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 Longyeardalen, central Svalbard and apply laboratory‐calibrated electrical resistivity tomography (ERT) to determine talus sediment thickness. Temperature–resistivity functions of two sandstone samples are measured in the laboratory and compared with borehole temperatures from the talus slope. The resistivity of the higher and lower‐porosity sandstone at relevant borehole permafrost temperatures defines a threshold range that accounts for the lithological variability of the dominant bedrock and debris material. This helps to estimate the depth of the transition from higher resistivities of ice‐rich debris to lower resistivities of frozen bedrock in the six ERT transects. The depth of the debris–bedrock transition in ERT profiles is confirmed by a pronounced apparent resistivity gradient in the raw data plotted versus depth of investigation. High‐resolution LiDAR‐scanning and ERT subsurface information were collated in a GIS to interpolate the bedrock surface and to calculate the sediment volume of the talus cones. The resulting volumes were referenced to source areas to calculate rockwall retreat rates. The rock mass strength was estimated for the source areas. The integral rockwall retreat rates range from 0.33 to 1.96 mm yr^–1, and are among the highest rockwall retreat rates measured in arctic environments, presumably modulated by harsh environmental forcing on a porous sandstone rock cliff with a comparatively low rock mass strength. Here, we show the potential of laboratory‐calibrated ERT to provide accurate estimates of rockwall retreat rates even in ice‐rich permafrost talus slopes. Copyright © 2012 John Wiley & Sons, Ltd.     

Conceptual model of fracture-limited sea cliff erosion: Erosion of the seaward tilted flyschs of Socoa,Basque Country,France

Sea cliff morphology and erosion rates are modulated by several factors, including rock control that reflects both lithology and rock structure. Erosion is anticipated to preferentially exploit ‘fractures’, broadly meant as any discontinuity in an otherwise continuous medium, where the rock mass is weakest. Unpicking the direct control of such fractures on the spatial and temporal pattern of erosion remains, however, challenging. To analyse how such fractures control erosion, we monitored the evolution of a 400 m-long stretch of highly structured sedimentary cliffs in Socoa, Basque Country, France. The rock is known as the Socoa flysch formation. This formation combines decimetre-thick turbidites composed of repeat triplets of medium to strong calcareous sandstone, laminated siltstones and argillaceous marls. The sequence plunges at 45° into the sea with a shore-parallel strike. The cliffs are cross-cut by two normal and reverse fault families, with 10–100 m alongshore spacing, with primary and secondary strata-bound fractures perpendicular to the bedding, which combined delimit the cliff rock mass into discrete blocks that are exploited by the erosion process. Erosion, and sometimes plucking, of such beds and blocks on the cliff face was monitored using ground-based structure-from-motion (SfM) photogrammetry, over the course of 5.7 years between 2011 and 2017. To compare with longer time change, cliff-top retreat rate was assessed using SfM-orthorectified archive aerial photographs spanning 1954–2008. We show that the 13,250 m² cliff face released 4500 blocks exceeding 1.45 × 10⁻³ m³, removing a total volume of 170 m³. This equates to an average cliff erosion rate of 3.4 mm/year, which is slightly slower than the 54-year-long local cliff-top retreat (10.8 ± 1.8 mm/year). The vertical distribution of erosion reflects the height of sea water inundation, where the maximum erosion intensity occurs ca. 2 m above high spring-tide water level. Alongshore, the distribution of rockfall scars is concentrated along bed edges bounding cross-cutting faults; the extent of block detachment is controlled by secondary tectonic joints, which may extend through several beds locally sharing similar mechanical strength; and rockfall depth is always a multiple of bed thickness. Over the longer term, we explain block detachment and resultant cliff collapse as a cycle. Erosion nucleates on readily exploitable fractures but elsewhere, the sea only meets defect-free medium-strong to strong rock slabs offering few morphological features for exploitation. Structurally delimited blocks are quarried, and with sufficient time, carve semi-elliptic scars reaching progressively deeper strata to be eroded. Lateral propagation of erosion is directed along mechanical weaknesses in the bedding, and large episodic collapses affect the overhanging slabs via sliding on the weak marl beds. Collapse geometry is confined to one or several triplets of turbidite beds, but never reaches deeper into the cliff than the eroded depth at the foot. We contend that this fracture-limited model of sea-cliff erosion, inferred from the Socoa site dynamics and its peculiar sets of fractures, applies more broadly to other fractured cliff contexts, albeit with site-specific geometries. The initiation of erosion, the propagation of incremental block release and the ultimate full failure of the cliff, have each been shown to be fundamentally directly controlled by structure, which remains a vital control in understanding how cliffed coasts have changed in the past and will change in the future.     

