glacial valley cross profile morphology tian shan mountains china abstract the morphology of glacial valley cross sections can be described in terms of power law y ax b or quadratic equations y a bx cx 2 fitted to empirical data the quadratic solution provides a more robust way of describing the morphology of glacial valley cross sections whereas the power law has more potential in understanding the cross sectional shapes and their evolution these two functions are used to study the cross sectional morphology of glacial valleys in the middle and western tian shan mountains and to discuss the comparison with fluvial channels the major conclusions are 1 power law equation parameters a and b are sensitive to the origin selection with larger sensitivity in vertical and a a ln a values conversely c values of the quadratic equation are more stable regardless of different origins selected 2 hirano and aniya earth surf processes landforms 13 1988 707 716 suggested two characteristic patterns in the relationship between the power law exponent b and the valley form ratio fr however glacial valleys in these areas do not fit the rocky mountain model for b fr values described by hirano and aniya for alpine glacial valleys this indicates that this rocky mountain model cannot be applied to all alpine glacial areas 3 the c values of the quadratic equation represent a curvi linear trend with its corresponding fr s at the same time power law parameters a and b fit a closed linear relationship both from these areas and others in the published literature 4 the cross sectional shapes of glacial valleys show clear differences with fluvial channels by comparing a b values of glacial valleys with the hydraulic geometry of fluvial channels this implies that the a b relationship and the variation range of b values commonly with 1 5 2 5 may be helpful to differentiate valleys formed by different processes

the rumble sound generated by a tidal bore event in the baie du mont saint michel a tidal bore is a sharp rise in free surface elevation propagating upstream in an estuarine system at the leading edge of the flood tide it generates a powerful noise that was sometimes compared to the sounds of a horse cavalcade herein the sounds generated by a tidal bore event in the baie du mont saint michel were carefully recorded the data showed three distinct periods these were the incoming tidal bore when the sound amplitude increased with the approaching bore front the passage of the tidal bore in front of the microphone where the impacts of the bore on the bank rocks or jetty generated powerful noises and the upstream propagation of the bore when the flood flow motion caused additional loud noises during the arrival of the tidal bore the sound levels were less energetic and a lower pitch sound was noted than during the subsequent record for the breaking bore process the analysis of the sound record indicated a dominant frequency around 76 77 hz the low pitch rumble had a frequency co

some forms of mountain peaks in the canadian rockies controlled by their rock structure abstract different mountain shapes in sedimentary sequences in the canadian rockies were enhanced by glacial erosion and have been modified post glacially by gravity driven slope processes slope modification by both glacial erosion and post glaciation landslides is related to slope geometry and rock structure particularly bedding dip five mountain peak shapes in monoclinal sequences each fall into different ranges of bedding dips 1 castellate and 2 matterhorn mountains occur in sub horizontal beds and their slopes are generally 35 65 and oblique to both bedding and joints 3 cuestas develop in gently to moderately dipping beds dip slopes and steeper normal escarpments form their cataclinal and anaclinal sides respectively with the dihedral angle between them about 90 4 hogbacks in moderately to steeply dipping beds have similar slope angles on both cataclinal and anaclinal slopes cataclinal slopes are either dip slopes or underdip slopes but anaclinal slopes are often steepened escarpments the dihedral angle between the slopes is usually slope gradients on a sample of 34 peaks are closely related to landslides particularly rock slides and rock topples these landslide modes are controlled by bedding dips

postglacial topographic evolution of glaciated valleys a stochastic landscape evolution model the retreat of valley glaciers has a dramatic effect on the stability of glaciated valleys and exerts a prolonged influence on the subsequent fluvial sediment transport regime we have studied the evolution of an idealized glaciated valley during the period following retreat of ice using a numerical model the model incorporates a stochastic process to represent deepseated landsliding non linear diffusion to represent shallow landsliding and an approximation of the bagnold relation to represent fluvial sediment transport it was calibrated using field data from several recent surveys within british columbia canada we present ensemble model results and compare them with results from a deterministic linear diffusion model to show that explicit representation of large landslides is necessary to reproduce the morphology and channel network structure of a typical postglacial valley our model predicts a rapid rate of fluvial sediment transport following deglaciation with a subsequent gradual decline similar to that inferred for holocene time we also describe how changes in the model parameters affect the estimated magnitude and duration of the paraglacial sediment pulse

