SHORELINE CHANGE AT THOMPSON ISLAND,BOSTON HARBOR,MASSACHUSETTS, 1938-1977

A photogrammetric and sediment analysis is presented to illustrate the relationship between beach erosion and seacliff recession on Thompson Island, Boston Harbor, Massachusetts. Aerial photographs taken in 1938, 1952, 1963, and 1977 were measured to determine rates of shoreline change around the island. The 39-year average rate of beach erosion is 0.3 m/yr ± 2% with an average rate of cliff recession at 0.2 m/yr ± 2%. Rates of beach erosion between six orientations that reflect principal wave approach to the island were not found to be significantly different (0.05) for the 39-year period. The rank order correlation between beach erosion and cliff recession for these six orientations was moderately well correlated (0.63). To determine possible controls of cliff erosion, 58 sediment samples were collected from glacial cliffs along the shoreline. The textural composition was determined, and then tested with discriminant function analysis. Partial correlation analysis between beach erosion and cliff recession holding a surrogate for sediment size constant improved the rank order from 0.63 to 0.84. The results indicate that coarser-grained cliffs recede at faster rates, but with less erosion occurring on adjacent beaches. Alternatively, finer-grained cliffs recede at slower rates, but with greater erosion occurring on adjacent beaches.     

The development of talus slopes around Lord Howe island and implications for the history of island planation

Lord Howe Island is a small eroded remnant of a Late Miocene shield volcano. A fringing coral reef dissipates wave energy along a portion of the shoreline, but the remainder of the coast is rugged with spectacular high basaltic sea cliffs. This paper investigates the evolution of talus slopes that occur beneath the loftiest cliffs, and places this analysis within the context of a longer history of island planation that has resulted in a wide truncated shelf around the island. During the Last Glacial, when the sea level was lower than at present, talus slopes accumulated around the extent of the island's cliffed coast because material eroded from cliffs by subaerial processes could not be removed by marine processes. The survival of these slopes during the Holocene has depended on a balance achieved between rates of subaerial and marine erosion. This balance is fundamentally influenced by cliff height, as cliffs higher than 200 m are plunging or veneered by talus slopes, whereas lower cliffs have erosional shore platforms. On comparison with published erosion rates from inland basalt scarps it appears that marine processes may account for over 90 per cent of the total cliff retreat that has occurred at Lord Howe Island, yet contemporary coastal morphology attests to the significance of subaerial processes in recent times. It is likely that marine cliffing was very rapid soon after volcanism ceased, but rates of erosion decreased through time as wave energy became increasingly attenuated across a widening planation surface, and as increasing cliff heights yielded greater quantities of talus that provided protection from rapid marine erosion.     

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.     

RATES OF SLOPE DECLINE,TALUS GROWTH AND CLIFF RETREAT ALONG THE SHOMYO RIVER IN CENTRAL JAPAN: A SPACE‐TIME SUBSTITUTION APPROACH

On the basis of a space–time substitution, rates of cliff retreat and talus development overthe pasttens ofthousands of years were quantified for valley‐side slopes along the Shomyo River with a known rate of waterfall recession. Detailed profiles of the valley‐side slopes were obtained at 19 sites along the river by map reading and field survey, and the morphologic characteristics of the slopes were then measured. By combining the locations of the slopes with the recession rate of Shomyo Falls, it is estimated that the growth rate of the talus slope is 3.9–7.2 mm/yr, the rate of decline of the cliff is 2.0–4.0 × 10^?4 deg/yr, and the retreat rate of the cliff top is 6.2–11.6 mm/yr.     

