investigations regarding alpine talus slopes using ground penetrating radar gpr in the bavarian alps germany the applicability of ground penetrating radar gpr for the investigation of loose debris was tested at two sites viererkar and zugspitzplatt a pulseekko 100 gpr system equipped with 25 mhz antennae was utilized the aim of the investigation was to record the base of the debris layer and thereby acquire an estimation of the backweathering rates of the adjacent rockwalls the study areas are situated in the northern alps near the german austrian border the sites are characterized by steep limestone rockwalls and extensive talus accumulations a total of six profiles was surveyed the method is suitable and effective for a quick survey in this dry high ohmic substrate the gpr system was able to deliver information about the subsurface stratigraphy to c 70 m depth the boundary line to the bedrock was discovered depending upon the profile surveyed 5 to 25 m below the surface the base of the debris material sometimes shows no distinct reflection buried features v shaped furrows zones overdeepened by ice action geological structures could be detected arched structures well below the talus bedrock interface can be interpreted as drainage systems in the karstic bedrock a thick scree layer of late glacial age was separated from a thinner layer on the talus surface which was related to the holocene the backweathering rates were fixed by a calculation of talus volume to c 100 mm 103 a during the holocene viererkar and 150 300 mm 103 a zugspitzplatt the detrital formation in north exposed sites is twice as intense as in south exposed sites these results match the rates of recent rockfall in the same area of investigation the calculated backweathering for the late glacial period is 150 730 mm 103 a the magnitude of the calculated rockwall retreat lies well within the range of previous measurements the discrepancy between some weathering rates highlights the fact that recent and past relief formation must be differentiated otherwise recent removal rates may be overestimated copyright 2001 john wiley sons ltd

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

economic potential of the ultramafic rocks of jamaica and tobago two contrasting geological settings in the caribbean ultramafic rocks in jamaica are dunites with minor lherzolite often serpentinised and are part of a dismembered ophiolite complex in tobago dunites wehrlites pyroxenites and hornblendites form the lower part of a plutonic complex of island arc affinity the mineral assemblages and chemistry reflect these differences chromite in jamaica is high in al and mg whereas in tobago it is rich in fe as in alaskan type intrusives ni cu pge assemblages in jamaica are pentlandite with later low temperature heazlewoodite awaruite and native copper the latter with pt and pd in tobago an assemblage of pentlandite pyrrhotite pyrite and chalcopyrite is much less affected by later alteration pge phases also occur the dunites in jamaica have sufficient mgo to be a potential source of olivine the higher fe in olivine from tobago indicates that olivine cumulates in plutonics from island arc settings are a less suitable source of the mineral ni laterites in jamaica are unlikely because of high topographic relief the prospect for ni laterites in tobago is low as there is little ni in the olivines chrysotile asbestos talc and magnesite are absent in both islands this is probably a consequence of the lack of secondary serpentine recrystallisation to form fibrous chrysotile veins the deep tectonic level and lack of hydrothermal circulation for magnesite to form and the absence of metamorphic metasomatic events and or late stage extension tectonics which might have yielded talc

structural and thermal controls of rockfall frequency and magnitude within rockwall talus systems swiss alps both from a systemic and natural hazard perspective it is essential to understand the causes and frequency of rockfalls in mountain terrain and to predict the block sizes deposited at specific locations commonly rockfalls are studied either retrospectively using talus slopes or directly by rockwall surveys nevertheless our understanding of rockfall activity particularly at the lower magnitude spectrum is still incomplete moreover the explanatory framework is rarely addressed explicitly in this study we investigate two rockwall talus systems in the swiss alps to estimate the rockfall frequency magnitude pattern and their key controls we present a holistic approach that integrates deductive geotechnical and thermal investigations of the source rockwalls with abductive talus based explanations of rockfall volume and frequency the rockwalls three dimensional 3d joint pattern indicates that 75 of the blocks may be released as debris fall 14 m3 and boulder falls 14 61 m3 which is mirrored in the corresponding talus material using two year records of near surface rockwall temperatures as input for a 1d heat conduction model underlines the destabilizing role of seasonal ice segregation deepest frost cracking of 300 cm may occur on the north northeast nne exposed snow rich rockwall with peaks at the outermost surface the synthesis of all data suggests that infrequent large planar slides approximately every 250 years overlain by smaller more frequent wedge and toppling failures approximately every 17 50 years as well as high frequency flake like clasts 3 6 events year characterize the rockfall frequency magnitude pattern at hungerli peak here we argue that small size rockfalls need more scientific attention particularly in discontinuous permafrost zones our study emphasizes that future frequency magnitude research should ideally incorporate site specific structural and thermal properties rather than just focusing on climatic or meteorological triggers we discuss how holistic rockwall talus approaches as proposed here could help to increase our process understanding of rockfalls in mountain environments copyright 2017 john wiley sons ltd

timing and significance of talus slope reworking trotternish skye northwest scotland exposures through the upper rectilinear or near rectilinear segment of vegetated relict talus slopes at the foot of the trotternish escarpment northern skye exhibit stacked debris flow diamictons intercalated with surface wash horizons and occasional buried organic palaeosols thus indicating a long history of reworking at this site radiocarbon dates for the tops of buried palaeosols indicate that reworking of sediment locally predates 5 9 5 6 cal ka bp and has been intermittently active throughout the late holocene broad coincidences of timing are evident for the onset of reworking of different sectors of the talus slope at c 5 9 5 6 cal ka bp c 2 3 1 7 cal ka bp and c 0 7 0 5 cal ka bp that may represent more widespread reworking analysis of subfossil palynomorphs within some buried palaeosols suggests that most reworking postdates the main period of local anthropogenic woodland clearance dated to the mid holocene buried palaeosols examined contain no evidence of local burning of veg