polynomial and power functions for glacial valley cross section morphology an empirical evaluation of glacial trough cross section shape is performed on seven vertical cross sections in three sierra nevada valleys glaciated during the late quaternary power and second order polynomial functions are fitted by statistical regression power functions are very sensitive to subtle valley bottom topographic features and require precise specification of the valley bottom centre location this dependency is problematic given under representation of valley bottoms by conventional contour sampling methods and the common alteration of valley bottom morphology by non glacial processes power function exponents vary greatly in response to these and other non genetic factors and are not found to be reliable indicators of overall valley morphology second order polynomials express overall valley shape in a single robust function they are applied to both bedrock and sediment floored glacial valleys with negligible statistical bias except where side slopes are stepped or convex upward or where valley form is asymmetrical they can describe alluviated or severely eroded valleys and can objectively identify indi vidual components of polymorphic valleys because valley bottom and centre locations need not be specified mathematical expressions of parameters useful for geomorphic measurements and glaciological modelling are analytically derived from the polynomials as functions of the three polynomial coefficients these parameter equations provide estimates of valley side slopes mean and maximum depth midpoint location width area boundary length form ratio and symmetry

structural tectonic and glaciological controls on the evolution of fjord landscapes abstract the fjord landscape of south america stretching 1500 km between golfo corcovado 43 s and tierra del fuego 56 s is the largest continuous fjord landscape on earth this paper presents the results of new structural geological and geomorphological mapping of this landscape using optical satellite images and digital elevation models first order geological structures are represented by strike slip faults forming lineaments up to hundreds of kilometres long the strike slip faulting has been active since late cretaceous times and is responsible for the presence of a conspicuous structural cleavage visible as lineaments up to 10 km long a detailed analysis of these second order lineaments from digital image data was carried out in three sectors in sector 1 located northwest of the north patagonian icefield there are three distinct mean orientations characterized by a main nearly orogen parallel orientation az 145 and two orogen oblique secondary orientations az 20 and az 65 in sector 2 located west of the south patagonian icefield there are also three separate mean orientations with most of the lineaments concentrated between azimuths 0 and 80 mean at 36 and two other orogen oblique means at azimuth 122 and 163 in sector 3 around the cordillera darwin there is a single main orogen parallel mean at 100 115 in all three sectors mapped fjord orientations bear a striking similarity to the structural data with fjords orientated preferentially in the same direction as structural lineaments we infer that successive glaciations followed the same ice discharge routes widening and deepening pre existing geological structures at the expense of the surrounding terrain to create the fjord landscape this study has broader implications for ice sheet reconstructions and landscape evolution beneath ice sheets because we demonstrate that the primary control on fjord development in glaciated areas is geological and not glaciological

valley formation by fluvial and glacial erosion cross valley profiles from the west slope of the olympic peninsula washington are used to investigate the relative effects of fluvial and glacial erosion on valley formation unlike most ranges where glaciers and rivers sequentially occupied the same valleys neighboring valleys in the olympic mountains developed in similar lithologies but were subject to different degrees of glaciation allowing comparison of the net effect of glacial and fluvial processes integrated over many glacial cycles upslope drainage area was used to normalize comparisons of valley width ridge crest to valley bottom relief and valley cross sectional area as measures of net differences in the mass of rock excavated from below ridgelines for 131 valley spanning transects valley width relief and cross sectional areas are similar for glaciated partly glaciated and unglaciated fluvial valleys with drainage areas of 10 km 2 but diverge downslope glaciated valleys draining 50 km 2 reach two to four times the cross sectional area and have to 500 m greater relief than comparable fluvial valleys partly glaciated valleys have intermediate dimensions at distances of 5 km from valley heads the cumulative upstream volume of rock removed to form valleys is two to four times greater in glacially incised valleys than in fluvial valleys the finding of strong differences in the net result of valley excavation by fluvial and glacial erosion supports the interpretation that alpine glaciers are more effective erosional agents than are rivers and implies that large alpine valleys deepened and enlarged in response to pleistocene glaciation

using parabolas to describe the cross sections of glaciated valleys it is widely accepted that the simple power curve y axb can be used to describe the cross section of glaciated valleys there are however good reasons to suppose that this particular form of equation is inappropriate for this purpose

investigating the spatial distribution of summit flats in the uinta mountains of northeastern utah usa isolated laterally extensive gently sloping surfaces known as summit flats are present at high elevations in many laramide ranges and are particularly well developed in the uinta mountains of northeastern utah to investigate the spatial distribution of these surfaces and to consider possible controls on this pattern a map of summit flats in the uintas was developed from digital elevation data summit flats were identified as unglaciated areas of the landscape above an elevation of 3400 m having a slope of less than 0 3 m m 1 and an area greater than 510 2 km 2 as defined summit flats comprise 43 of the unglaciated land area above 3400 m in the uintas with the largest individual flat covering nearly 34 km 2 to quantitatively evaluate the distribution of summit flats in the uintas the area of summit flats was normalized to the total unglaciated area above 3400 m in 10 km wide swaths oriented normal to the range axis values of percent summit flats obtained by this method decrease dramatically westward from a high of more than 60 at the eastern end of the uintas to 0 at the western end given that individual summit flats can be diminished through lateral erosion by surrounding valley glaciers and that the summit flats themselves were apparently never glaciated this result suggests that glacial erosion has been more effective in the western uintas over the course of the quaternary focused glacial erosion at the upwind end of the range is consistent with the hypothesis that the proximity of lake bonneville enhanced precipitation over the western uintas during the last glacial maximum munroe j s and mickelson d m 2002 last glacial maximum equilibrium line altitudes and paleoclimate northern uinta mountains utah u s a journal of glaciology 48 257 266