Catastrophic rock avalanche 3600 years BP from El Capitan,Yosemite Valley,California

Large rock slope failures from near‐vertical cliffs are an important geomorphic process driving the evolution of mountainous landscapes, particularly glacially steepened cliffs. The morphology and age of a 2·19 × 10⁶ m³ rock avalanche deposit beneath El Capitan in Yosemite Valley indicates a massive prehistoric failure of a large expanse of the southeast face. Geologic mapping of the deposit and the cliff face constrains the rock avalanche source to an area near the summit of ～8·5 × 10⁴ m². The rock mass free fell ～650 m, reaching a maximum velocity of 100 m s^?1, impacted the talus slope and spread across the valley floor, extending 670 m from the base of the cliff. Cosmogenic beryllium‐10 exposure ages from boulders in the deposit yield a mean age of 3·6 ± 0·2 ka. The ～13 kyr time lag between deglaciation and failure suggests that the rock avalanche did not occur as a direct result of glacial debuttressing. The ～3·6 ka age for the rock avalanche does coincide with estimated late Holocene rupture of the Owens Valley fault and/or White Mountain fault between 3·3 and 3·8 ka. The coincidence of ages, combined with the fact that the most recent (AD 1872) Owens Valley fault rupture triggered numerous large rock falls in Yosemite Valley, suggest that a large magnitude earthquake (≥M7.0) centered in the south‐eastern Sierra Nevada may have triggered the rock avalanche. If correct, the extreme hazard posed by rock avalanches in Yosemite Valley remains present and depends on local earthquake recurrence intervals. Published in 2010 by John Wiley & Sons, Ltd.     

Sediment dynamics below retreating cliffs

The retreat of cliffs may constitute the dominant erosional response to base‐level fall in arid settings underlain by horizontally‐bedded sedimentary rock. These vertical cliffs typically loom above a relatively straight bedrock slope (‘plinth’) that is mantled with a thin layer of sediment and perched near the angle of repose. In detail, a plinth consists of a system of quasi‐parallel ridges and channels. We ask how the sediment supplied from a retreating cliff influences the erosion of the plinth hillslopes and channels, and how this affects the rate of cliff retreat. Motivated by field observations and high‐resolution topographic data from two sites in western Colorado, we develop a two‐dimensional (2D), rules‐based numerical model to simulate the erosion of channels draining a plinth and diffusive erosion of the intervening interfluves. In this model, retreat of a cliffband occurs when the height of the vertical cliff exceeds a threshold due to incision by channels on the plinth below. Debris derived from cliff retreat is distributed over the model plinth according to the local topography and distance from the source. This debris then weathers in place, and importantly can act to reduce local bedrock erosion rates, protecting both the plinth and ultimately the cliff from erosion. In this paper, we focus on two sets of numerical model experiments. In one suite, we regulate the rate of rockfall to limit the cliff retreat rate; in most cases, this results in complete loss of the plinth by erosion. In a second suite, we do not impose a limit on the cliff retreat rate, but instead vary the weathering rate of the rockfall debris. These runs result in temporally steady cliff‐plinth forms and retreat rates; both depend on the weathering rate of the debris. Copyright © 2011 John Wiley & Sons, Ltd.     