rock slope instability and erosion toward improved process understanding rock slopes in a range of environments are among the landscape elements most sensitive to climate change the latter affecting rock mass properties altering slope boundary conditions and changing geosystem configurations major climate dependent influences promoting destabilization include stress redistribution with changing glacial ice extents degradation of mountain permafrost altered slope hydrology and weathering environments loading and unloading due to deposition and erosion and changes in the spectrum of magnitude and frequency of driving forces in steep bedrock terrain erosional processes control slope morphology by modulating rates of i weathering in response to climate and pre disposition ii rock slope retreat in response to magnitude and frequency of detachment and iii channel incision or valley infilling in response to variable sediment supply modelling landscape evolution and anticipating natural hazards in these environments thus requires deeper insights into the processes driving rock slope instability and erosion this special issue emphasizes new understanding of rock slope processes through a collection of manuscripts at the forefront of research in the field copyright 2014 john wiley sons ltd

late pleistocene glaciations in the northwestern sierra nevada california pleistocene fluvial landforms and riparian ecosystems in central california responded to climate changes in the sierra nevada yet the glacial history of the western sierra remains largely unknown three glacial stages in the northwestern sierra nevada are documented by field mapping and cosmogenic radionuclide surface exposure crse ages two crse ages of erratic boulders on an isolated till above bear valley provide a limiting minimum age of 76 400 3800 10be yr another boulder age provides a limiting minimum age of 48 800 3200 10be yr for a broad crested moraine ridge within bear valley three crse ages producing an average age of 18 600 1180 yr were drawn from two boulders near a sharp crested bouldery lateral moraine that represents an extensive tioga glaciation in bear valley nine crse ages from striated bedrock along a steep valley transect average 14 100 1500 yr and suggest rapid late glacial ice retreat from lower fordyce canyon with no subsequent extensive glaciations these ages are generally consistent with glacial and pluvial records in east central california and nevada

chronology for fluctuations in late pleistocene sierra nevada glaciers and lakes mountain glaciers because of their small size are usually close to equilibrium with the local climate and thus should provide a test of whether temperature oscillations in greenland late in the last glacial period are part of global scale climate variability or are restricted to the north atlantic region correlation of cosmogenic chlorine 36 dates on sierra nevada moraines with a continuous radiocarbon dated sediment record from nearby owens lake shows that sierra nevada glacial advances were associated with heinrich events 5 3 2 and 1

origin of sheet structure 1 morphology and boundary conditions abstract sheet structure or large scale exfoliation is the division of a rock mass into lenses plates or sheets approximately parallel to the earth s surface sheet fractures which separate the plates are characterized by surface markings resembling those formed during the brittle fracture of metals glass and ceramics and those on joints in rock lineations or hackle marks parallel to the direction of fracture propagation are common branching occurs during propagation so that the fractures appear as many echelon segments in profile the opposing surfaces of sheet fractures observed in quarry walls and natural exposures are typically in contact damage if any to rock adjacent to sheet fractures is generally limited to a zone less than one centimeter wide sheets tend to parallel preferred orientations of microcracks in many rock masses most sheets that are exposed today are of prehistoric origin but in some rock masses such as chelmsford granite in massachusetts new sheets continue to form sheet structure forms in environments of high differential stress dominated by large scale compression parallel to an exposed rock surface in parts of the chelmsford granite the magnitude of this compression is greater than 30 mpa in the zone of sheeting high differential stresses near the ground surface can result from several natural agents including contemporary tectonic forces vertical unloading of a rock mass that formed at depth under high triaxial compression and suppression of expansion that would otherwise result from temperature increases or chemical alteration of the rock exfoliation of boulders during forest fires and spheroidal weathering of boulders appears to occur under similar states of stress although the agents responsible for the stresses differ

pacing the post last glacial maximum demise of the animas valley glacier and the san juan mountain ice cap colorado during the last glacial maximum lgm a 5000 km 2 ice cap covered the san juan mountains of southwest colorado the largest valley glacier draining this ice cap occupied the animas valley and flowed 91 km to the south to characterize the post lgm demise of the animas valley glacier we employ cosmogenic 10 be to date the lgm terrace outside the terminal moraines and a suite of seven glacially polished bedrock samples the 10 be depth profile within the terrace sediments suggests abandonment at 19 4 1 5 ka as deglaciation began the ponding of glacial lake durango behind the terminal moraines shut off fluvial sediment supply and caused terrace abandonment the age of the terrace therefore records the initiation of lgm retreat negligible 10 be inheritance in the terrace profile suggests that glacial erosion of the bedrock valley floor from which sediments were derived erased all cosmogenic inventory glacial polish exposure ages monotonically decrease up valley from 17 1 to 12 3 ka with the single exception of a sample collected from a quartzite rib yielding an average retreat rate of 15 4 m yr this trend and the lack of inherited cosmogenic nuclides in the terrace sediments imply that polish ages accurately record the glacial retreat history retreat of the animas lobe began at a time of regional drying recorded in sediments and shoreline elevations of large lakes deglaciation lasted for 7 2 k y and was complete by 12 3 1 0 ka the retreat history followed the pattern of increasing insolation and was perhaps fastest during a time of regional drying

the influence of alpine glaciation on the relief of tectonically active mountain belts a surface processes model incorporating large scale fluvial processes local hill slope processes and glacial erosion is used to investigate the effect of glaciation on the relief of a fast growing narrow orogen the results of several simulations show that glaciers concentrate erosion near peaks fluvial erosion is reduced downstream of glaciers glaciers with frozen bases can lead to an increase in relief and the net affect of glaciation in the absence of widespread basal freezing is to reduce relief

quantification of soil production and downslope creep rates from cosmogenic 10be accumulations on a hillslope profile average soil transport rates over a period of 3500 yr on a convex soil mantled hillslope have been quantified using a mass balance model that incorporates the soil concentration of the cosmogenic isotope 10 be the 10 be model results support the assumption used in most geomorphic models that the soil creep rate is proportional to surface gradient the predicted diffusion coefficient is 360 55 cm 3 yr 1 cm 1 contour length and the average rate of soil production is 0 026 0 007 cm yr within the uncertainty of this technique the data do not reject g k gilbert9s hypothesis that some hillslopes may exist in a condition of dynamic equilibrium with a uniform soil production rate however the model does not require an assumption of dynamic equilibrium and may be an approach that uniquely allows the quantification of a local soil production rate law