the seismic stratigraphy of okanagan lake british columbia a record of rapid deglaciation in a deep fiord lake basin this paper presents the first detailed data regarding the newly discovered deep infill of okanagan lake okanagan lake 50 00 n 119 30 w is 120 km long 3 5 km wide and occupies a glacially overdeepened bedrock basin in the southern interior of british columbia this basin and other elongate lakes of the region e g shuswap kootenay kalamalka canim and mahood lakes mark the site of westward flowing ice streams within successive cordilleran ice sheets an air gun seismic survey of okanagan lake shows that the bedrock floor is nearly 650 m below sea level more than 2000 m below the rim of the surrounding plateau the maximum thickness of pleistocene sediment in okanagan lake basin approaches 800 m forty six seismic reflection traverses and an axial profile show a relatively simple stratigraphy composed of three seismic sequences argued to be no older than the last glacial cycle 30 ka a discontinuous basal unit sequence i characterized by large scale diffractions and up to 460 m thick infills the narrow v shaped bedrock floor of the basin and is interpreted as a boulder gravel deposited by subglacial meltwaters overlying seismic sequence ii is composed of two sub sequences sub sequence iia is a chaotic to massive facies up to 736 m thick lakeshore exposures close to where this unit reaches lake level show deformed and chaotically bedded glaciolacustrine silts containing gravel lens and large ice rafted boulders the surface topography of this sub sequence is irregular and in general mimics the form of the underlying bedrock as a result of compaction this sequence passes laterally into stratified facies sub sequence iib at the northern end of the basin seismic sequence ii appears to record rapid ice proximal dumping of glaciolacustrine silt as the okanagan glacier backwasted upvalley in a deep lake a thin 60 m max laminated seismic sequence iii drapes the hummocky surface of sequence ii and represents postglacial sedimentation from fan deltas the extreme thickness of sequences i and ii in okanagan lake reflects the focussing of large volumes of meltwater and sediment into the basin during deglaciation pre existing sediments that pre date the last glacial cycle appear to have been completely eroded glaciological conditions during sedimentation may have been similar to marine based outlet glaciers calving in deep water in fiord basins in contrast to marine settings where ice bergs are free to disperse large volumes of dead ice were trapped within the basin structural evidence for sedimentation around dead ice blocks has been previously used to argue that the cordilleran ice sheet downwasted in situ we emphasize in contrast the trapping of dead ice left behind by rapidly calving lake based outlet glaciers

ice sheet erosion patterns in valley systems in northern sweden investigated using cosmogenic nuclides erosion patterns associated with glaciation of trunk and hanging valley systems in northern sweden were investigated using cosmogenic nuclide 10be apparent exposure ages and inferred nuclide inheritance sequences of samples taken across valleys known to have been covered repeatedly by the fennoscandian ice sheet revealed two primary patterns of erosion in vavlavagge the exposure age pattern is consistent with 2 m of glacial erosion during the last glacial cycle along the entire profile at ravtasvaggi dievssavaggi and alisvaggi exposure ages in the valley bottom contrast with apparent exposure ages two to four times older on the valley sides the older ages on the valley sides reflect cosmogenic nuclide inheritance due to limited 2 m bedrock erosion of the valley sides during the last glacial cycle the pattern and scale of erosion in these valleys indicates that glacial valley formation is a result of multiple glacial cycles rather than the result of topographic modification during a single glacial cycle initial data comparing hanging valley and trunk valley sites do not show distinct differences in apparent exposure ages slightly older ages for samples from hanging valley bottoms may suggest nuclide inheritance indicating lower erosion than in trunk valley bottoms as would be expected given the marked topographic step between hanging and trunk valleys although quantifying the amount of erosion depends on the assumed cosmogenic nuclide inheritance prior to the onset of erosion the pattern of erosion is independent of this hence the observed pattern of cosmogenic nuclide concentrations provides constraints on spatial patterns of erosion and helps to refine understanding of the timing and extent of landform modification by glaciation copyright 2005 john wiley sons ltd