An empirical–conceptual gully evolution model for channelled sea cliffs

Incised coastal channels are a specific form of incised channel that are found in locations where stream channels flowing to cliffed coasts have the excess energy required to cut down through the cliff to reach the outlet water body. The southern coast of the Isle of Wight, southern England, comprises soft cliffs that vary in height between 15 and 100 m and which are retreating at rates ≤ 1.5 m a^− 1, due to a combination of wave erosion and landslides. In several locations, river channels have cut through the cliffs to create deeply (≤ 45 m) incised gullies, known locally as ‘Chines’. The Chines are unusual in that their formation is associated with dynamic shoreline encroachment during a period of rising sea-level, whereas existing models of incised channel evolution emphasise the significance of base level lowering. This paper develops a conceptual model of Chine evolution by applying space for time substitution methods using empirical data gathered from Chine channel surveys and remotely sensed data. The model identifies a sequence of evolutionary stages, which are classified based on a suite of morphometric indices and associated processes. The extent to which individual Chines are in a state of growth or decay is estimated by determining the relative rates of shoreline retreat and knickpoint recession, the former via analysis of historical aerial images and the latter through the use of a stream power erosion model.     

Estuarine shore platforms in Whanganui Inlet, South Island, New Zealand

Whanganui Inlet is a low mesotidal environment where wave energy at the shoreline is limited due to a small fetch, a narrow entrance and tidal flat accretion to intertidal elevations. Wave energy is therefore only an erosive force at high tide and under storm conditions. Despite this low-energy environment extensive shore platforms occur within the inlet. They are sub-horizontal and range in width from 4.1 to 185.2 m with an average of 44.9 m. All the platforms are formed in sandstone of low resistance (mean N-type Schmidt Hammer rebound value of 17 ± 8) and have their seaward edges buried by intertidal sediment flats. The majority of platforms occur at around MHWN level, corresponding to the elevation of those flats. Where wave energy is highest, opposite the inlet's entrance and at those sites with the largest fetch, platforms develop to 0.5–1.0 m below MSL. A higher platform level is also found at MHWS elevations, however it appears to be relict with active erosion of its seaward edge occurring and therefore is most likely related to a higher mid-Holocene sea level. Apart from the location of the lowest platforms little correspondence is found between platform morphology and wave energy. Platform evolution appears to be intrinsically linked to the intertidal sediment flats which determine the degree of surface saturation of the bedrock and, hence, the number of wetting and drying cycles the platforms may undergo. As the seaward edge is buried platform development is primarily through retreat of the landward cliff. This process can, however, be complicated by the migration of intertidal water channels onto the seaward edge of the platforms or relative sea level fall which may rejuvenate landward retreat of the low-tide cliff.     

Generalised observations of wave characteristics on near‐horizontal shore platforms: Synthesis of six case studies from the North Island,New Zealand

Most field studies of wave processes on shore platforms in front of eroding cliffs focus on a single site, revealing aspects of wave dynamics at that location. Here, we analyse data from six platforms around northeastern New Zealand and describe the fundamental control of shore platform width, gradient and elevation on wave processes, including greater attenuation of short‐period waves at lower tidal stages and increases in longer period wave energy towards the cliff toe. These data suggest that empirical formulae developed from coral‐reef environments provide better predictions of wave height on platforms than formulae currently used in shore platform models.     

Effect of the development of notches and tension cracks on instability of limestone coastal cliffs in the Ryukyus, Japan

Notches cut by waves are currently developing at the base of vertical coastal limestone cliffs in Okinawa, Japan. The cliff height varies from 3.0–22.1 m, and the maximum notch depth is 8.8 m. Many rectangular or cubic blocks, which appear to have originated from cliff failures, are found on platforms in front of the cliffs. On the flat top surface of the cliff, tension cracks often run parallel to the cliff face. The vertical face of the cliffs displays small undulations but no sliding striation, suggesting that cliff failures have been caused by toppling rather than by shearing or sliding. We use slope stability analysis to determine the critical condition for toppling failure. Physical and mechanical properties of the cliff material were first obtained from laboratory tests. The results indicate that the strength of limestone shows a scale effect, such that the strength decreases with increasing size of the test specimens. Based on this result, we estimated the strength of a rock mass corresponding to the size of the coastal cliff. Cliff stability was then analyzed using a cantilever beam model. Comparison of the stability analysis and the dimension of fallen blocks indicates that toppling failure is strongly associated with the development of notches and tension cracks.     

