Cliff Profile Research Articles

Species distribution and habitat attributes guide translocation planning of a threatened short-range endemic plant

The success of plant translocations depends on defining habitat attributes critical for establishment and survival, and on locating this habitat in the landscape. We used species distribution modelling, coupled with fine-scale characterisation of local habitat attributes to characterise and identify potential sites for translocation of Tetratheca erubescens J.P.Bull (Elaeocarpaceae), a threatened shrub restricted to cliff faces on one banded ironstone range in semi-arid south-west Western Australia. Here, we 1) constructed a maximum entropy species distribution model (SDM) from known occurrence locations and environmental data, and projected this onto a broader surrounding landscape to seek suitable unoccupied habitat; 2) characterised the local habitat attributes (LHA) of both occupied and predicted but unoccupied habitat (locality types), determining potential characteristics of habitat suitability at the scale of individual plants (microsites) not captured by the landscape-scale models and; 3) assessed unoccupied areas of modelled high habitat suitability to identify ‘potential translocation sites’ that may have the characteristics to support T. erubescens. The SDM resolved two management considerations by identifying suitable habitat outside the known extent of threatened species to locate areas to search for additional natural populations (none found) or to verify as ‘potential translocation sites’ (24 sites identified). Using field surveys to assess LHA at a finer-scale, we detected differences between plant microsites and other random points in occupied habitat. As such, T. erubescens was more likely to grow in wider rock cracks, located in relatively water gaining points in the cliff profile, in soils with organic (rather than mineral) content. Our approach systematically addressed the complex challenges involved in identifying and selecting sites for translocation of a threatened plant species, a first for a semi-arid environment. We used the outcomes of this study to present a conceptual model for practitioners and regulators that outlines our approach for identifying ‘potential translocation sites’ and ranking their suitability within the context of three assessment filters: species requirements; management-operational constraints and regulatory considerations. This application is relevant to translocation programs that seek to return plant species into natural habitat areas.

Sedimentary Coastal Cliff Erosion in Greenland

AbstractClimate change will increase the duration of annual sea‐ice‐free periods and shift precipitation patterns across the Arctic. Those factors are likely to increase erosion rates along its coasts. Large parts of the Arctic coast consist of hard rock. However, glacial, deltaic, and coastal sedimentary deposits occur in deglaciated areas and isostatic uplift following glaciations has created beach ridge plains and pocket beaches with coarse soft‐sediment cliffs. Hitherto, very little was known about the spatial distribution, erosion rates, and morphodynamics of soft sediment cliffs along the coast of Greenland. Here, we investigate a 3‐km sedimentary cliff section on the south coast of Qeqertarsuaq (Disko Island). We measured 2D cliff top erosion over 50 years between 1964 and 2014 as well as 3D cliff profile change over 2 years between 2019 and 2021. Morphometric indices of the gravel beach and cliff were calculated based on a series of cross‐shore elevation profiles. Wave run‐up at the beach fronting the cliff was modeled with XBeach‐G for a series of storm events under present day sea‐ice conditions and for a reduced sea‐ice scenario. Cliff top erosion rates varied along the cliff with maximum rates of 0.3 m y−1. The investigated coastal cliff erodes by two coupled processes: (a) precipitation‐driven surface runoff downslope the cliff and (b) wave‐driven erosion at the cliff toe. In a continuously warming climate, this study shows that erosion of soft coastal cliffs in Greenland thus can accelerate due to increased storminess and prolonging open water periods.

Organic matter characteristics of a rapidly eroding permafrost cliff in NE Siberia (Lena Delta, Laptev Sea region)