a reassessment of the role of ice sheet glaciation in the long term evolution of the east greenland fjord region abstract the long term evolution of the east greenland fjord region is investigated using geomorphological and stratigraphical evidence to 1 assess the nature of landscape modification caused by late cenozoic ice sheet glaciation and 2 relate patterns of glacial landscape modification to first order i e continent margin scale topography and geology geomorphological and stratigraphical evidence demonstrates evolution of the first order topography and incision of at least part of the present first order fjord system by 55 ma this hypothesis is tested using apatite u th he ages for samples from two bedrock profiles near kong oscar fjord the thermochronology supports landscape evolution before 55 ma followed by relative tectonic stability because it indicates rapid denudation around the time of rifting that occurred prior to continental breakup i e between 75 and 55 ma the nature of landscape modification caused by late cenozoic glacial erosion appears to have been controlled by first order topography and geology with selective ice sheet erosion in areas of high elevation caledonian basement and apparently little glacial erosion of low elevation mesozoic sedimentary strata nevertheless fjord morphometry demonstrates systematic evolution of the first order fjord system from confined and overdeepened fjords in caledonian basement to wider and disproportionately larger fjords in mesozoic strata the latter indicates that changes in lithological strength enabled the development of more efficient fjord morphology under full glacial conditions that may have promoted fast ice flow

early east antarctic ice sheet growth recorded in the landscape of the gamburtsev subglacial mountains the gamburtsev subglacial mountains are regarded as a key nucleation site for the antarctic ice sheet and they retain a unique long term record of pre glacial and early glacial landscape evolution here we use a range of morphometric analyses to constrain the nature of early glaciation and subsequent ice sheet evolution in the interior of east antarctica using a new digital elevation model of the gamburtsev subglacial mountains derived from an extensive airborne radar survey we find that an inherited fluvial landscape confirms the existence of the gamburtsev subglacial mountains prior to the onset of glaciation at the eocene oligocene climate boundary ca 34 ma features characteristic of glaciation at a range of scales are evident across the mountains high elevation alpine valley heads akin to cirques identified throughout the mountains are interpreted as evidence for early phases of glaciation in east antarctica the equilibrium line altitudes associated with these features combined with information from fossil plant assemblages suggest that they formed at or prior to 34 ma it cannot be ruled out that they may have been eroded by ephemeral ice between the late cretaceous and the eocene 100 34 ma hanging valleys overdeepenings truncated spurs and steep sided linear valley networks are indicative of a more widespread alpine glaciation in this region these features represent ice growth at or before 33 7 ma and provide a minimum estimate for the scale of the east antarctic ice sheet between ca 34 and 14 ma when dynamic fluctuations in ice extent are recorded at the coast of antarctica the implications are that the early east antarctic ice sheet grew rapidly and developed a cold based core that preserved the alpine landscape the patterns of landscape evolution identified provide the earliest evidence for the development of the east antarctic ice sheet and can be used to test coupled ice climate evolution models

dem analysis on longitudinal and transverse profiles of steep mountainous watersheds longitudinal and transverse characteristics of watersheds are fundamental to describe watershed topography only a limited number of studies however have analyzed the characteristics of transverse profiles in watersheds and almost no studies have examined longitudinal and transverse profiles at the same time how transverse profiles change within a watershed and how longitudinal and transverse characteristics of watersheds are related remain to be examined to address these issues this study employs dems gis and computer programming techniques to acquire a series of longitudinal and transverse profiles of watersheds basic morphometric parameters representing the shape of these profiles were analyzed methods to classify the profiles and to identify points where the morphometric characteristics change abruptly were also developed these methods were then applied to severely dissected steep watersheds in the japanese alps the results show that spatial change in transverse characteristics reflects two factors general topography of the watersheds and local differences in channel development the form of the transverse profiles is also affected by erosion processes that create mature v shaped valleys with a characteristic slope angle the longitudinal and transverse profiles play different roles in determining watershed topography and the overall longitudinal slope exerts a major influence on the mean watershed slope the discontinuity of the longitudinal and transverse profiles however tends to occur at the same location which may reflect the occurrence of knickzones and their influences on the development of valley side slopes

controls on spatial variations of glacial erosion in the qilian shan northeastern tibetan plateau abstract understanding interactions between topography and glacial erosion is a key to advancing knowledge of mountain landscape evolution and is often hampered by the lack of quantitative data for glacial erosion here we examine spatial variations in glacial erosion in the qilian shan using a power law model y axb fit to cross profiles of glacial valleys we further analyze the impacts of glacial dynamics topography and lithology on the degree of glacial erosion using qualitative and quantitative methods although the distribution of glacial erosion is controlled by the combined effects of glacial dynamics topography and lithology average glacier size or ice flux and mean slope above the equilibrium line altitude are dominant controls in addition the topographic data analyzed through a b fr diagram suggests two opposite types of glacial valley cross profile development temperate maritime glacier valleys are mainly dominated by overdeepening while continental glacier valleys are mainly dominated by a widening process although this varies with lithology