Critical notch depths for failure of coastal limestone cliffs: case study at Kuro‐shima Island,Okinawa, Japan

Development of a notch at the base of a cliff reduces cliff stability and often induces a collapse. Pleistocene limestone coastal cliffs of elevation 5?m in Kuro‐shima, Ryukyu Islands, have a prominent notch with a depth of 3–4?m at their bases. Around these coastal cliffs, collapses different from previous studies of cliff collapses in the Ryukyu Islands were found; collapses in Kuro‐shima have a horizontal failure surface. The horizontal failure surface, situated at the height of the failure surface corresponding to the retreat point of the notch, is bounded by vertical joints cutting the whole cliff and the reef flat in front of the cliff. Two types of horizontal failure surface were found, triangular and quadrangular; the distinction appears to depend on the angle between the vertical joints and the front face of the cliff. Prior to collapse, these cliffs appear to have been separated from the adjacent cliffs by the development of vertical joints. Consequently, a cliff that will collapse can be identified in advance; cliff instability is strongly dependent on the development of a notch. To study the effect of notch development on cliff collapse, the notch depth at which collapse occurs was calculated using stability analysis. Instability of a cliff increases with notch depth; collapse occurs at the horizontal failure surface when the ratio of the notch depth to the seaward length of the cliff is approximately 0·5–0·7 for a triangular failure surface, and 0·7–0·9 for a quadrangular failure surface. Copyright © 2010 John Wiley & Sons, Ltd.     

Marine control over negative power law scaling of mass wasting events in chalk sea cliffs with implications for future recession under the UKCP09 medium emission scenario

Coastal cliff erosion represents a significant geohazard for people and infrastructure. Forecasting future erosion rates is therefore of critical importance to ensuring the resiliency of coastal communities. We use high precision monitoring of chalk cliffs at Telscombe, UK to generate monthly mass movement inventories between August 2016 and July 2017. Frequency–magnitude analysis of our inventories demonstrate negative power law scaling over 7 orders of magnitude and, for the first time, we report statistically significant correlations between significant wave height (H_s) and power law scaling coefficients (r² values of 0.497 and 0.590 for β and s respectively). Applying these relationships allows for a quantitative method to predict erosion at the site based on H_s probabilities and sea level forecasts derived from the UKCP09 medium emission climate model (A1B). Monte‐Carlo simulations indicate a range of possible erosion scenarios over 70 years (2020–2090) and we assess the impact these may have on the A259 coastal road which runs proximal to the cliffs. Results indicate a small acceleration in erosion compared with those based on current conditions with the most likely scenario at the site being 21.7 m of cliff recession by 2090. However, low‐probability events can result in recession an order of magnitude higher in some scenarios. In the absence of negative feedbacks, we estimate an ~11% chance that the A259 will be breached by coastal erosion by 2090. Copyright © 2018 John Wiley & Sons, Ltd.     

Sea stack formation and the role of abrasion on beach‐mantled headlands

Sea stacks are common and striking coastal landforms, but few details are known about how, how quickly, and under what conditions they form. We present numerical and analytical models of sea stack formation due to preferential erosion along a pre‐existing headland to address these basic questions. On sediment‐rich rocky coasts, as sea cliffs erode and retreat, they produce beach sediment that is distributed by alongshore sediment transport and controls future sea cliff retreat rates. Depending on their width, beaches can encourage or discourage sea cliff erosion by acting either as an abrasive tool or a protective cover that dissipates wave energy seaward of the cliff. Along the flanks of rocky headlands where pocket beaches are often curved and narrow due to wave field variability, abrasion can accelerate alongshore‐directed sea cliff erosion. Eventually, abrasion‐induced preferential erosion can cut a channel through a headland, separating it from the mainland to become a sea stack. Under a symmetrical wave climate (i.e. equal influence of waves approaching the coastline from the right and from the left), numerical and analytical model results suggest that sea stack formation time and plan‐view size are proportional to preferential erosion intensity (caused by, for example, abrasion and/or local rock weakness from joints, faults, or fractures) and initial headland aspect ratio, and that sea stack formation is discouraged when the sediment input from sea cliff retreat is too high (i.e. sea cliffs retreat quickly or are sand‐rich). When initial headland aspect ratio is too small, and the headland is ‘rounded’ (much wider in the alongshore direction at its base than at its seaward apex), the headland is less conducive to sea stack formation. On top of these geomorphic and morphologic controls, a highly asymmetrical wave climate decreases sea stack size and discourages stack formation through rock–sediment interactions. Copyright © 2014 John Wiley & Sons, Ltd.     