Abstract. Organic carbon (OC) stored in Arctic permafrost represents one of Earth's largest and most vulnerable terrestrial carbon pools. Amplified climate warming across the Arctic results in widespread permafrost thaw. Permafrost deposits exposed at river cliffs and coasts are particularly susceptible to thawing processes. Accelerating erosion of terrestrial permafrost along shorelines leads to increased transfer of organic matter (OM) to nearshore waters. However, the amount of terrestrial permafrost carbon and nitrogen as well as the OM quality in these deposits is still poorly quantified. We define the OM quality as the intrinsic potential for further transformation, decomposition and mineralisation. Here, we characterise the sources and the quality of OM supplied to the Lena River at a rapidly eroding permafrost river shoreline cliff in the eastern part of the delta (Sobo-Sise Island). Our multi-proxy approach captures bulk elemental, molecular geochemical and carbon isotopic analyses of Late Pleistocene Yedoma permafrost and Holocene cover deposits, discontinuously spanning the last ∼52 kyr. We showed that the ancient permafrost exposed in the Sobo-Sise cliff has a high organic carbon content (mean of about 5 wt %). The oldest sediments stem from Marine Isotope Stage (MIS) 3 interstadial deposits (dated to 52 to 28 cal ka BP) and are overlaid by last glacial MIS 2 (dated to 28 to 15 cal ka BP) and Holocene MIS 1 (dated to 7–0 cal ka BP) deposits. The relatively high average chain length (ACL) index of n-alkanes along the cliff profile indicates a predominant contribution of vascular plants to the OM composition. The elevated ratio of iso- and anteiso-branched fatty acids (FAs) relative to mid- and long-chain (C ≥ 20) n-FAs in the interstadial MIS 3 and the interglacial MIS 1 deposits suggests stronger microbial activity and consequently higher input of bacterial biomass during these climatically warmer periods. The overall high carbon preference index (CPI) and higher plant fatty acid (HPFA) values as well as high C/N ratios point to a good quality of the preserved OM and thus to a high potential of the OM for decomposition upon thaw. A decrease in HPFA values downwards along the profile probably indicates stronger OM decomposition in the oldest (MIS 3) deposits of the cliff. The characterisation of OM from eroding permafrost leads to a better assessment of the greenhouse gas potential of the OC released into river and nearshore waters in the future.

Multi-Temporal Cliff Erosion Analysis Using Airborne Laser Scanning Surveys

Rock cliffs are a significant component of world coastal zones. However, rocky coasts and factors contributing to their erosion have not received as much attention as soft cliffs. In this study, two rocky-cliff systems in the southern Baltic Sea were analyzed with Airborne Laser Scanners (ALS) to track changes in cliff morphology. The present contribution aimed to study the volumetric changes in cliff profiles, spatial distribution of erosion, and rate of cliff retreat corresponding to the cliff exposure and rock resistance of the Jasmund National Park chalk cliffs in Rugen, Germany. The study combined multi-temporal Light Detection and Ranging (LiDAR) data analyses, rock sampling, laboratory analyses of chemical and mechanical resistance, and along-shore wave power flux estimation. The spatial distribution of the active erosion areas appear to follow the cliff exposure variations; however, that trend is weaker for the sections of the coastline in which structural changes occurred. The rate of retreat for each cliff–beach profile, including the cliff crest, vertical cliff base, and cliff base with talus material, indicates that wave action is the dominant erosive force in areas in which the cliff was eroded quickly at equal rates along the cliff profile. However, the erosion proceeded with different rates in favor of cliff toe erosion. The effects of chemical and mechanical rock resistance are shown to be less prominent than the wave action owing to very small differences in the measured values, which proves the homogeneous structure of the cliff. The rock resistance did not follow the trends of cliff erosion revealed by volume changes during the period of analysis.

Innovative use of GIS and drone photogrammetry for cliff stability modelling

Evaluating the condition and stability of coastal cliffs and assessing longevity of significant overhangs and undercuts in an active marine environment is a challenge. As part of cliff management studies undertaken for the Coastal Zone Management Unit of the government of Barbados, a programme of innovative cliff profile mapping was conducted around the island of Barbados. A combination of land-based field mapping information and fixed wing aircraft collected high-resolution LiDAR imagery was utilised for cliff geometry assessments, supplemented for specific cliff profiles by fill-in high-precision three-dimensional (3D) digital terrestrial imagery, captured by low level wide wingspan UAV drone flights. Comprehensive photogrammetric processing of this imagery, combined with detailed geographic information system software evaluation of the collected point cloud data, allowed generation of full 3D wireframe digital terrain and digital elevation models (DTMs and DEMs). Multiple representative cliff areas around the island were identified, allowing thorough 3D stability assessment to be accomplished of key problematic areas using Flac3D. Vertical cross-section profiles were also cut so that undercut and notched cliffs could be analysed in detail in two dimensions using Voronoi tessellation approaches applied within the universal distinct element code UDEC as a means to replicate the characteristics of the vuggy coralline limestone cliffs.