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

a discussion of hirano and aniya s 1988 1989 explanation of glacial valley cross profile development in a rational explanation of cross profile morphology for glacial valleys and of glacial valley development hirano and aniya 1988 present an explanation for the apparent catenary form of glaciated valley cross profiles based on the assumption that glacial erosion works towards minimizing friction between a glacier and its channel however as there is as yet no convincing justification for the application of such extremum principles to this problem it is not clear how useful this type of analysis might be in more detail hirano and aniya s analysis includes assumptions about patterns of basal water pressures and the controls on friction inconsistent with other work in this area furthermore as hirano and aniya 1989 point out the catenary solution they derive is the maximum rather than the minimum friction solution the underlying reason for this surprising result is that the variational principle hirano and aniya use is inappropriately structured for the problem being addressed as it does not involve a cross section area i e ice discharge constraint hirano and aniya also discuss the progressive development of glacial valley cross profiles but their analysis is hindered by the fact that most of the data they present are purely morphological and thus cannot be used to assess form development as a function of either time or extent of glacial erosion the only possible exceptions are graf s 1970 data which give morphological parameters as a function of valley order but these do not unequivocally support hirano and aniya s conclusions and are used with implicit assumptions about the relationship between valley order and time of glacial occupation or magnitude of glacial erosion that are incorrect or uncorroborated respectively however if we accept the common assumption that glacial erosion results in a progression from initial v shaped to broadly u shaped parabolic or catenary forms hirano and aniya s argument that there are two distinct types of form development incision in alpine situations versus valley widening under ice sheets is justified and is an important observation that needs to be addressed in future models of glacial valley development

climate controlled variations in scree production southern alps new zealand the interaction of fluvial glacial and hillslope processes controls the development of mountain belts and their response to tectonic and climatic forcing studies on the contribution of hillslope processes to mountain erosion have focused on bedrock landslides as they have a profound and readily observed impact on sediment yield and slope morphology despite the ubiquity of scree or talus mantled slopes in mountainous terrain the role of frequent low magnitude 100 m 3 rockfall events is seldom addressed in the context of landscape evolution here we quantify the contribution of rockfall erosion across an 80 by 40 km transect in the southern alps new zealand by analyzing the spatial extent of scree slopes mapped from aerial photographs and estimating long term 10 15 k y rockfall erosion rates from the accumulation of slope deposits below bedrock headwalls and in debris and alluvial fans along the rapidly uplifting high rainfall western margin where high rates of bedrock landsliding have been previously documented scree mantled slopes are sparse rainfall decreases exponentially east of the main divide and the proportion of slopes mantled by scree increases monotonically attaining a maximum value of 70 the systematic distribution of scree deposits cannot be attributed to lithologic variation seismicity or the legacy of glaciation instead climate may serve as a primary control on scree production as nearly 70 of the mapped scree deposits in our transect are confined to a narrow elevation range of 1200 1600 m above sea level masl our analysis of altitudinal controls on annual temperature variations indicates that scree production via frost cracking processes may be maximized between elevations of 1600 and 2300 masl as higher elevations are subject to persistent permafrost which obviates the frost cracking process rates of rockfall erosion near the rapidly uplifting main divide are low 0 1 mm yr whereas rates in the scree dominated eastern areas average 0 6 mm yr and may approximately balance rock uplift

the relative efficiency and influence of glacial and fluvial erosion on tibetan plateau landscapes abstract driven by climate change periods of dominating glacial and fluvial erosion have both sculpted mountain ranges during the quaternary to understand the evolution of mountain landforms there is considerable interest in knowing which erosion agent leads to more incision how glacial thermal regime impacts erosion and landscape development and whether glacial erosion can result in increased relief to investigate the relative efficiency of glacial and fluvial erosion and the influence of glacial thermal regime on topography we compare two different mountain ranges the qionglai shan monsoonal temperate glaciation and lenglongling subcontinental glaciation in western china we use a 1 d model to construct presumptive fluvial longitudinal profiles if glaciation had not occurred and compare these with the actual glacial profiles to obtain relative incision rates of fluvial and glacial erosion results demonstrate that glacial incision is smaller than fluvial incision in lenglongling and larger than fluvial incision in the qionglai shan which may be influenced by different tectonics and glacier types topographic analysis of glacial and fluvial valleys with similar drainage areas show that glacial erosion results in valley widening local valley relief increase induced by isostatic uplift and slope gradient increase in lenglongling however there is no local valley relief and slope increase observed in the qionglai shan which may reflect slope process responses to threshold hillslope limiting the height above valley bottoms

glacial valley cross profile development the influence of in situ rock stress and rock mass strength with examples from the southern alps new zealand abstract the evolution of the glacial valley cross profile form is commonly attributed primarily to the glaciological variables that control the erosion of the channel however studies in the new zealand southern alps suggest that the rock mass strength rms of the eroded rock mass is a major control on slope stability and hence on the final form of the trough rms of the slope rock will alter with time and erosional excavation and oversteepening of the valley slopes the in situ rock stress field induced by 1 the extreme topography in the axial ranges of the new zealand southern alps and 2 tectonically by collision of the australian and pacific crustal plates may play a role in valley slope development by controlling the location of rock failure and reducing rms this provides weakened rock that may provide sites for selective glacial erosion of the rock mass hence valley form evolution models should also take into account the rms and the in situ stress field acting in the eroded rock mass this study has implications for the development and modification of alpine glacial troughs in similar tectonic settings elsewhere