influence of rock mass strength on the erosion rate of alpine cliffs collapse of cliff faces by rockfall is a primary mode of bedrock erosion in alpine environments and exerts a first order control on the morphologic development of these landscapes in this work we investigate the influence of rock mass strength on the retreat rate of alpine cliffs to quantify rockwall competence we employed the slope mass rating smr geomechanical strength index a metric that combines numerous factors contributing to the strength of a rock mass the magnitude of cliff retreat was calculated by estimating the volume of talus at the toe of each rockwall and projecting that material back on to the cliff face while accounting for the loss of production area as talus buries the base of the wall selecting sites within basins swept clean by advancing last glacial maximum lgm glaciers allowed us to estimate the time period over which talus accumulation occurred i e the production time dividing the magnitude of normal cliff retreat by the production time we calculated recession rates for each site our study area included a portion of the sierra nevada between yosemite national park and lake tahoe rockwall recession rates determined for 40 alpine cliffs in this region range from 0 02 to 1 22 mm year with an average value of 0 28 mm year we found good correlation between rockwall recession rate and smr which is best characterized by an exponential decrease in erosion rate with increasing rock mass strength analysis of the individual components of the smr reveals that joint orientation with respect to the cliff face is the most important parameter affecting the rockwall erosion rate the complete smr score however best synthesizes the lithologic variables that contribute to the strength and erodibility of these rock slopes our data reveal no strong independent correlations between rockwall retreat rate and topographic attributes such as elevation aspect or slope angle copyright 2009 john wiley sons ltd

the value of rock mass classification systems for weak rock masses a case example from huntly new zealand three published rock mass classification systems rmr smr rms were applied to weak waikato coal measure mudrocks in order to assess their value as indicators of rock mass conditions and stable slope angles the smr classification gives the most sensitive measure of rock mass conditions for the slopes studied but none of the systems adequately predicts observed slope angles regression analysis indicates that where conditions for failure by sliding along discontinuities exist the slopes are most sensitive to the discontinuity parameters of parallelism dip angle and spacing further when the discontinuities are oriented favourably with respect to the slope slope angles are most sensitive to intact rock strength and groundwater this is supported by geomorphic evidence low angle natural slopes developed on unfavourable discontinuity sets are well predicted by published equations associated with both the rmr and rms classification systems steeper slopes developed on favourable discontinuities are at a much lower angle than the equations predict and show evidence of extensive creep application of the present rock mass classification systems to these weak rocks is thus only appropriate when conditions exist under which the rocks fail by sliding on unfavourably oriented discontinuities where this does not occur the contribution of intact strength to the rock mass strength is greatly overestimated by all of the rock mass classification systems studied development of a separate rock mass classification for these conditions is not seen as appropriate hence recognition of the favourability or otherwise of the discontinuity sets is crucial to dealing with these rocks a stereonet overlay that facilitates this division is presented

the assessment of rockfall hazard at the base of talus slopes fragmental rockfall is characterized by the independent movement of individual rock fragments after detachment from a rock face the continued operation of the process leads to the accumulation of talus slopes on talus slopes the rockfall shadow extends beyond the base of the talus and consists of scattered boulders that have run out beyond the base of the slope the landing probability of boulders in the shadow is examined return periods of the order of 1000 years relative to a house site are typical rockfall behaviour particularly with respect to run out into the shadow can be assessed using geological evidence empirical methods physical modelling and computer based analytical models an empirical minimum shadow angle of 27 5 i e the angle between the distal limit of the shadow and the top of the talus slope is suggested and would be useful in rockfall vulnerability studies at the base of talus slopes as a first approximation to shadow limits it is preferable to the use of the rockfall fahrb

tectonics fracturing of rock and erosion 1 we argue that by fracturing rock not by raising it relative to base level tectonics plays its most important role in causing rapid incision of valleys and rapid erosion of hillslopes tectonic deformation riddles the upper crust with fractures which not only provide avenues for water flow and thus promote weathering and further disintegration of rock but also fragment bedrock into debris that is readily extracted and transported by surface processes bends in active faults require straining of adjacent rock masses aftershocks that occur subsequent to slip on primary faults reflect penetrative brittle deformation of the upper crust at least some aftershocks must nucleate or lengthen cracks which contribute to the comminution of these rock masses scaling rules suggest that dimensions of ruptures for very small m 2 earthquakes can be meters or less the gutenberg richter recurrence relationship implies that such earthquakes are common as high magnification seismographs in low noise environments confirm moreover large differences among fault plane solutions for aftershocks show that the small faults on which they occur are not parallel to one another some faults must intersect thus the upper crust in tectonically active regions should be fragmented into blocks down to the scale of boulders or smaller dismembered rock arrives at the earth s surface already prepared to be transported away as a corollary both deeply exhumed lower crust and posttectonic igneous rock never deformed under brittle conditions and not deformed recently should be less susceptible to detachment and subsequent transport than fractured rock

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

holocene rockwall retreat in svalbard a triple rate evolution volumetric calculations of slope deposits direct measurements of rockwall retreat and chronological control based on lichenometry provide a wide range of rockwall retreat rates in svalbard 0 1580 mm ka 1 that appears consistent with previous evaluations from other arctic areas in northwest and central spitsbergen 79 n a triple rate rockwall retreat is suggested for the last two millennia very slow biogenic flaking 2 mm ka 1 moderate retreat due to frost shattering 100 ka 1 and rapid retreat associated with post glacial stress relaxation c 1000 mm ka 1 examination of the distribution of various processes indicates that the holocene retreat of most rockwalls has not exceeded one or two metres bedrock conditions appear to be the main control on retreat rates the massiveness of igneous and metamorphic outcrops widespread in arctic shield areas largely accounts for the slowness of rockwall retreat which on these lithologies is primarily due to chemical and biological processes more rapid rates are usually associated with stress relaxation following glacial surges or with local frost susceptibility of bedrock often where faulting has induced high joint density at such sites rockwall retreat rates are of the same order of magnitude as those reported from alpine areas 1000 3000 mm ka 1 where both bedrock weakening due to tectonic stresses and the greater height of steep rockwalls account for the more rapid rockwall retreat rate 1997 by john wiley sons ltd