Coastal cliff behaviour: Observations on the relationship between beach levels and recession rates

Beaches dissipate wave energy and regulate the frequency that the cliff foot is subject to wave attack. The relationship between beach levels and cliff recession rates has been established for Pleistocene soft rock cliffs along the North Norfolk and Suffolk coasts, UK. The results suggest that over a decadal timescale, there is a non-linear increase in the average recession rate as the beach profile area above High Water Mark (HWM) decreases. Small changes in beach level can result in significant differences in the recession rates. The impact of a unit change in beach level on the recession rate depends on the initial beach level. On a year-by-year basis, it is possible to divide the beach level and recession relationship into a series of zones with characteristic types of behaviour. At low beach levels there is high to extremely high recession with considerable variability, whereas at high beach levels there is almost zero recession with limited variability. It is concluded that historical recession rates are the product of both the past forcing events and changes in cliff–beach state. Extrapolation of historical rates can be extremely unreliable unless it is supported by an understanding of the dynamic behaviour of the cliff–beach system and the energy inputs over the observation period.     

Lateglacial and Holocene talus slope development and rockwall retreat on Mynydd Du, UK

Modification of Lateglacial and Holocene talus sheets by debris flows and gully incision on Mynydd Du, Wales, has resulted in a convergence of upper slope form characterised by an upper rectilinear slope gradient of 36°±3° and a range of concavities of c. 0.1–0.2. In most cases, gully incision and accumulation of debris cones have led to an increase in slope concavity. Evidence for talus erosion, reworking and redeposition on the upper slope emphasises secondary reworking processes, as well as primary talus accumulation on the upper slope, and permits construction of a model of talus development at Mynydd Du. On the basis of talus volume, calculation of the first rockwall retreat data set for southern Britain suggests that c. 7.1 m (84%) of overall rockwall retreat (8.5 m) took place during the Lateglacial, and only c. 1.4 m (16%) occurred during the Holocene. These figures imply that Lateglacial retreat rates ranged from 1.01 to 2.44 m ka⁻¹, with an overall mean rate of 1.23 m ka⁻¹. In contrast, Holocene rockwall retreat rates range from 0.10 to 0.17 m ka⁻¹, with a mean rate of 0.12 m ka⁻¹. Approximately 27% of cliff retreat is attributed to microgelivation. While similar to Holocene and present-day alpine environments, these Lateglacial retreat rates are one order of magnitude higher than most equivalent values for arctic sites. This reflects both ‘alpine-style’ diurnal freeze–thaw activity on Mynydd Du during the Younger Dryas and paraglacial rock-mass instability following deglaciation. Assuming an exponential decline in rockwall sediment release, it is estimated that approximately half the talus had accumulated within c. 1 ka of deglaciation. At one site, paraglacial talus accumulation appears to have contributed significantly to the glacial sediment transport system of a subsequent ice advance. Present-day rates of rockwall retreat and talus accumulation by rockfall are estimated to be 0.014 and 0.022 mm yr⁻¹ (m ka⁻¹), respectively, similar to values for other British sites and markedly lower than Holocene rates of cliff recession due to microgelivation. By implication, the geomorphic significance of microgelivation may have been greatly underestimated in studies of inland rock-slope evolution in temperate, mid-latitude environments.     

丹霞地貌概念讨论

关于丹霞地貌概念上的不同意见,主要是：①岩性岩相上是较单一的“陆相碎屑岩”还是涵盖多种岩性岩相的“沉积岩”;②地貌上是“丹崖”,还是“赤壁丹崖”,或是“赤壁丹崖群”。根据目前已知的中外丹霞地貌的基本特征,初步认为,丹霞地貌是由砂岩砾岩为主的沉积岩经侵蚀作用所形成的赤壁丹崖群地貌。近10a来,一些丹霞地貌概念,基本上属于“中国丹霞地貌”的概念,不完全符合世界丹霞地貌的实际情况。     

Radiocarbon Chronology of Ross Sea Drift, Eastern Taylor Valley, Antarctica: Evidence for a Grounded Ice Sheet in the Ross Sea at the Last Glacial Maximum