development of glacial valley cross sections under conditions of spatially variable resistance to erosion abstract a simulation model that examines the development of valley cross profiles as a result of glacial erosion has been constructed by placing a finite element model for ice flow within an iterative program that modifies channel cross section form on the basis of an erosion equation the simulation model has previously been used to examine how different flow laws erosion laws and ice discharge histories affect medium scale landform development this allows us to better understand the development of glacial landforms and also provides a critical test for erosion and flow laws within the erosion component of the simulation program it is possible to specify spatial variations in bedrock erodibility in order to examine the effect of spatially variable lithologies on form development as an initial test of form development under these conditions a central area of enhanced erodibility is placed in the valley center in this case active glacial channels develop that are narrower and deeper than those which occur with homogenous bedrock this change in form occurs as long as the weaker zone is small enough that the active channel does not eventually narrow to a stage such that it is entirely contained within the weaker material and as long as it is large enough that it continues to have a significant impact on overall discharge through the section form variation occurs because the weaker central zone enhances the lateral gradient of the erosion rate which is critical for form development a central area of more erodible bedrock changes not only the form of the active glacial channel but also the convexity of side slopes that are left above the glacier as it erodes downwards into the landscape the absence of a classic u shaped cross section should not be taken a priori to suggest either absence of glacial erosion or ineffective glacial erosion rather the combination of active glacial and hillslope processes in a landscape with spatially and temporally variable ice discharge and rock characteristics can create a diverse set of cross section forms

assessing glacial modification of bedrock valleys using a novel approach abstract analysis of valley cross sections is an established technique for evaluating the effects of glacial erosion here we present a semiautomated approach for assessing glacial modification of bedrock valleys by quantifying morphological variability in valley cross sections extracted in bulk from digital elevation models dems this method can be tailored to derive cross sections of user specified width and spacing and can generate cross sectional data sets of multiple orders of magnitude we demonstrate the technique by extracting 27 000 cross sections from the sierra nevada california and computing measures of valley morphology including a form ratio a quadratic curve fit and a power law curve fit for each cross section in addition we introduce a novel metric the v index which is calculated by comparing valley cross sectional area to that of an ideal v shaped cross section and is thus an expression of the deviation from an ideal v shaped form statistical comparisons of the different measures suggest that the v index is a robust alternative to traditional morphological measures because it can accommodate irregular valley cross sections and discriminate between glacial and nonglacial forms we document the distribution of valley forms throughout the sierra nevada using the v index to identify range scale patterns in cross sectional morphology and to investigate the role of lithology on glacial modification of bedrock valleys

an extended perfect plasticity method for estimating ice thickness along the flow line of mountain glaciers 1 direct measurement of the thickness of mountain glaciers is difficult over large areas yet knowledge of the thickness is essential for calculating their volumes and future evolution we develop a new method for estimating the ice thickness along glacier flow lines using the perfect plasticity rheological assumption that relates the thickness and surface slope to a yield stress previous studies have used this assumption with the shallow ice approximation to estimate the ice thickness but the standard approach neglects the effect of side drag on glacier stress balance our method addresses this shortcoming and extends the standard method by accounting for the side drag via the glacier width besides the assumed yield stress the inputs for our method are the outline and surface topography of the glacier surface velocity and mass balance data are unnecessary we validated the extended method on five glaciers in northwest china where thickness data are available from radio echo soundings finding that it can reproduce measured thicknesses with a mean absolute error of 11 8 like the standard method moreover for long glacier tongues confined to flow between parallel valley sides this method is found to give more accurate thickness estimates than does the standard method with a mean absolute error of as low as 5 3 sensitivity analysis shows that the estimated ice thickness depends strongly on yield stress and surface slope and less strongly on glacier width because this method is physically more realistic than the standard method and its inputs are easily derivable from remote sensing observations it has the potential to be used for processing large glacier data sets

a rational explanation of cross profile morphology for glacial valleys and of glacial valley development the fact that the cross profile of the glacial valley could be well approximated by parabolas y axb b 2 0 is explained by the variation principle assuming that the glacier erosion works towards minimizing thefriction between ice and bedrock the variation principle proves that the ideal or fully developed morphology of the glacial valley should be a catenary the curve which a chain hanging from two fixed points forms maclaurin s series expansion of the catenary equation shows that a parabola is a very good approximation of the catenary hence the good approximation of the cross profile by parabolas different catenaries are generated by changing the form ratio depth rim width and are then approximated by y axb by the method of last squares the b values obtained become only fractionally larger than 2 0 with invreasing form ratios of up to 1 0 indicating that b values would range in practice between 1 0 and about 2 0 two types of trend in the relationship between b values and the form ratio were obtained from several glaciers for one type the b value becomes larger with increasing form ratios and for the other the opposite the first type is called the rocky mountain model after its source of data and represents overdeepening of the glacial valley development the second type is caalled the patagonia antarctica model representing a widening instead of a deepening process of development these differences are attributed to the nature of the glaciers which produced these valleys i e alpine glaciers and continental ice sheets