direct observation of frost wedging in alpine bedrock width and temperature of rock joints were automatically monitored in the japanese alps three years of monitoring on a sandstone rock face shows two seasonal peaks of joint widening in autumn and spring the autumn events are associated with short term freeze thaw cycles and the magnitude of widening reflects the freezing intensity and water availability the short term freezing can produce wedging to a depth of at least 20 cm the spring events follow a rise in the rock surface temperature to 0 c beneath the seasonal snowcover and likely originate from refreezing of meltwater entering the joint some of these events contribute to permanent enlargement of the joint two other joints on nearby rock faces experience only sporadic widening accompanying freeze thaw cycles and insignificant permanent enlargement observations indicate that no single thermal criterion can explain frost weathering the temperature range at which wedging occurs varies with the bedrock conditions water availability and duration of freezing copyright 2001 john wiley sons ltd

bedrock fracture density controls on hillslope erodibility in steep rocky landscapes with patchy soil cover southern california usa abstract although many steep landscapes comprise a patchwork of soil mantled and bare bedrock hillslopes models typically assume hillslopes are entirely soil mantled or bare bedrock making it challenging to predict how rock properties influence hillslope erodibility and landscape evolution here we study headwater catchments across the san gabriel mountains sgm and northern san jacinto mountains nsjm in southern california two steep landscapes with similar climate and lithology but with distinctly different bedrock fracture densities 5 higher in the sgm we combine new and published detrital in situ cosmogenic 10be derived erosion rates with analysis of high resolution imagery and topography to quantify how the morphology and abundance of bare bedrock and soil mantled hillslopes vary with erosion rate within and between the two landscapes for similar mean hillslope angles 35 46 catchments in the nsjm erode at rates of 0 1 0 6 m kyr 1 compared to 0 2 2 2 m kyr 1 in the sgm in both landscapes bare bedrock hillslopes increase in abundance with increasing erosion rate however more and steeper bedrock is exposed in the nsjm indicating that wider bedrock fracture spacing reduces soil production efficiency and supports steeper cliffs additionally higher erosion rates in the sgm require a 3 higher soil transport efficiency reflecting an indirect control of bedrock fracture density on the size of sediment armoring hillslopes our data highlight how hillslope morphodynamics in steep landscapes depend on the strength of soil and bedrock and the efficiency of soil production and transport all of which are variably sensitive to rock properties and influence the partitioning of soil and bare bedrock on hillslopes

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

a theoretical model of the fracture of rock during freezing we present a mathematical model for the breakdown of porous rock by the growth of ice within cracks the model is founded upon well established principles of fracture mechanics and recent advances in soil physics ijong with the assumption that progressive crack growth results from water migrating to ice bodies in cracks much as water migrates to ice lenses in freezing soil our model predicts crack growth rates compatible with empirical data calculations for a granite and a marble indicate that sustained freezing is most effective in producing crack growth when temperatures range from 4 c to 15 c at higher temperatures thermodynamic limitations prevent ice pressure from building up sufficiently to produce significant crack growth at lower temperatures the migration of water necessary for sustaining crack growth is strongly inhibited in hydraulically open systems in which pore water pressure remains near atmospheric pressure during the freezing process crackgrowth rates during continuous cooling will generally be greatest at low rates of cooling less than 0 1 0 5 c h at higher rates of cooling the influx of water to growing cracks is significantly inhibited the model delineates clearly the role of material parameters elastic moduli fracture mechanical properties grain size and shape and crack size environmental conditions temperature temperature gradient water pressure and time in frost damage to rocks our calculations along with recent experimental work on water migration in freezing rocks fukuda 1983 lead us to question the widely accepted importance of two phenomena freezing of water in sealed cracks and freeze thaw cycling in the fracture of rock exposed to natural freezing conditions

rockfall triggering by cyclic thermal stressing of exfoliation fractures exfoliation of rock deteriorates cliffs through the formation and subsequent opening of fractures which in turn can lead to potentially hazardous rockfalls although a number of mechanisms are known to trigger rockfalls many rockfalls occur during periods when likely triggers such as precipitation seismic activity and freezing conditions are absent it has been suggested that these enigmatic rockfalls may occur due to solar heating of rock surfaces which can cause outward expansion here we use data from 3 5 years of field monitoring of an exfoliating granite cliff in yosemite national park in california usa to assess the magnitude and temporal pattern of thermally induced rock deformation from a thermodynamic analysis we find that daily seasonal and annual temperature variations are sufficient to drive cyclic and cumulative opening of fractures application of fracture theory suggests that these changes can lead to further fracture propagation and the consequent detachment of rock our data indicate that the warmest times of the day and year are particularly conducive to triggering rockfalls and that cyclic thermal forcing may enhance the efficacy of other more typical rockfall triggers some rockfalls occur without obvious triggers such as seismicity or freeze thaw conditions temperature and deformation patterns on a granite cliff suggest that cyclical thermal forcing can progressively open fractures and trigger rockfalls