More than 250 radiocarbon dates of lacustrine algae and marine shells afford a chronology for Ross Sea drift in eastern Taylor Valley. Dates of algae that lived in ice-dammed Glacial Lake Washburn show that grounded Ross Sea ice blocked the mouth of Taylor Valley between 8340 and 23,800 ¹⁴C yr bp . Ross Sea ice was at its maximum position at the Hjorth Hill moraine between 12,700 and 14,600 ¹⁴C yr bp and was within 500m distance of this position as late as 10,794 ¹⁴C yr bp . The implication is that the flow line of the Ross Sea ice sheet which extended around northern Ross Island and across McMurdo Sound to Taylor Valley must have remained intact, and hence that a grounded ice sheet must have existed east of Ross Island as late as 8340 ¹⁴C yr bp . Evidence from ice-dammed lakes in Taylor Valley and from shells from McMurdo Sound suggests grounding-line retreat from the vicinity of Ross Island between 6500 and 8340 ¹⁴C yr bp . If this is correct, then most recession to the present-day grounding line on the Siple Coast took place subsequently in the absence of significant deglacial sea-level rise. Rising sea level may have triggered internal mechanisms within the ice sheet that led to retreat, but did not in itself drive continued ice-sheet recession. Ice retreat, once set in motion, continued in the absence of sea-level forcing. If correct, this hypothesis implies that the grounding line could continue to recede into the interior reservoir of the West Antarctic Ice Sheet.     

Shore platform development around Lord Howe Island, southwest Pacific

This paper describes the morphology of near-horizontal basalt and calcarenite shore platforms around Lord Howe Island as well as the lithological and process environment in which they occur. The morphology of platforms around the island is highly varied. For instance, they occur at a wide range of elevations between low-tide level and several metres above the highest tides, and their width ranges between just a few metres to over 100 m. However, common to most platforms is that they have near-horizontal surfaces that terminate abruptly in a steep scarp at their seaward edge. Correlations indicate that the variability in platform elevation and width is attributable to variability in key parameters of erosion, such as rock resistance and shoreline water depth. Whereas wider platforms occur in rocks of lesser resistance, platform elevation is shown to increase both as rock resistance and the depth of water at the shoreline increases. Wave exposure has a significant control on platform width in some instances, but an over-arching relationship was not detected. Most platforms around the island appear to have developed over the Holocene and a conceptual model is proposed to account for the varied morphology of platforms that have developed over this period. Resistant plunging cliffs occur along about 18% of the rocky coastline of Lord Howe Island and represent the starting point for the model. Where cliffs have yielded to wave erosion platforms have been initiated at an elevation controlled predominantly by rock resistance and shoreline water depth. At Lord Howe Island narrow platforms up to about 20 m wide in exposed locations generally occur about 4 to 6 m above high tide level, and have apparently been subject to very infrequent erosion. Shore platforms about 20 to 40 m wide have generally formed in less resistant rocks, are somewhat lower at about 2 to 3 m above high tide level, and have been subject to more frequent erosion over the Holocene. Most basalt platforms around the island fall within these first two categories, but there are some instances of platforms about 50 m wide. These platforms have formed in rocks of comparatively low resistance and are relatively low in elevation occurring close to the level of high tide. An interesting feature of some of these wide platforms is that, unlike the morphology of narrower platforms, raised ramparts sometimes occur on their outer edge.     

Sea cliffs resulting from late Miocene extensional tectonics: the Serra Gelada case study (Betic Cordillera, Spain)

The Serra Gelada sea cliffs are carved in Mesozoic carbonate rocks belonging to the External Zones of the eastern Betic Cordillera (Alicante, SE Spain). Several normal faults with vertical slips of more than a hundred metres have played an important role in the origin of this coastline. Some previous studies propose that the present cliff morphology was mainly originated by Quaternary fault activity. However, the integration of geomorphological features, stratigraphical and sedimentological data, together with the results of the tectonic analysis of fractures occurring in Serra Gelada, and a detailed study of seismic reflection profiles carried out in the adjacent continental shelf, indicate that these normal faults were active mainly during the late Miocene. Therefore, the Serra Gelada sea cliffs represent a tectonically controlled long-term landscape. Thus, normal faults have not significantly modified the Serra Gelada relief since then. Furthermore, the northern part of the Serra Gelada cliff may be considered as an inherited pre-Quaternary relict palaeocliff since it has only undergone very little erosive recession.     