A New Approach of Coastal Cliff Monitoring using Mobile Laser Scanning

Abstract The article proposes a new approach of coastal cliff monitoring which bases on a new comprehensive monitoring system, being a combination of mobile scanning from the sea with the geotechnical stability analysis. Mobile laser scanning is an innovative solution for 3D data collection which allows the monitored object geometry to be precisely measured, thus providing opportunities for series of analyses contributing to the development of an improved monitoring system. The authors present selected cliff profiles obtained from mobile scanning in the area of Jastrzebia Gora, complemented by relevant geotechnical analyses. The analysis of current cliff geometry is accompanied by calculations of hypothetical cliff profile changes resulting from possible action of nature forces. Finally, a comprehensive strategy for coastal cliff monitoring is proposed.

RocFall Analysis: The Distance and Size of Rock Fall from Brighton to Eastbourne

A study was performed along 30 km distance of the coast of South England to estimate the size of falling blocks and their extent using RocFall and other computer programmes. In addition to computer analysis, observational analysis was used to identify many factors affecting the size and the end point of falling rocks. The analysis was based along the coastal Chalk cliffs from Brighton to Eastbourne. It involved measurements of dip and dip direction, classification and characteristics of rocks as well as noting the history of previous rock fall in relation to the extent and size. Nine areas were examined and reported according to their location except for Brighton which was divided according to the cliff protection methods used. For each area a cliff profile was drawn and information about its height, cliff sloping angle, geological formations, and type of cliff protection used, if any, were listed. Types of common failures were also identified from observation during site reconnaissance and from performed failure analysis using Dips programme. As a result, falling block size and rocks end point, to the worst case, could be estimated and judgment about the efficiency of protection methods used in protected areas has been made.

Short- and long-term thermo-erosion of ice-rich permafrost coasts in the Laptev Sea region

Abstract. Permafrost coasts in the Arctic are susceptible to a variety of changing environmental factors all of which currently point to increasing coastal erosion rates and mass fluxes of sediment and carbon to the shallow arctic shelf seas. Rapid erosion along high yedoma coasts composed of Ice Complex permafrost deposits creates impressive coastal ice cliffs and inspired research for designing and implementing change detection studies for a long time, but continuous quantitative monitoring and a qualitative inventory of coastal thermo-erosion for large coastline segments is still lacking. Our goal is to use observations of thermo-erosion along the mainland coast of the Laptev Sea, in eastern Siberia, to understand how it depends on coastal geomorphology and the relative contributions of water level and atmospheric drivers. We compared multi-temporal sets of orthorectified satellite imagery from 1965 to 2011 for three segments of coastline ranging in length from 73 to 95 km and analyzed thermo-denudation (TD) along the cliff top and thermo-abrasion (TA) along the cliff bottom for two nested time periods: long-term rates (the past 39–43 yr) and short-term rates (the past 1–4 yr). The Normalized Difference Thermo-erosion Index (NDTI) was used as a proxy to qualitatively describe the relative proportions of TD and TA. Mean annual erosion rates at all three sites were higher in recent years (−5.3 ± 1.3 m a−1) than over the long-term mean (−2.2 ± 0.1 m a−1). The Mamontov Klyk coast exhibits primarily spatial variations of thermo-erosion, while intrasite-specific variations caused by local relief were strongest at the Buor Khaya coast, where the slowest long-term rates of around −0.5 ± 0.1 m a−1 were observed. The Oyogos Yar coast showed continuously rapid erosion up to −6.5 ± 0.2 m a−1. In general, variable characteristics of coastal thermo-erosion were observed not only between study sites and over time, but also within single coastal transects along the cliff profile. Varying intensities of cliff bottom and top erosion are leading to diverse qualities of coastal erosion that have different impacts on coastal mass fluxes. The different extents of Ice Complex permafrost degradation within our study sites turned out to influence not only the degree of coupling between TD and TA, and the magnitude of effectively eroded volumes, but also the quantity of organic carbon released to the shallow Laptev Sea from coastal erosion, which ranged on a long-term from 88 ± 21 to 800 ± 61 t per km coastline per year and will correspond to considerably higher amounts, if recently observed more rapid coastal erosion rates prove to be persistent.