rock fall dynamics and deposition an integrated analysis of the 2009 ahwiyah point rock fall yosemite national park usa we analyzed a combination of airborne and terrestrial lidar high resolution photography seismic and acoustic data in order to gain insights into the initiation dynamics and talus deposition of a complex rock fall a large 46 700 m3 rock fall originated from near ahwiyah point in eastern yosemite valley and fell a total of 730 m to the valley floor on 28 march 2009 analyses of remote sensing seismic and acoustic data were integrated to reconstruct the rock fall which consisted of 1 the triggering of a 25 400 m3 rock block in an area of intersecting and sometimes highly weathered joint planes 2 the sliding and subsequent ballistic trajectory of the block from a steeply dipping ledge 3 dislodging of additional rock from the cliff surface from beneath the rock fall source area 4 a mid cliff ledge impact that detached a volume of rock nearly equivalent in volume to the initial block 5 sliding of the deteriorating rock mass down the remainder of the cliff and 6 final impact at the base of the cliff that remobilized the existing talus downward and outward and produced an airblast that knocked down hundreds of trees the depositional geomorphology indicates that the porosity of the fresh talus is significantly lower than that expected for typical blocky talus slopes likely because the rock debris from this event was pulverized into smaller more poorly sorted fragments and densified via dynamic compaction when compared to less energetic fragmental type rock falls these results suggest that accumulation of individual rock fall boulders tends to steepen talus slopes whereas large energetic rock falls tend to flatten them detachment and impact signals were recorded by seismic and acoustic instruments and highlight the potential use of this type of instrumentation for generalized rock fall monitoring while lidar and photography data were able to quantify the cliff geometry rock fall volume source and impact locations and geomorphological changes to the cliff and talus published in 2012 this article is a us government work and is in the public domain in the usa

quantifying 40 years of rockfall activity in yosemite valley with historical structure from motion photogrammetry and terrestrial laser scanning abstract rockfalls and rockslides are often dominant geomorphic processes in steep bedrock landscapes but documenting their occurrence can be challenging requiring frequent monitoring and well resolved spatial data repeat application of remote sensing methods such as terrestrial laser scanning tls and structure from motion sfm photogrammetry can detect even very small rockfalls but typically these acquisitions span only years and may not record rockfall activity representative of longer term rates of cliff erosion inventory databases can extend rockfall records but are commonly incomplete and prone to observation bias we employed tls and sfm on two adjacent cliffs el capitan and middle brother in yosemite valley integrating semi annual data collections from 2010 to 2017 with historical archival sfm models derived from oblique photographs taken in 1976 comparing the 1976 sfm models against more recent data allows for more accurate and precise rockfall detection and volume measurement over a 40 year period change detection indicates that 235 rockfalls occurred from the two cliffs more than twice as many events as are recorded in yosemite s inventory database although individual rockfall volumes reported in the inventory database vary from those measured by sfm tls reported cumulative volumes are similar to measured volumes likely because the large volume events that account for most of the cumulative volume tend to be widely observed and well documented volume frequency relationships indicate that the cliffs erode predominantly by less frequent larger volume rockfalls at rates of 0 9 to 1 7 mm yr our study demonstrates how integrated sfm and tls measurements especially utilizing sfm models derived from historical imagery allow detection and quantification of rockfalls spanning several decades complementing and improving inventory databases informing rockfall hazard assessment and providing longer term rates of cliff erosion

controls on the stability and inclinations of hillslopes formed on hard rock in identifying controls on rock slope form a distinction is made between 1 rock slopes with joints which dip steeply out of a cliff and hence are subject to mass failure of the rock mass above a critical joint and 2 rock slopes with inclinations which are either in equilibrium with the mass strength of their rocks or have profiles which will develop towards strength equilibrium as cross joints open in the first class of slope stability results not just from the basic frictional resistance of the rock but also from the frictional roughness along the critical joint and from the normal stress acting across that joint stability may be reduced by weathering and loss of strength of the joint wall rock as a result of normal stress variations with depth induced by overburdens high cliffs which are not undercut have a concave profile the second group of slopes includes those with inclinations controlled at the scale of individual joint blocks buttressed slopes and those on unjointed rock masses buttressed and unjointed rock masses develop towards a condition of mass strength equilibrium as cross joints open strength equilibrium slopes may be recognized by application of a rock mass strength classification proposed for geomorphic purposes eleven propositions are formulated which identify controls on rock slope development and some consequences of these controls

a numerical model of landform development by glacial erosion the jagged peaks knife edged ridges and steep sided valleys of alpine landscapes are well known products of glaciation yet little is known about how the dynamics of ice flow and glacial erosion give rise to such landforms by linking a finite element model for ice flow through a valley with a model for glacial erosion we have simulated the gradual evolution of one of the most striking of glacial landforms the u shaped valley incorporating realistic glacier sliding laws and spatial variations in rock resistance our simulations illustrate how ice flow erosion patterns and topography interact in the long term evolution of glacial landforms in addition our results indicate for the first time that the formation of a steady state u shaped valley takes approximately 105 years

rockfall activity from an alpine cliff during thawing periods abstract rockfall activity during thawing periods was investigated by collecting rockfall debris fallen on the snow covered talus slope in a cirque in the japanese alps near surface rock temperature was also monitored on the cirque wall maximum rockfall activity occurs on average about 10 days after the meltout of the cirque wall the intensive activity is rarely associated with precipitation events or diurnal frost cycles on the rockface a thermal conduction model suggests that rockfalls at this site occur most frequently when the seasonal thawing front penetrates to a depth of about 1 m in the bedrock while the freeze thaw penetration controls the maximum dimensions of detachable rock mass joint spacing on the rockface affects the size distribution of rockfall debris boulder falls resulting from seasonal frost weathering are considered to be the most important process responsible for the contemporary modification of the cirque wall the rockfall volume during a thawing period is usually 1 3 m3 which is equivalent to the rate of cirque wall retreat of the order of 0 01 mm year 1 however big boulder falls that occur once per decade can increase the retreat rate significantly

