate that these well sorted palaeoflood swd beds were deposited by the suspended sediment load of the floodwater sourced from the loess tablelands in the middle reaches of the yellow river four episodes of extraordinary palaeoflood events have been recorded in the lianghekou lhk site these hydro climatic events were dated to 3200 2900 a by using the optically stimulated luminescence osl method in combination with pedo stratigraphic correlations with the previously studied holocene pedo stratigraphy yhg site in the yellow river drainage basin the peak discharges were estimated to be 41 268 47 415 m 3 s 1 by using the slope area method of streamflow measurement they are about twice the largest gauged flood 24 000 m 3 s 1 that has ever been measured in the wubu gauge station these extraordinary palaeoflood events are correlated with holocene climatic variability characterized by cooling and drying during the period 3200 2900 a this study provides sdata for hydraulic engineering development of water resources and mitigating flood hazard in the yellow river drainage basin and also gives insight into understanding the sensitivity of regional hydro climatic systems and the effects of climate change