quantified time scale for glacial valley cross profile evolution in alpine mountains space is used as a proxy for time in determining the role of glaciation in shaping valley cross profiles in the two thumb range southern alps new zealand the range is undergoing rapid tectonic transport and uplift as it is advected toward the alpine fault repeated cycles of glacial erosion during the quaternary have fashioned u shaped valleys in the north of the range close to the main divide while valleys in the less glaciated south of the range have rounded divides and convex fluvial cross profiles tectonic transport and uplift rates coupled with an offshore oxygen isotope 18o record and glacial geological reconstructions allow time constraints to be applied to valley development along the range valley width and depth measurements and power law equations were used to quantify the shape of cross profile transects in 37 valleys valleys evolved into a characteristic u shaped cross profile formed over 400 k y of glacial occupancy this regardless of other models of glacial flattening of valley long profiles suggests that alpine glaciation excavates greater volumes of rock indicating that glaciers are a more effective erosional agent than fluvial activity in alpine mountain belts

hawaiian magma reservoir processes as inferred from the petrology of gabbro xenoliths in basalt kahoolawe island gabbro xenoliths in a tholeiitic lava of kahoolawe island hawaii a 1 3 1 4 ma shield volcano are 1 3 cm in size and comprised of plagioclase clinopyroxene and orthopyroxene gabbro textures while intergranular and in part subophitic are open due to 28 48 vol of vesicular basalt occupying xenolith space vesicles in and around the xenoliths are lined or filled with rhyolitic glass segregation vesicles the host is evolved tholeiite mgo 6 1 wt with phenocrysts microphenocrysts and glomerocrysts of olivine clinopyroxene orthopyroxene and plagioclase and megacrysts 1 cm of plagioclase the sr isotope ratio of one xenolith is 0 70489 the host basalt ratio is 0 70460 xenolith isotope composition grain resorption and clinopyroxene fs12 5 15wo38 35 5 orthopyroxene fs19 5 24wo4 1 and plagioclase an68 65or0 8 1 2 compositions suggest that these gabbros crystallized from kahoolawe tholeiitic magma of essentially the same composition as the host basalt but pre dating the magma represented by the host based on the absence of intergranular fe ti oxide phases from the pl cpx opx assemblages and the open vuggy textures we envision crystallization on a reservoir roof at temperatures 1100 c entrainment of gabbro assemblages and plagioclase megacrysts from a roof mush suspension zone occurred during convection associated with replenishment of the magma reservoir these open textured gabbro xenoliths are therefore not fragments of preexisting coarse grained bodies such as sills or segregation veins rhyolitic glass in vesicles represents a gas effervescence filtration process that forced fractionated residual liquids from the groundmass into voids associated with the xenoliths

the shapes of some mountain peaks in the canadian rockies different mountain shapes in sedimentary sequences in the canadian rockies were enhanced by glacial erosion and have been modified postglacially by gravity driven slope processes slope modification by both glacial erosion and postglaciation landslides is related to rock structure particularly bedding dip rock mass strength and slope geometry five mountain peak shapes in monoclinal sequences each fall into different ranges of bedding dips castellate 1 and matterhorn 2 mountains occur in sub horizontal beds and their slopes on all sides follow combinations of bedding planes and joints the overall slopes are generally 37 to 65 and oblique to both bedding and joints slopes in sub horizontal beds may be controlled by their rock mass strength cuestas 3 develop in gently to moderately dipping beds dip slopes and steeper normal escarpments form their cataclinal and anaclinal sides respectively with the dihedral angle between them about 90 hogbacks 4 in moderately to steeply dipping beds have similar slope angles on both cataclinal and anaclinal slopes cataclinal slopes are either dip slopes or underdip slopes but anaclinal slopes are often steepened escarpments the dihedral angle between the slopes is usually less than 90 dogtooth 5 mountains occur in steeply dipping to sub vertical beds and the dihedral angle can be as low as 60 slope gradients in inclined beds are closely related to landslides whose modes are controlled by bedding dips copyright 1999 john wiley sons ltd