PROCESS–RESPONSE COASTAL BLUFF RECESSION MODEL, APPLICATION TO HOLDERNESS COAST (UK)

Coastal erosion and coastal instability, threatens property, businesses and life. Because of the great concentration of natural resources in coastal zones, it is imperative that coastal change is well understood to allow for effective management and, where necessary, engineering intervention. The development of cliff erosion predictive models is mainly limited to geomorphological data because of the complex interactions between coupled processes acting in time and space result in large-scale variations. There have been few reliable process-response models of cliff recession published. These have been based on functional relationships between the dominant physical processes covering the shoreface, beach and cliff or bluff, avoiding the geotechnical retreat mechanisms and behaviour within the cliff. Under this procedure, the resulting simulations of cliffs of differing behaviour can produce identical annual retreat characteristics despite the potential responses to a changing environment being unequal.
 
 According to this inconvenience, a new process-response recession model has been developed. It incorporates the behavioural characteristics of coastal rock cliffs, the geology of which is dominated by overconsolidated clays and an associated protective talus wedge. This work brings together classic geotechnical stability analyses for rotational landslides called “Swedish” or “Fellenius” Method of slices among with collapse stability analysis for toppling mass failure. Once cliff material has fallen (landslide or toppling failure), colluvium formation at cliff foot is incorporated into the model using three different procedures, according to the geomechanical features of the weathered debris material. The first procedure is based on the friction angle for weathered materials; the second one is based on the angle of reach for landslides and the last one is based on field observations. The irregular recession cyclic pattern continues with the colluvium wedge erosion and the cliff profile steepening before a new failure occurs. Unlike previous models, the calibration parameter is restricted to account for hydrodynamic uncertainties from marine action and not includes the material strength. This new model introduces geotechnical parameters to evaluate the rock mass instability and failure through a safety factor, including RMR, cohesion, friction angle, uniaxial compressive strength of soil or rock mass of each geological component within the cliff, groundwater, etc. Among the capabilities of the model, it provides precise and stable responses to some of the inherent uncertainties in cliff or bluff recession processes, including those morphodynamics caused by different failure mechanisms, such as colluvium generation, groundwater influence, erosive tidal cycles and the cliff behaviour under changing climate conditions (i.e. sea level rise).
 
 In this work, the presented model is validated through profile evolution assessment at various locations of Holderness Coast, UK. The model reflects that the coastal oversteeping is a key factor of instability in coastal cliffs, accompanied by cliff height that determines the failure mechanism present in the study area. Higher sea levels would remove the previous failed material at the base of the cliff more quickly and cause the slope to become unstable more frequently. Sea level rise during this century produce an increasing erosion rates over soft cliffs. Additionally, higher groundwater level also produces an increase in the number and size of the slope failure. The results represent an important step-forward in linking material properties to the processes of cliff recession and the subsequent long-term response under changing environmental conditions.

Deriving mechanisms and thresholds for cliff retreat in soft-rock cliffs under changing climates: Rapidly retreating cliffs of the Suffolk coast, UK

Understanding changing thresholds and mechanisms for retreat in soft rock cliffs is important under changing climates. This can be achieved through combining detailed field observation, long-term process and morphological monitoring and numerical modelling. The cliffs of the Suffolk coast, southern North Sea have exhibited long-term (1883–2010) recession rates of 3.5ma−1, rising to 4.7ma−1 in the period 1993–2010. Annual to biannual ground survey data, and the application of GIS techniques to digitised records of changing shoreline position from historic maps and aerial photography, reveal considerable decadal-scale variations in cliff recession, within which are nested inter-annual fluctuations in rates of retreat. Archival datasets on significant periods of onshore winds and their interaction with high water levels (including the incidence of storm surges) and rainstorm events are used to determine thresholds for cliff base erosion and its propagation upwards through the cliff profile. In addition, the ‘GEO-Slope’ dynamic coupled hydrology-stability model is used to establish thresholds for cliff face failures driven by variations in rainfall inputs. Retreat mechanisms are complex, governed by cliff geology, both as a primary control on suction loss and through its interaction with basal marine conditions. The study allows a general model of cliff retreat for soft rock cliffs to be put forward, whereby a resistant basal platform is overlain by more erodible, weakly and moderately cemented sands and gravels. In this model, the varying balance between marine and terrestrial forcing factors are reflected in low (<4ma−1), intermediate (4–7ma−1) and high (>7ma−1) modes of cliff retreat.