influence of rock strength properties on escarpment retreat across passive continental margins we examine the influence of bedrock strength properties imparted by jointing and fracturing on the retreat of erosional escarpments across passive continental margins two types of escarpment are identified and focus is placed on the gorge head knickpoint form of escarpment where upland streams drain across the escarpment along the macleay river system of eastern australia major knickpoints associated with escarpment retreat are currently located about 200 km upstream from the river mouth despite differences in bedrock lithology and order of magnitude differences in drainage areas upstream of the gorge heads implying an average escarpment retreat rate of 2 km m y since the margin formed 85 m y ago field observations from apsley river gorge a macleay river tributary indicate that bedrock jointing profoundly affects the evolution of the gorge headwall and sidewalls where dominant jointing dips into hillslopes failure occurs by block and column toppling conversely where jointing dips out of hillslopes mass wasting occurs by joint parallel rock sliding allowing slopes to remain steep as material is shed from the gorge sidewalls the toppling slopes develop lower overall gradients the gorge becomes asymmetric in cross section and the channel remains nearer to the steeper sidewall affected by joint parallel sliding coupling between fluvial and hillslope processes is essential to maintain headward propagation of the knickpoint gorge head and to facilitate mass wasting from the gorge sidewalls peak stream discharge across the gorge head must exceed a threshold sufficient to transport debris away from the gorge head to keep the knickpoint rock face free of talus to incise the stream channel and to erode the toes of the sidewalls if these conditions on fluvial processes are met then hillslope processes including rockslope failure mechanisms are the rate controlling processes for knickpoint propagation and therefore determine the rate of escarpment retreat across the passive margin

quantification of rock control in geomorphology abstract rock control the influence of rock properties on landform development has for long been a relatively under researched topic but numerous techniques have been developed especially by geotechnical engineers and stone conservationists that are now being applied to geomorphological issues by geomorphologists these techniques range in scale from the quantification of major discontinuities through to analysis of micro pores rock control studies that have taken place over the last few decades have contributed important insights into our understanding of such diverse phenomena as 1 river terraces bedrock channel forms and gradients pot hole development drainage density and gully badland development 2 slope forms and failures especially with respect to rock mass strength 3 the form of glacial troughs and of subglacial erosional landforms 4 weathering controls on rock durability and the association of weathering with porosity 5 classic karst features including various types of karren 6 sandstone landforms 7 inselbergs and tors and 8 cliffs and shore platforms

reconnaissance sediment budgets for lynn valley british columbia holocene and contemporary time scales volumes of erosional and depositional landforms were estimated by reconnaissance methods in a steep forested drainage basin in the coast mountains british columbia to examine what insight might be gained into the sediment budget in the absence of direct measurements of sediment transfer processes sediment transfers in postglacial and contemporary time were inferred on hillslopes from hillslopes to stream channels and within stream channels data were collected by aerial photographic analysis and field observation postglacial sediment sources are major gullies created by debris slides and flows failures in glaciolacustrine terraces and stream channel degradation depositional landforms consist of talus slopes and colluvial and alluvial fans contemporary sediment sources include debris slides and flows in established gullies and minor processes on hillslopes debris slide and debris flow volumes were calculated and other processes were estimated from regional values erosion rate averaged over postglacial time is 276 t km 2 a 1 0 15 mm a 1 surface lowering with gullies and stream channel degradation contributing 170 and 82 t km 2 a 1 respectively a terminal alluvial fan provides an independent check of the results in contemporary time erosion rates are calculated to be 350 t km 2 a 1 with debris flows and slides contributing nearly all of this sediment the contemporary rate is probably perturbed by recent land use history mass movement processes appear to be the dominant mechanism of sediment transfer and contributions from pleistocene valley deposits have declined significantly during holocene time resume les volumes de terrain erodes et deposes ont ete estimes par des methodes de reconnaissance dans un bassin de drainage boise a pentes abruptes des montagnes cotieres de la colombie britannique afin d etudier quelles connaissances sur le budget sedimentaire pourraient etre acquises en l absence de mesures directes du processus de transfert de sediments les transferts de sediments a l epoque post glaciaire et contemporaine ont ete inferes sur les flancs des cotes des flancs des cotes vers les chenaux des rivieres et dans les chenaux des rivieres les donnees proviennent de l analyse de photographies aeriennes et d observations sur le terrain les sources de sediments post glaciaires sont des ravines creees par des glissements et des coulees de debris des effondrements de terrasses glacio lacustres et la degradation de chenaux les formes de terrain creees par deposition comprennent des tabliers d eboulis et des deltas colluviaux et alluvionnaires les sources contemporaines de sediments comprennent des glissements et des coulees de debris dans des ravines etablies et des processus mineurs sur des pentes les volumes des glissements et des coulees de debris ont ete calcules et d autres processus ont ete evalues a partir de valeurs regionales la moyenne du taux d erosion pour la periode post glaciaire est de 276 tonnes km 2 a 1 abaissement de la surface de 0 15 mm a les ravines et la degradation des chenaux des ruisseaux contribuant respectivement 170 et 82 tonnes km 2 a 1 un delta alluvionnaire terminal permet une verification independante des resultats a l epoque contemporaine les taux d erosion sont calcules comme etant 350 tonnes km 2 a 1 et les glissements et des coulees de debris y contribueraient pour la presque totalite le taux contemporain est probablement perturbe par l utilisation recente du terrain les processus de mouvements de masse semblent etre le mecanisme dominant de transfert de sediments et les contributions provenant des depositions dans les vallees au pleistocene ont diminue de facon significative durant l holocene traduit par la redaction campbell and church 713

rates of surface processes on slopes slope retreat and denudation results taken from 270 publications on rates are summarized and collated with those from 149 publications reviewed previously young 1969 1974 the data are classified by major climatic zone normal or steep relief and consolidated or unconsolidated rocks representative rates and their ranges are given for soil creep solifluction surface wash solution chemical denudation rock weathering slope retreat cliff free face retreat marine cliff retreat and denudation the last being compared with representative rates of uplift solifluction is of the order of 10 times faster than soil creep but both cause only very slow ground loss solution is an important cause of ground loss for siliceous rocks on which it may be half as rapid as on limestones total denudation brought about mainly by surface wash reaches a maximum in the semi arid and probably also the tropical savanna zones acceleration of natural erosion rates by human activities ranges from 2 3 times with moderately intense land use to about 10 times with intensive land use and considerably higher still where there is recognized accelerated soil erosion where there is active uplift typical rates are of the order of 10 times faster than denudation although in some high steep mountain ranges these may approach equality