glacial and gully erosion on mars a terrestrial perspective the mid to high latitudes of mars host assemblages of landforms consistent with a receding glacial landscape on earth these landforms are postulated to have formed 5 ma under a different climate regime when mars orbital obliquity was on average 10 higher than today here we investigate the spatiotemporal relationship between gullies and glacial landforms both common in the mid latitudes gullies are kilometre scale landforms with a source alcove transportation channel and depositional apron the glacial landforms comprise 1 extant viscous flow features vff that extend from the base of crater walls into the interior of crater floors and are widely interpreted as debris covered glaciers containing extant ice and 2 landforms such as arcuate ridges at the base of crater walls that have been interpreted as relicts of more recent less extensive glacial advances focussed on crater walls we measure headwall retreat associated with glacial landforms and date their host craters to constrain minimum headwall retreat rates we record headwall retreat rates up to 102 m my 1 for the youngest suite of glacial landforms equivalent to erosion rates of wet based glaciers on earth and to headwall retreat rates associated with martian bedrock gully systems we find extensive evidence for a single erosional episode dating 5 10 ma which postdates emplacement of the majority of vff but seems to predate formation of the gullies we propose that the wet based glacial episode was associated with glaciation focussed on the crater walls rather than melting of the glacial ice deposits on the crater floors vff this is consistent with our observations of crater wall morphologies including the presence of arcuate ridges consistent with terrestrial glaciotectonic features that require liquid water to form textural alteration of the eroded bedrock surface consistent with ice segregation and frost shattering and the presence of downslope pasted on terrain tentatively interpreted here as glacial till deposits sourced from glacial erosion of the crater wall the pasted on terrain is usually interpreted as a thicker latitude dependant mantle located on sloping terrain formed from airfall of ice nucleated on dust but we suggest that it has been reworked by glaciation and is predominantly glacial in origin although our results cannot substantiate that gullies are produced by meltwater the discovery of this wet glacial event does provide evidence for widespread meltwater generation in mars recent history

comparison of late pleistocene and modern glacier extents in central nepal based on digital elevation data and satellite imagery abstract late pleistocene and modern ice extents in central nepal are compared to estimate equilibrium line altitude ela depressions new techniques are used for determining the former extent of glaciers based on quantitative objective geomorphic analyses of a 90 m resolution digital elevation model dem for every link of the drainage network valley form is classified as glacial or fluvial based on cross valley shape and slope statistics down valley transitions from glacial to fluvial form indicate the former limits of glaciation in each valley landsat multispectral scanner imagery for the same region is used to map current glacier extents for both full glacial and modern cases elas are computed from the glacier limits using the dem and a toe to headwall altitude ratio of 0 5 computed ela depressions range from 100 900 m with a modal value of 650 m and a mean of 500 m values consistent with previously published estimates for the central himalaya but markedly smaller than estimates for many other regions we suggest that this reflects reduced precipitation rather than a small temperature depression consistent with other evidence for a weaker monsoon under full glacial conditions

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

on the mathematical description of glaciated valley cross sections morphometric analyses of glaciated valleys typically involve attempts to fit empirical data from valley cross profiles to quadratic parabolic or power law equations with the choice of equation depending on the goal of the analysis in assessing the relative merits of these two types of equations some confusion has arisen because of apparent variations in coordinate system datums between studies which have made use of the power law equation however we show that this confusion relates to a simple mathematical error rather than any true difference in methodology of more significance however is the fact that in fitting a power law equation to observed profiles significant bias is introduced by the use of a logarithmic data transformation because of this implicit bias power law equation parameters are influenced most strongly by data points close in elevation to the centre of the valley and so failure to account for the depositional elements which frequently occur in these areas can lead to significant errors if the aim of the analysis is to characterize the form of the erosional profile

high resolution imaging science experiment hirise observations of glacial and periglacial morphologies in the circum argyre planitia highlands mars 1 the landscape of the argyre planitia and adjoining charitum and nereidum montes in the southern hemisphere of mars has been heavily modified since formation of the argyre impact basin this study examines morphologies in the argyre region revealed in images acquired by the high resolution imaging science experiment hirise camera and discusses the implications for glacial and periglacial processes distinctive features such as large grooves semicircular embayments in high topography and streamlined hills are interpreted as glacially eroded grooves cirques and whalebacks or roche moutonnee respectively large boulders scattered across the floor of a valley may be ground moraine deposited by ice ablation glacial interpretations are supported by the association of these features with other landforms typical of glaciated landscapes such as broad valleys with parabolic cross sections and stepped longitudinal profiles lobate debris aprons interpreted as remnant debris covered glaciers or rock glaciers and possible hanging valleys aligned boulders observed on slopes may also indicate glacial processes such as fluting alternatively boulders aligned on slopes and organized in clumps and polygonal patterns on flatter surfaces may indicate periglacial processes perhaps postglaciation that form patterned ground at least portions of the argyre region appear to have been modified by processes of ice accumulation glacial flow erosion sediment deposition ice stagnation and ablation and perhaps subsequent periglacial processes the type of bedrock erosion apparent in images suggests that glaciers were at times wet based the number of superposed craters is consistent with geologically recent glacial activity but may be due to subsequent modification