Comparison of short-term seacliff retreat measurement methods in Del Mar, California

Seacliff retreat has been variously characterized as the recession rate of the cliff top, of the cliff base, and as the bulk recession rate based on volumetric changes of the entire cliff face. Here, these measures of retreat are compared using nine semi-annual airborne LiDAR (Light Detection And Ranging) surveys of southern California seacliffs. Changes in the cliff base location (where the steeply sloping cliff face intersects the beach) include cliff retreat owing to basal erosion, but also reflect changes in beach sand level and basal talus deposits. Averaged over the 2.5 km alongshore study span, the cliff base actually prograded seaward about 12 cm during the 4-year study. Cliff top change was dominated by few, relatively large (several meters) localized retreats. Cliff face changes, that include failures and deposits anywhere on the cliff profile, had a relatively small mean magnitude compared to cliff top changes and were more widely distributed alongshore. However, the similar alongshore averaged, cumulative cliff top and net bulk cliff face end-point retreat (14 and 19 cm, respectively) suggest that mean cumulative cliff top retreat can potentially be a viable surrogate for mean net cumulative cliff-wide erosion (and vice versa) over relatively short time periods. Cliff face erosion occurred repeatedly at some locations, confirming the presence of seacliff erosion hot-spots during the study period.

Seasonal ‘fall out’ of sessile macro-fauna from submarine cliffs: quantification, causes and implications

Submarine cliffs are typically crowded with sessile organisms, most of which are ultimately exported downwards. Here we report a 24 month study of benthic fauna dropping from such cliffs at sites of differing cliff angle and flow rates at Lough Hyne Marine Nature Reserve, Co. Cork, Ireland. The magnitude of ‘fall out’ material collected in capture nets was highly seasonal and composed of sessile and mobile elements. Sponges, ascidians, cnidarians, polychaetes, bryozoans and barnacles dominated the sessile forms. The remainder (mobile fauna) were scavengers and predators such as asteroid echinoderms, gastropod molluscs and malacostracan crustaceans. These were probably migrants targeting fallen sessile organisms. ‘Fall out’ material (including mobile forms) increased between May and August in both years. This increase in ‘fall out’ material was correlated with wrasse abundance at the cliffs (with a one month lag period). The activities of the wrasse on the cliffs (feeding, nest building and territory defence) were considered responsible for the majority of ‘fall out’ material, with natural mortality and the activity of other large mobile organisms (e.g. crustaceans) also being implicated. Current flow rate and cliff profile were important in amount of ‘fall out’ material collected. In low current situations export of fallen material was vertical, while both horizontal and vertical export was associated with moderate to high current environments. Higher ‘fall out’ was associated with overhanging than vertical cliff surfaces. The ‘fall out’ of marine organisms in low current situations is likely to provide an important source of nutrition in close proximity to the cliff, in an otherwise impoverished soft sediment habitat. However, in high current areas material will be exported some distance from the source, with final settlement again occurring in soft sediment habitats (as current speed decreases).

Mathematical Modeling of Basal Coastal Cliff Erosion in Uniform Strata: A Theoretical Approach

This article utilizes the general assumption that cliff erosion is dependent on the strength of wave assailing forces and the resistance of the cliff material. It considers the possibility of understanding and describing coastal cliff erosion processes, particularly basal erosion, through a mathematical analysis of one of the clearest manifestations of its action, i.e., the development of the basal cliff notch through mechanical wave action. It analyzes profile change through time, assuming a cliff composed of uniform strata. Deterioration of the cliff is expressed by means of the erosion function, which in this study is related to the cliff erosion rate. An equation, termed the Belov, Davies, Williams (BDW) equation, has been formulated for a moving surface, and this is proposed as a mathematical model of cliff profile change. Explicit solutions of the BDW equation are found for both steady state and time‐dependent erosion rates. In both cases, it is assumed that wave erosion intensity decreases exponentially with height. Solutions obtained showed that cliff retreat, related to basal notching, depends upon the erosion intensity and magnitude of the geophysical parameters. The model predicted the occurrence of cliff basal notching, the depth of which is comparable to field observations and can be applied to other locations. It is believed the approach and model has the potential to be developed further for other structural contexts such as locations with a variable lithology.