WAVE ENERGY ESTIMATES AND BLUFF RECESSION ALONG LAKE MICHIGAN'S SOUTHEAST SHORE*

Recession rates for unconsolidated bluffs at 23 sites along Lake Michigan's southeast shore are compared with deep water wave energy probabilities to estimate the relative degradational importance of recurrent 5, 10, 20, 50, and 100-year storms. Recession rates are based on measured bluff crest retreat while wave energies are calculated using standard water wave theory and data interpolated from a meteorologically based hindcast model. Correlation and regression tests suggest the following: (1) although wave energy is certainly a destructive factor, it explains less than half of the variation in bluff crest recession, supporting the interpretation that shorezone erosion here results from the interaction of numerous factors; and (2) over long time periods the total effect of more frequent moderate intensity storms is greater than that from rare, especially high energy events.     

Former glacial lakes of the Bunger Hills,Antarctica

The Bunger Hills in East Antarctica occupy a land area of approximately 400 km². They have been exposed by Holocene retreat of the Antarctic ice sheet and its outlet glaciers. The accompanying sea level rise flooded the marine inlets that now separate the northern islands and peninsulas from the major part of the hills. During deglaciation the continental ice sheet margin retreated south‐eastwards with several temporary halts, during which ice‐dammed lakes were formed in some valleys. These lakes were maintained long enough to permit formation of beaches of sand and gravel, and for the erosion of shore platforms and low cliffs in bedrock. Around the western end of Fish Tail Bay impressive shoreline features 20 m above sea level define a former ice‐dammed lake that was 5.5 km long. A similar 7 km long former ice‐dammed lake was formed at Lake Dolgoe. The more extensive and deeper glacial lake is revealed by well‐developed and preserved shoreline features cut at 29 m which is 16 m above present lake level. In addition, several small ice‐dammed lakes existed temporarily near Lake Shchel and in the valley to the west. Lake Fish Tail existed more than 6,900 ¹⁴C years ago and Lake Shchel probably more than 6,680 ¹⁴C years ago. It is inferred that the shore platforms and beaches were formed by lake ice and wave action over considerable periods when the lakes were impounded by steep cold ice margins. There appears to have been a balance between meltwater input and evaporative loss from the lakes in the cold dry continental climate. There is no evidence for rapid lake level fluctuations, and there was very little input of clastic sediment. This resulted in poor development of deltaic and rhythmically laminated lake floor deposits. It is suggested that such deposits are more characteristic of ice‐dammed lakes formed in association with wet‐based temperate ice than those associated with dry‐based polar ice.     

Beach morphology and change along the mixed grain-size delta of the dammed Elwha River, Washington

Sediment supply provides a fundamental control on the morphology of river deltas, and humans have significantly modified these supplies for centuries. Here we examine the effects of almost a century of sediment supply reduction from the damming of the Elwha River in Washington on shoreline position and beach morphology of its wave-dominated delta. The mean rate of shoreline erosion during 1939–2006 is ~ 0.6 m/yr, which is equivalent to ~ 24,000 m³/yr of sediment divergence in the littoral cell, a rate approximately equal to 25–50% of the littoral-grade sediment trapped by the dams. Semi-annual surveys between 2004 and 2007 show that most erosion occurs during the winter with lower rates of change in the summer. Shoreline change and morphology also differ spatially. Negligible shoreline change has occurred updrift (west) of the river mouth, where the beach is mixed sand to cobble, cuspate, and reflective. The beach downdrift (east) of the river mouth has had significant and persistent erosion, but this beach differs in that it has a reflective foreshore with a dissipative low-tide terrace. Downdrift beach erosion results from foreshore retreat, which broadens the low-tide terrace with time, and the rate of this kind of erosion has increased significantly from ~ 0.8 m/yr during 1939–1990 to ~ 1.4 m/yr during 1990–2006. Erosion rates for the downdrift beach derived from the 2004–2007 topographic surveys vary between 0 and 13 m/yr, with an average of 3.8 m/yr. We note that the low-tide terrace is significantly coarser (mean grain size ~ 100 mm) than the foreshore (mean grain size ~ 30 mm), a pattern contrary to the typical observation of fining low-tide terraces in the region and worldwide. Because this cobble low-tide terrace is created by foreshore erosion, has been steady over intervals of at least years, is predicted to have negligible longshore transport compared to the foreshore portion of the beach, and is inconsistent with oral history of abundant shellfish collections from the low-tide beach, we suggest that it is an armored layer of cobble clasts that are not generally competent in the physical setting of the delta. Thus, the cobble low-tide terrace is very likely a geomorphological feature caused by coastal erosion of a coastal plain and delta, which in turn is related to the impacts of the dams on the Elwha River to sediment fluxes to the coast.     