The role of beach morphology on coastal cliff erosion under extreme waves

Erosion of hard‐rock coastal cliffs is understood to be caused by a combination of both marine and sub‐aerial processes. Beach morphology, tidal elevation and significant wave heights, especially under extreme storm conditions, can lead to variability in wave energy flux to the cliff‐toe. Wave and water level measurements in the nearshore under energetic conditions are difficult to obtain and in situ observations are rare. Here we use monthly cliff‐face volume changes detected using terrestrial laser scanning alongside beach morphological changes and modelled nearshore hydrodynamics to examine how exposed cliffs respond to changes in extreme wave conditions and beach morphology. The measurements cover the North Atlantic storms of 2013 to 2014 and consider two exposed stretches of coastline (Porthleven and Godrevy, UK) with contrasting beach morphology fronting the cliffs; a flat dissipative sandy beach at Godrevy and a steep reflective gravel beach at Porthleven. Beach slope and the elevation of the beach–cliff junction were found to influence the frequency of cliff inundation and the power of wave–cliff impacts. Numerical modelling (XBeach‐G) showed that under highly energetic wave conditions, i.e. those that occurred in the North Atlantic during winter 2013–2014, with H_s = 5.5 m (dissipative site) and 8 m (reflective site), the combination of greater wave height and steeper beach at the reflective site led to amplified wave run‐up, subjecting these cliffs to waves over four times as powerful as those impacting the cliffs at the dissipative site (39 kWm^‐1 compared with 9 kWm^‐1). This study highlighted the sensitivity of cliff erosion to extreme wave conditions, where the majority (over 90% of the annual value) of cliff‐face erosion ensued during the winter. The significance of these short‐term erosion rates in the context of long‐term retreat illustrates the importance of incorporating short‐term beach and wave dynamics into geomorphological studies of coastal cliff change. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Submarine platform development by erosion of a Surtseyan cone at Capelinhos,Faial Island,Azores

Erosion of volcanic islands ultimately creates shallow banks and guyots, but the ways in which erosion proceeds to create them over time and how the coastline retreat rate relates to wave conditions, rock mass strength and other factors are unclear. The Capelinhos volcano was formed in 1957/58 during a Surtseyan and partly effusive eruption that added an ~2.5 km² tephra and lava promontory to the western end of Faial Island (Azores, central North Atlantic). Subsequent coastal and submarine erosion has reduced the subaerial area of the promontory and created a submarine platform. This study uses historical information, photos and marine geophysical data collected around the promontory to characterize how the submarine platform developed following the eruption. Historical coastline positions are supplemented with coastlines interpreted from 2004 and 2014 Google Earth images in order to work out the progression of coastline retreat rate and retreat distance for lava- and tephra-dominated cliffs. Data from swath mapping sonars are used to characterize the submarine geometry of the resulting platform (position of the platform edge, gradient and morphology of the platform surface). Photographs collected during SCUBA and ROV dives on the submarine platform reveal a rugged surface now covered with boulders. The results show that coastal retreat rates decreased rapidly with time after the eruption and approximately follow an inverse power-law relationship with coastal retreat distance. We develop a finite-difference model for wave attenuation over dipping surfaces to predict how increasing wave attenuation contributed to this trend. The model is verified by reproducing the wave height variation over dipping rock platforms in the UK (platform gradient 1.2° to 1.8°) and Ireland (1.8°). Applying the model to the dipping platform around Capelinhos, using a diversity of cliff resistance predicted from known lithologies, we are able to predict erosion rate trends for some sectors of the edifice. We also explore wider implications of these results, such as how erosion creates shallow banks and guyots in reef-less mid-oceanic archipelagos like the Azores. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