The characteristics and rates of the various slope degradation processes in the Barton Clay Cliffs of Hampshire

Summary Research is in progress to determine the nature and pattern of the degradation processes in an actively eroding stretch of the Barton Clay cliffs in the Naish Farm area of Highcliffe, Hampshire. In response to loss of beach material the rate of recession has accelerated and is currently averaging 1.9 m/year, resulting in steepening of the overall cliff angle from an average of 13° in 1947 to a maximum recorded average of 19° in 1976. The presence of three preferred bedding plane shear surfaces at various elevations within the cliff produce a benched type of cliff profile, comparable with that seen in the Gault and Lias Clay controlled cliffs of Fairy Dell, Dorset but in contrast with the published descriptions of London Clay cliffs. The degradation processes include scarp slumping, spalling (including toppling and soil falls), bench sliding (involving movement of colluvium over a preferred bedding plane), debris sliding (including movement of screes over clay scarps), mud sliding (i.e., the traditional lobate ‘mudflows’), mud runs (true flows), stream (or gully) erosion and man-related processes. Regular surveying and monitoring using a variety of techniques and having to surmount difficult field conditions, has begun to elucidate the characteristics, rates and inter-relationships of these processes. A flow chart representing the systematic transfer of soil from the in situ state via various colluvial modes (including the three bench levels) en route to the sea and an accompanying colluvial soil budget for the year July 1981 to July 1982 has been drawn up. It is shown that bench sliding is by far the most significant process in terms of volume of colluvium moved through the under-cliff, accounting for 930f the total volume of colluvium contained in the area. Mud and debris sliding, although important processes, are of relatively minor significance.

Toppling failures from alpine cliffs on ben Lomond, Tasmania

AbstractLarge cliff failures involving forward toppling over a basal hinge have occurred on more than half of the plateau edge of Ben Lomond, northeastern Tasmania. This mode of failure, which is readily identified from the columnar structure of the dolerite involved, has affected up to 107 m3 of rock at a time and a total of more than 50 × 106 m3 in all the cases which can still be identified. It represents perhaps the most important form of cliff retreat, amounting to a rate of 0.2 mm yr−1 over the last 100,000 years.Topographic evidence and joint surveys suggest that two different mechanisms have produced the topples on Ben Lomond. One has involved failure in the sediments underlying the dolerite with consequent foundering and cambering of large sections of the plateau edge. This mechanism accounts for relatively few of the Ben Lomond topples, though it includes the largest individual cases. The second mechanism, dominant in most of the topples, involved slab failure in the cliffs. Both modes of failure have been facilitated by vertical weaknesses within the bedrock and both require an initially steep cliff profile. Because of the latter requirement, which is not met on the other mountains of northeastern Tasmania, large‐scale topples are found only on Ben Lomond, and only there where glacial steepening of the cliff has been possible. Following the initial failure, topples of both types have migrated downslope by block sliding for distances up to 2 km.

AUTECOLOGICAL STUDIES OF TRIFOLIUM MOLINERII AT THE LIZARD PENINSULA, CORNWALL

Summary Trifolium molinerii (T. incarnatum ssp. molinerii) is shown to be a strongly maritime plant growing typically in head deposits on schistose cliffs and a member of the Thero‐Sedetum anglici Association of Malloch, (1971). Principle co‐ordinates analysis of quadrat data separated eight groupings, which are interpreted in terms of environmental factors in particular in relation to position on the cliff profile. The status of the clover is discussed and, although biotic influences and human interference are important factors in its present distribution, the plant is considered to be native.

Zonation of intertidal molluscs on rocky shores of Southern Somalia

The zonation of two species of chitons, 70 species of gastropods and 19 species of bivalves inhabiting the rocky cliffs and erosion platforms of four southern Somalian localities exposed in varying degrees to wave action, was plotted on the basis of quantitative collections along transects. Also included are data on the coastal morphology and algal covering. The composition and zonation of the mollusc community, which is clearly related to wave exposure and cliff profile, corresponds to that of other known continental and insular localities in the Western Indian Ocean.