大同-阳原盆地古泥河湾湖的岸线变化

本文通过大同-阳原盆地古湖岸线遗迹的分析,阐述第四纪古泥河湾湖的出现、发展和消亡,指出湖泊变迁是构造运动和气候变化等因素共同作用的结果,构造运动是主要因素。     

Processes and rates of rock fragment displacement on cliffs and scree slopes in an amba landscape, Ethiopia

Distinct rock fragment displacements occur on the ambas, or structurally determined stepped mountains of the Northern Ethiopian Highlands. This paper describes the rock fragment detachment from cliffs by rockfall, quantifies its annual rate, and identifies factors controlling rock fragment movement on the scree slopes. It further presents a conceptual model explaining rock fragment cover at the soil surface in these landscapes. In the May Zegzeg catchment (Dogu'a Tembien district, Tigray), rockfall from cliffs and rock fragment movement on debris slopes by runoff and livestock trampling were monitored over a 4-year period (1998–2001). Rockfall and rock fragment transport mainly induced by livestock trampling appear to be important geomorphic processes. Along a 1500-m long section of the Amba Aradam sandstone cliff, at least 80 t of rocks are detached yearly and fall over a mean vertical distance of 24 m resulting in a mean annual cliff retreat rate of 0.37 mm y^− 1. Yearly unit rock fragment transport rates on scree slopes ranged between 23.1 and 37.9 kg m^− 1 y^− 1. This process is virtually stopped when exclosures are established. Corresponding mean rock fragment transport coefficients K are 32–69 kg m^− 1 y^− 1 on rangeland but only 3.9 kg m^− 1 y^− 1 in densely vegetated exclosures. A conceptual model indicates that besides rockfall from cliffs and argillipedoturbation, all factors and processes of rock fragment redistribution in the study area are of anthropogenic origin.     

Geomorphologic and Palaeoenvironmental Development of Holocene Perched Coastal Dune Systems in Brittany, France

Two perched dune systems are investigated in the Baie d'Audierne (Finistère, Brittany, northwest France). A maximum age of 4545–4160 cal. yr bp for the onset of sand accretion is provided through radiocarbon (¹⁴C) analysis of an organic-rich basal palaeosol, corresponding closely to deceleration, but before stabilisation, of regional Holocene relative sea levels (RSLs). Molluscan analysis through the dune sand indicates an initial bare sand or sparsely vegetated surface, subsequently maturing and colonised by scrub vegetation, and finally returning to more open dune grassland conditions. These data suggest a three-stage Perched Dune Development Model (PDDM), which may be applicable to perched dunes on at least a regional scale: (1) initial sand sheet inundation as RSL rises; (2) stabilisation of RSL allowing dune to mature; and (3) sea-level stabilisation stimulates cliff formation, perching and progressive landward retreat of the dune shore. This study recognises that perched dunes hold greater potential for establishing chronologies for regional Holocene dune development than the more widely studied lowland dune systems, where evidence for early sand inundation may be obscured or lost by modest rises in sea level.