The role of groundwater in cliff instability: an example at Cape Blanc‐Nez (Pas‐de‐Calais,France)

Cliff retreat in northern Boulonnais is described. The distribution of the amount of retreat is related to structural and topographic factors. Both control the runoff–infiltration balance and therefore the mechanical behaviour of the rocks, which determines the modes of failure. The spatial variability of the retreat rate is explained, but predicting the temporal variability of the retreat rate, which is central to risk management, is much more difficult. Rainfall and piezometric surface data enhanced a ‘piston flow’ mechanism during November 2000. The result of its occurrence on the stability of the cliff and conditions of its recurrence are examined with a view to better understanding the rate of recession of coastal cliffs and wiser management of risk. Copyright © 2005 John Wiley & Sons, Ltd.     

Landscape development in an hyperarid sandstone environment along the margins of the Dead Sea fault: Implications from dated rock falls

In this study, we explored the spatial and temporal relations between boulders and their original in-situ locations on sandstone bedrock cliffs. This was accomplished by combining field observations with dating methods using cosmogenic isotopes (¹⁰Be and ¹⁴C) and optically stimulated luminescence (OSL). Our conclusions bear both on the landscape evolution and cliff retreat process in the hyperarid region of Timna and on the methodology of estimating exposure ages using cosmogenic isotopes.

We recognize three discrete rock fall events, at 31 ka, 15 ka, and 4 ka. In this hyperarid region, the most plausible triggering mechanism for rock fall events is strong ground acceleration caused by earthquakes generated by the nearby Dead Sea fault (DSF). Our record, however, under represents the regional earthquake record implying that ongoing development of detachment cracks prior to the triggering event might be slower than the earthquake cycle.

Cliff retreat rates calculated using the timing of rock fall events and estimated thickness of rock removed in each event range between 0.14 m ky^− 1 and 2 m ky^− 1. When only full cycles are considered, we derive a more realistic range of 0.4 m ky^− 1 to 0.7 m ky^− 1. These rates are an order of magnitude faster than the calculated rate of surface lowering in the area. We conclude that sandstone cliffs at Timna retreat through episodic rock fall events that preserve the sharp, imposing, landscape characteristic to this region and that ongoing weathering of the cliff faces is minor.

A 10%–20% difference in the ¹⁰Be concentrations in samples from matching boulder and cliff faces that have identical exposure histories and are located only a few meters apart indicates that cosmogenic nuclide production rates are sensitive to shielding and vary spatially over short distances. However, uncertainties associated with age calculations yielded boulder and matching cliff face ages that are similar within 1 σ . The use of external constraints in the form of field relations and OSL dating helped to establish each pair's age. The agreement between calculated ¹⁴C and ¹⁰Be ages indicates that the accumulation of ¹⁰Be at depth by the capture of slow deep-penetrating muons was properly accounted for in the study.     