A Discussion on valley slopes and cliffs in southern England: morphology, mechanics and Quaternary history - The development of an abandoned cliff in London Clay at Hadleigh, Essex

Hadleigh Cliff forms part of a line of abandoned London Clay slopes, rising to a height of generally + 40 m o.d. or more, which extends westwards from Southend-on- Sea. The cliff, with its toe level originally at about —19 m o.d., was formed by strong fluvial erosion in the Middle and Late Devensian. By the latter part of the Late Devensian erosion had virtually ceased and since then the cliff has degraded in an episodic manner, largely in response to climatic changes. Four main stages of degradation, with intermediate periods of relative stability, have been recognized and dated, as follows: (1) Late-glacial, periglacial mudsliding, associated with a toe level of —19 m o.d. (2) Early Atlantic, temperate mudsliding, associated with a toe level which was rising with the continuing Flandrian aggradation, but lay on average at about —9m o.d. (3) Early Sub-Atlantic, temperate mudsliding, taking place to the present toe level of about + 3 m o.d. (4) A late 19th century, moderately deep-seated landslide in the crest of the slope, possibly caused in part by human interference. The times at which the first three of these stages of degradation occurred are believed to represent periods of generally increased mass movement activity in much of Britain and Europe. The present morphology of Hadleigh Cliff comprises a straight 20 scarp at the crest, an irregular and actively unstable 11° degradation zone, fronted by a smoother, quasi-stable accumulation zone inclined at about 8°. From a knowledge of the volumes and dates of the various colluvial units mantling the slope, reconstructions of earlier positions of the cliff profile are made. These indicate that during the last 10 000 years the inclination of the combined degradation zone and crest scarp has declined from about 19° to 13°, while that of the accumulation zone has remained relatively constant. The accompanying recession of the cliff crest has been approximately 50 m. From the pattern and dating of the various stages of colluviation, which increase both in age and in degree of fabric breakdown from crest to toe of the slope, it is clear that the cliff is degrading from the top. This is also reflected in the fact that the zone of weathered, in situ London Clay beneath the colluvium diminishes in thickness, in general, from bottom to top of the slope and is entirely absent beneath the late 19th century landslide. In an average year the potential evaporation at Hadleigh exceeds the rainfall. As a result soil moisture deficits are unusually high and appreciable pore-water tensions in the capillary zone probably exist even at times of maximum seasonal piezometric levels. Account is taken of these in the stability analyses that are carried out, which indicate that the accumulation zone has a factor of safety of around 1.05 in com­ parison with the value of unity obtaining in the currently unstable degradation zone. A comparison between the values of (pT indicated by the back analyses and those measured on the Hadleigh colluvium in ring shear shows the latter to be appreciably the lower: the discrepancy is reduced if the effects of pore-water tensions in the capillary zone are allowed for.

The degradation of the Barton Clay Cliffs of Hampshire

Summary The Barton Clay crops out along a 4·8 km stretch of the Hampshire coast and together with its capping of Plateau Gravels forms a cliff profile averaging around 30 to 37 m high. The original classic work on the geology of the Barton Clay had suggested a total thickness of only 34·5 m; more recent work involving boring and levelling on the cliff face has shown the formation to be 46·4 m thick. Active degradation of the outcrop creates a markedly stepped profile, relatively flat bench levels being separated from one another by steeper scarp slopes. The benches are underlain by up to 6 m of slipped debris or bench rubble. The scarps are formed by exposures of the solid Barton Clay and Plateau Gravel in the case of the top scarp. The interface between the bench rubble and Barton Clay is formed by a shear surface, an underground continuation of the scarp surfaces, which has been polished and grooved as a result of the seaward movement of the bench rubble. These shear surfaces are developed along particular, identifiable stratigraphic horizons within the Barton Clay and tend to develop along these horizons irrespective of the height of the latter within the cliff profile. Variations in the morphology occur laterally along the outcrop and can be related to the erosional history and partly to the geology. Five individual processes of degradation are particularly active on the undercliffs; these comprise bench sliding which is sliding of the bench rubble over the shear surfaces, collapse of the scarp slopes or scarp slumping, wearing down of the scarp slope by weathering or scarp spalling, and erosion by mud-flows and running water. Both mud-flows and streams when well developed cut through bench rubble and into the scarp slope and locally destroy the stepped profile. The current activity of these processes is due to marine erosion of the toe, the net result of the processes being a progressive retreat of the cliff top averaging over one metre per year in some places.