mantle related carbonados geochemical insights from diamonds from the dachine komatiite french guiana abstract carbonado is a unique type of polycrystalline diamond characterised among others by 13c depleted isotope compositions 13c 25 vs pdb little advanced nitrogen aggregation ib iaa and sintered ceramic like diamond grains its origin remains an enigma with models proposing a formation either in the earth s crust or even within an exploding super nova the possibility that carbonado formed in the earth s mantle is often rejected because diamond with carbonado like geochemical features has never been found in rocks such as kimberlites that carry diamonds from the mantle in this study it is shown that the c and n stable isotope compositions nitrogen contents and nitrogen speciation of diamonds from the dachine komatiite french guyana exhibit unambiguous similarities with carbonados these include c isotopes from 32 6 to 0 15 mode at 27 n aggregation only ib iaa diamonds from 2 to 76 of n pairs and n isotopes from 4 1 to 6 9 average 2 1 2 9 which all strikingly match the carbonado data this evidence illustrates that the main geochemical arguments usually called to reject a mantle origin of carbonado are no longer valid a model linking carbonado crystallisation from komatiite volatiles is developed in this model the sintering is produced by the high temperature of the komatiite magma thus accounting for their absence in colder kimberlites the low 13c compositions of carbonados would be inherited from the transition zone 300 km depths which is known to yield diamonds with distinct c isotope distributions compared to most lithospheric diamonds 150 300 km depths this model can account for most available observations of carbonados including their large size sintering photoluminescense cathodoluminescence features and geochemical characteristics

arctic rockwall retreat rates estimated using laboratory calibrated ert measurements of talus cones in longyeardalen svalbard holocene rockwall retreat rates quantify integral values of rock slope erosion and talus cone evolution here we investigate holocene rockwall retreat of exposed arctic sandstone cliffs in longyear

forensic analysis of rockfall scars abstract we characterise and analyse the detachment scar surfaces of rockfalls to understand the mechanisms that underpin their failure rockfall scars are variously weathered and comprised of both discontinuity release surfaces and surfaces indicative of fracturing through zones of previously intact rock known as rock bridges the presence of rock bridges and pre existing discontinuities is challenging to quantify due to the difficulty in determining discontinuity persistence below the surface of a rock slope rock bridges form an important control in holding blocks onto rockslopes with their frequency extent and location commonly modelled from the surface exposure of daylighting discontinuities we explore an alternative approach to assessing their role by characterising failure scars we analyse a database of multiple rockfall scar surfaces detailing the areal extent shape and location of broken rock bridges and weathered surfaces terrestrial laser scanning and gigapixel imagery were combined to record the detailed texture and surface morphology from this scar surfaces were mapped via automated classification based on rgb pixel values our analysis of the resulting data from scars on the north yorkshire coast uk indicates a wide variation in both weathering and rock bridge properties controlled by lithology and associated rock mass structure importantly the proportion of rock bridges in a rockfall failure surface does not increase with failure size rather larger failures display fracturing through multiple rock bridges and in contrast smaller failures fracture occurs only through a single critical rock bridge this holds implications for how failure mechanisms change with rockfall size and shape additionally the location of rock bridges with respect to the geometry of an incipient rockfall is shown to determine failure mode weathering can occur both along discontinuity surfaces and previously broken rock bridges indicating the sequential stages of progressively detaching rockfall our findings have wider implications for hazard assessment where rock slope stability is dependent on the nature of rock bridges how this is accounted for in slope stability modelling and the implications of rock bridges on long term rock slope evolution

climatic controls on frost cracking and implications for the evolution of bedrock landscapes 1 in mountainous landscapes the role of periglacial processes in producing sediment is poorly defined despite evidence of abundant talus slopes ice growth in rock has long been recognized as an efficient erosion mechanism but the effects have not been readily applied to landscape evolution in response to tectonic and climatic forcing here we quantify how and where ice driven mechanical erosion occurs in cold bedrock dominated landscapes using a simple one dimensional numerical heat flow model in our model ice grows by water migration to colder regions in shallow rock by the reduction in chemical potential associated with intermolecular forces between ice and mineral surfaces a process called segregation ice growth the depth and intensity of frost cracking is primarily dependent on mean annual temperature mat with positive mat sites characterized by intense cracking in the top meter of the rock mass and a maximum frost penetration of 4 m in contrast negative mat areas have less intense cracking that primarily occurs at depths between 50 and 800 cm we compare the depth and intensity of frost cracking predicted by our model with measures of the intensity of frost processes determined in three studies the first measured the timing of rockfall in the canadian rockies niagara escarpment and japanese alps the second analyzed scree deposits in the southern alps new zealand and the third documented rockfall frequency in utah these natural examples show that rockfalls tend to nucleate at elevations that coincide with zones of intense frost cracking predicted by our model as such climatic variations associated with interglacial glacial cycles may impart a significant influence on the denudation of mountainous landscapes

timing and significance of late glacial and holocene cirque glaciation in the sierra nevada california mapping and radiocarbon dates of cirque moraines in the sierra nevada demonstrate that the last significant pre little ice age glacier advance in the range the recess peak resulted from snowline lowering roughly twice that of the matthes little ice age advance and that the recess peak advance is late pleistocene in age we mapped recess peak and matthes deposits in 64 cirques along a profile of the main sierran crest that spans the north south limits of neoglacial deposits in the range equilibrium line altitudes for the reconstructed recess peak glaciers vary greatly but coherently with those of the matthes advance the variability of both sets of deposits reflects strong topographic influences on snow accumulation and ablation patterns in their deep cirques tephrochronology and radiocarbon dates from lake sediment cores provide limits on the timing of the two advances previous work documenting the absence of a young regionally extensive tephra on matthes moraines in the central sierra demonstrates that they formed after 700 14c years bp 650 cal years bp the age of the recess peak advance has been less certain we therefore collected and dated sediment cores from lakes dammed behind terminal moraines correlating to the recess peak advance in four widely separated drainages along the sierran crest north to south south fork american river lee vining creek middle fork san joaquin river and bishop creek twenty three high precision ams radiocarbon dates on gyttja peat and macrofossils from the cores are internally consistent and demonstrate that the recess peak advance previously thought to be of late holocene age 2500 years bp ended before 11 190 70 14c years bp 13 100 85 cal years bp recess peak is therefore late pleistocene in age and probably predates the north atlantic younger dryas climatic reversal the absence of any glacial deposits on the bedrock between the recess peak and matthes deposits indicates that 1 any advance related to the younger dryas event in central california was smaller than the matthes advance 2 the matthes advance was the most extensive and possibly the only neoglacial event in the range and 3 climate in the sierra between 13 000 cal years bp and 650 cal years bp was apparently too warm and or dry to support glaciers larger than those of the little ice age other mapping indicates that the recess peak is the first significant glacier advance after retreat of tioga local late wisconsin maximum glaciers these results suggest a regionally variable climate in western north america during the younger dryas event because glaciers appear to have expanded in the canadian rockies at that time the new recess peak age limits combined with other dated lake cores indicate that the sierra was essentially deglaciated by 14 000 15 000 cal years bp 12 000 13 000 14c years bp substantially earlier than previously estimated this finding indicates that current production rates of some in situ cosmogenic nuclides calibrated on an assumed deglaciation of the range at 11 000 cal years bp 10 000 14c years bp may be systematically too high by as much as 20