Emergent characteristics of rockfall inventories captured at a regional scale

High-resolution rockfall inventories captured at a regional scale are scarce. This is partly owing to difficulties in measuring the range of possible rockfall volumes with sufficient accuracy and completeness, and at a scale exceeding the influence of localized controls. This paucity of data restricts our ability to abstract patterns of erosion, identify long-term changes in behaviour and assess how rockfalls respond to changes in rock mass structural and environmental conditions. We have addressed this by developing a workflow that is tailored to monitoring rockfalls and the resulting cliff retreat continuously (in space), in three-dimensional (3D) and over large spatial scales (>10⁴ m). We tested our approach by analysing rockfall activity along 20.5 km of coastal cliffs in North Yorkshire (UK), in what we understand to be the first multi-temporal detection of rockfalls at a regional scale. We show that rockfall magnitude–frequency relationships, which often underpin predictive models of erosion, are highly sensitive to the spatial extent of monitoring. Variations in rockfall shape with volume also imply a systemic shift in the underlying mechanisms of detachment with scale, leading us to question the validity of applying a single probabilistic model to the full range of rockfalls observed here. Finally, our data emphasize the importance of cliff retreat as an episodic process. Going forwards, there will a pressing need to understand and model the erosional response of such coastlines to rising global sea levels as well as projected changes to winds, tides, wave climates, precipitation and storm events. The methodologies and data presented here are fundamental to achieving this, marking a step-change in our ability to understand the competing effects of different processes in determining the magnitude and frequency of rockfall activity and ultimately meaning that we are better placed to investigate relationships between process and form/erosion at critical, regional scales. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

A wave tank experiment on the erosional mechanism at a cliff base

A laboratory experiment using a two-dimensional wave tank was designed to investigate the mechanism of erosion at a cliff base by waves armed with rock fragments. The experiment was performed under constant wave conditions by systematically changing the amount of beach sand at the foot of steep model cliffs of the same slope and strength. Cliff erosion occurred when the beach material at the cliff/beach junction was moved by waves. Turbulence created by bores rushing up on the beach mobilized the sand and exerted a mechanical shearing force on the cliff face using the sand as an abrasive. The analysis of results indicated that the effect of the abrasive doubled when the cliff/beach junction was located above Still Water Level (SWL) as compared to when it was below SWL. The assailing force of the sediment-laden water masses was proportional to the square of the bore speed immediately in front of the cliff face. The factor of proportionality is related to the quantity of beach sand entrapped in the turbulent fluid.     

Overestimation of short-term coastal cliff retreat rates in the eastern Mediterranean resolved with a sediment budget approach

Our understanding of sea-cliff erosion processes and their response to recent and/or projected environmental changes such as sea-level rise, climate change and anthropogenic development hinges on our ability to quantify sea-cliff retreat rates and their variability through time. Here, we focus on Israel's Mediterranean ‘Sharon’ sea-cliff as a case study for examining the significance of recent short-term (i.e. annual to decadal) cliff-top retreat rates that appear to exceed longer-term rates of ‘background’ (i.e. centennial to millennial) retreat by 1–2 orders of magnitude. We demonstrate that an inherent sampling bias in rate estimates inferred from observation intervals shorter than process episodicity can also explain such a pattern. This potential ambiguity leads to a striking paradox where despite highly accurate and robust documentation of recent cliff-top retreat, such as that obtained from aerial photographs and/or instrumental surveys, the short-term retreat rates of episodically retreating sea cliffs remain poorly constrained. To address this key data gap along the Sharon sea cliff we employed a sediment budget approach that focuses on quantifying the continuous wave scouring of cliff-collapsed material from the shore platform as a rate-limiting process for episodic retreat of the cliff above. We used four high-resolution (0.5 m/pixel) airborne LiDAR data sets acquired between 2006 and 2015 to determine short-term maximum retreat rates of up to ~0.08 m/yr during this nine-year period. These modern retreat rates compare to the cliff's background retreat rate of 0.03 to 0.09 m/yr since the mid-Holocene, as determined herein from multiple geologic and archeological observations. Our results demonstrate that previously reported twentieth century cliff-top retreat rates for this sea cliff, which range up to values of several meters per year, are biased and that sea-cliff erosion rates have not yet been significantly impacted by recent environmental changes in the eastern Mediterranean basin, such as the restriction of sediment supply following emplacement of the Nile's Aswan dam system. © 2018 John Wiley & Sons, Ltd.     

The significance of scarp retreat for cenozoic landform evolution on the Colorado Plateau,U.S.A.

Since the beginning of the Cenozoic period several hundreds of metres of the sedimentary cover have been removed from the Colorado Plateau. Palaeoclimatic considerations show that the Colorado Plateau has been dominated by dry climates throughout the Cenozoic with the possible exception of the early Palaeocene. Today in the still prevailing arid climate, which strongly accentuates differences in rock resistance, the relief shows a structurally controlled cuesta scarp topography in the slightly deformed strata of alternating resistance. In examining whether the denudational efficiency of scarp retreat was sufficient to account for the wide erosional gaps in the sedimentary cover, rates of scarp retreat were determined by using the information of dated volcanic material and by applying a new method, which calculates the amount of retreat from the width of beheaded valleys of known age. Rates of retreat range from 0·5 to 6·7 km my^?1. The results show that the rates of retreat are controlled by the thickness and resistance of the caprocks. A model of Cenozoic scarp retreat demonstrates that the rates of recession calculated for the scarps in Upper Cretaceous rocks were sufficient to bring them into their present positions from the centre of the Monument Uplift on the central Colorado Plateau. The late Eocene positions of the cliffs in the Early Tertiary formations give an indication of their maximal extent. After the Upper Cretaceous sediments had been removed from the uplifts, erosion cut through successively older rocks, and activated scarps in stratigraphically lower positions. Scarp retreat can operate simultaneously and independently at different levels, which enabled this erosional mechanism to remove great proportions of the sedimentary cover of the Colorado Plateau during the Cenozoic.     

Aeolian deposits on a Scottish mountain summit: characteristics,provenance, history and significance

The summit plateau of The Storr (719 m) in northern Skye is mantled by a sheet of aeolian sediment up to 2·9 m thick, covering an area of 33 000 m² with a volume of 41 000 m³. The deposits are of massive, poorly sorted sand with significant components of silt and fine gravel, and contain clasts up to 109 mm in length. The thickness and coarseness of the deposits decline westwards and northwards away from the highest cliffs, implying that the sediment comprises particles dislodged from rockwalls and blown upwards in an accelerating vertical or near-vertical airflow, settling through a lower-velocity flow onto the plateau surface where they are trapped by vegetation. Radiocarbon dating of soils buried under and within the deposits suggests that accumulation began after 7·2–6·9 calendar ka BP but before 5·6–5·3 calendar ka BP , and was probably initiated by exposure of the present rockwall by a massive landslide at c. 6·5 ± 0·5 calendar ka BP . Pollen analyses of buried organic horizons suggest that a vegetation mat dominated by grasses and sedges was present throughout the period of sediment deposition. Sediment accumulation over much of the plateau averaged 10–20 mm per century throughout the late Holocene, but reached c. 60 mm per century in the area of the thickest deposits. The volume of the deposits implies the removal of 420–480 mm of rock (averaged over the face) during the late Holocene, and suggests that small-scale granular disaggregation and release of small clasts constitute a major component of rockwall retreat under present conditions. The origin of the Storr deposits suggests that plateau-top aeolian sediments on other Scottish mountains accumulated in a similar way, but have been eroded and redeposited on lee slopes following breakage of vegetation cover. © 1998 John Wiley & Sons, Ltd.     

The structure and sedimentology of relict talus,Trotternish, northern Skye,Scotland