permafrost in steep bedrock slopes and its temperature related destabilization following climate change permafrost in steep bedrock is abundant in many cold mountain areas and its degradation can cause slope instability that is unexpected and unprecedented in location magnitude frequency and timing these phenomena bear consequences for the understanding of landscape evolution natural hazards and the safe and sustainable operation of high mountain infrastructure permafrost in steep bedrock is an emerging field of research knowledge of rock temperatures ice content mechanisms of degradation and the processes that link warming and destabilization is often fragmental in this article we provide a review and discussion of existing literature and pinpoint important questions ice filled joints are common in bedrock permafrost and possibly actively widened by ice segregation broad evidence of destabilization by warming permafrost exists despite problems of attributing individual events to this phenomenon with certainty convex topography such as ridges spurs and peaks is often subject to faster and deeper thaw than other areas permafrost degradation in steep bedrock can be strongly affected by percolating water in fractures this degradation by advection is difficult to predict and can lead to quick and deepdevelopment of thaw corridors along fractures in permafrost and potentially destabilize much greater volumes of rock than conduction would although most research on steep bedrock permafrost originates from the alps it will likely gain importance in other geographic regions with mountain permafrost

lithological controls on hillslope sediment supply insights from landslide activity and grain size distributions the volumes rates and grain size distributions of sediment supplied from hillslopes represent the initial input of sediment delivered from upland areas and propagated through sediment routing systems moreover hillslope sediment supply has a significant impact on landscape response time to tectonic and climatic perturbations however there are very few detailed field studies characterising hillslope sediment supply as a function of lithology and delivery process here we present new empirical data from tectonically active areas in southern italy that quantifies how lithology and rock strength control the landslide fluxes and grain size distributions supplied from hillslopes landslides are the major source of hillslope sediment supply in this area and our inventory of 2800 landslides reveals that landslide sediment flux is dominated by small shallow landslides we find that lithology and rock strength modulate the abundance of steep slopes and landslides and the distribution of landslide sizes outcrop scale rock strength also controls the grain sizes supplied by bedrock weathering and influences the degree of coarsening of landslide supply with respect to weathering supply finally we show that hillslope sediment supply largely determines the grain sizes of fluvial export from catchments and that catchments with greater long term landslide rates deliver coarser material therefore our results demonstrate a dual control of lithology on hillslope sediment supply by modulating both the sediment fluxes from landslides and the grain sizes supplied by hillslopes to the fluvial system

analysis of crack movements observed in an alpine bedrock cliff crack widths and rock temperatures were monitored on an andestic bedrock cliff in the summit area of the daisetsu mountains hokkaido northern japan sequential data recorded the gradual widening of a crack to the point of critical crack extension which resulted in catastrophic rock breakage the data indicate that a combination of liquid water in ltration into crack tip and subsequent freezing is the most signi cant factor contributing to critical crack extension the recorded sub critical crack movements involved a number of minor crack extensions and contractions the timing of which correlates well with the magnitude of the reconstructed thermal stresses at the crack tip derived from thermal deformation of the plate shaped rock fragment larger crack extensions occurred when stress at the crack tip exceeded a threshold value possibly re ecting the control of rock fracture mechanics by which cracks are thought to propagate when the stress intensity factor at the crack tip exceeds the threshold values for stress corrosion cracking and the fracture toughness of the material copyright 2004 john wiley sons ltd

evidence for nonlinear diffusive sediment transport on hillslopes and implications for landscape morphology steep soil mantled hillslopes evolve through the downslope movement of soil driven largely by slope dependent transport processes most landscape evolution models represent hillslope transport by linear diffusion in which rates of sediment transport are proportional to slope such that equilibrium hillslopes should have constant curvature between divides and channels on many soil mantled hillslopes however curvature appears to vary systematically such that slopes are typically convex near the divide and become increasingly planar downslope this suggests that linear diffusion is not an adequate model to describe the entire morphology of soil mantled hillslopes here we show that the interaction between local disturbances such as rainsplash and biogenic activity and frictional and gravitational forces results in a diffusive transport law that depends nonlinearly on hillslope gradient our proposed transport law 1 approximates linear diffusion at low gradients and 2 indicates that sediment flux increases rapidly as gradient approaches a critical value we calibrated and tested this transport law using high resolution topographic data from the oregon coast range these data obtained by airborne laser altimetry allow us to characterize hillslope morphology at 2 m scale at five small basins in our study area hillslope curvature approaches zero with increasing gradient consistent with our proposed nonlinear diffusive transport law hillslope gradients tend to cluster near values for which sediment flux increases rapidly with slope such that large changes in erosion rate will correspond to small changes in gradient therefore average hillslope gradient is unlikely to be a reliable indicator of rates of tectonic forcing or baselevel lowering where hillslope erosion is dominated by nonlinear diffusion rates of tectonic forcing will be more reliably reflected in hillslope curvature near the divide rather than average hillslope gradient