Sections up to 3·5 m deep cut through the upper rectilinear segment of relict, vegetated talus slopes at the foot of the Trotternish escarpment reveal stacked debris-flow deposits intercalated with occasional slopewash horizons and buried organic soils. Radiocarbon dating of buried soil horizons indicates that reworking of sediment by debris flows predates 5·9–5·6 Cal ka BP , and has been intermittently active throughout the late Holocene. Particle size analyses of 18 bulk samples from these units indicates that c. 27–30 per cent of the talus deposit is composed of fine (<2 mm) sediment. Sedimentological comparison with tills excludes a glacigenic origin for the talus debris, and the angularity of constituent clasts suggests that in situ weathering has been insignificant in generating fine material. We conclude that the fine sediment within the talus is derived primarily by granular weathering of the rockwall, with syndepositional accumulation of both fine and coarse debris, implying that c. 27–30 per cent of rockwall retreat since deglaciation reflects granular weathering rather than rockfall. The abundance of fines within the talus deposits is inferred to have been of critical importance in facilitating build-up of porewater pressures during rainstorms, leading to episodic failure and flow of debris on the upper parts of the slope. A wider implication of these findings is that the mechanical properties of talus slopes cannot be regarded as those of free-draining accumulations of coarse clasts, and that models that treat talus slopes as such have limited value in explaining their form and evolution. Our findings lend support to models that envisage the upper straight slope on talus accumulations as the product of mass-transport as well as rockfall, and indicate that episodic debris flow has been the primary agent of mass-transport at this site. © 1998 John Wiley & Sons, Ltd.     

Bedrock erosion and relief production in the northern Flinders Ranges,Australia

Cosmogenic ¹⁰Be concentrations in exposed bedrock surfaces and alluvial sediment in the northern Flinders Ranges reveal surprisingly high erosion rates for a supposedly ancient and stable landscape. Bedrock erosion rates increase with decreasing elevation in the Yudnamutana Catchment, from summit surfaces (13·96 ± 1·29 and 14·38 ± 1·40 m Myr^?1), to hillslopes (17·61 ± 2·21 to 29·24 ± 4·38 m Myr^?1), to valley bottoms (53·19 ± 7·26 to 227·95 ± 21·39 m Myr^?1), indicating late Quaternary increases to topographic relief. Minimum cliff retreat rates (9·30 ± 3·60 to 24·54 ± 8·53 m Myr^?1) indicate that even the most resistant parts of cliff faces have undergone significant late Quaternary erosion. However, erosion rates from visibly weathered and varnished tors protruding from steep bedrock hillslopes (4·17 ± 0·42 to 14·00 ± 1·97 m Myr^?1) indicate that bedrock may locally weather at rates equivalent to, or even slower than, summit surfaces. ¹⁰Be concentrations in contemporary alluvial sediment indicate catchment‐averaged erosion at a rate dominated by more rapid erosion (22·79 ± 2·78 m Myr^?1), consistent with an average rate from individual hillslope point measurements. Late Cenozoic relief production in the Yudnamutana Catchment resulted from (1) tectonic uplift at rates of 30–160 m Myr^?1 due to range‐front reverse faulting, which maintained steep river gradients and uplifted summit surfaces, and (2) climate change, which episodically increased both in situ bedrock weathering rates and frequency–magnitude distributions of large magnitude floods, leading to increased incision rates. These results provide quantitative evidence that the Australian landscape is, in places, considerably more dynamic than commonly perceived. Copyright © 2006 John Wiley & Sons, Ltd.     

Geometrical simulation studies of coastal cliff failures in liassic strata,South Wales,U.K.

Three numerical models of cliff failure have been developed, based on analyses of rockfalls along the 22 km Liassic coastline of the Glamorgan Heritage Coast, U.K. Detailed field investigation of the bucklandi (limestone dominated) and angulata (mudstone dominated) Lias series at four locations confirm the veracity of the models. Translation failure was correctly predicted at the mainly limestone buttressed cliff sites in the bucklandi and at locations where the angulata series formed a high proportion of the cliff mass. Toppling was predicted for vertical and overhanging cliffs with basal undercutting. The factor of safety reduces as the ratio of undercutting depth (d) to tension fracture distance (D) from the cliff face increases. Instability can be triggered by thrust forces generated by wave/tide impact, freeze/thaw and expansion/contraction associated with clay infill. Thrust forces can be modest, ranging from 1·7 to 2·7 MN m², but can exceed the cross-joint strength as weathering proceeds.     

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.