Talus Accumulation Research Articles

Late Pleistocene to Holocene glacial, periglacial, and paraglacial geomorphology of the upper Río Limarí basin (30–31° S) in the Andes of central Chile

ABSTRACT We present a field-based reconstruction of the geomorphology in the Subtropical Andean mountains of the Limarí basin, semiarid central Chile (30–31° S). Fieldwork campaigns and remote-sensing analysis served for detailed geomorphological mapping at four formerly glaciated valleys in the heads of the Combarbalá and Río Hurtado sub-basins. We identify a mosaic of glacial, periglacial, and paraglacial landforms. Glacial landforms include a massive dead-ice moraine complex, with thermokarst and debris-filled fractures suggesting former ice-cored moraine degradation. This landform is superimposed by transversal and arcuate ridges suggesting active-ice processes. Periglacial landforms such as rock glaciers, gelifluction, and protalus lobes occur in cirques and U-shaped valleys, but also on moraine deposits. Paraglacial processes are indicated by talus accumulation in those formerly glaciated slopes. The geomorphological imprint is evidence for the interaction and succession between glacial, periglacial, and paraglacial dynamics from the Late Pleistocene to the present.

Holocene warming of alpine rockwalls decreased rockwall erosion rates

Alpine rockwall erosion studies suggest that deglaciated rockwalls in the European Alps are eroding slower today than at earlier times in the Holocene. Explanations for this have included a waned glacial debuttressing effect since the retreat of Last Glacial ice loads and the establishment of more moderate climates, but seldom have such explanations been robustly tested. We combine field data with modelling to reconstruct changes in rockwall erosion and associated climate drivers, in an alpine valley of the European Alps since the beginning of the Holocene. Paleo (Holocene to decadal-scale) erosion rates were calculated from talus accumulation below rockwalls, and compared with recent rates (2016–2019) measured from repeat laserscan surveys of the same rockwalls. We reconstructed the glacial retreat history in the valley, and modelled Holocene changes in permafrost distribution and frost cracking using calibrated rockwall temperature reconstructions. We found that rockwalls that have been free of glacier ice since ∼10 ka experienced higher Holocene-averaged erosion rates compared to recent erosion rates. Our modelling suggests this relates to periods of higher intensities of frost cracking and cycles of permafrost aggradation and degradation in the Holocene, relative to today. For a recently deglaciated high-elevation rockwall, erosion rates were 1–2 orders of magnitude greater than the lower-elevation sites, but decayed rapidly over time since deglaciation. A high, but rapidly decaying rate results from short-lived paraglacial adjustment, permafrost thaw and high frost cracking activity. Our findings suggest that periglacial activity strongly influences the rates and patterns of erosion of deglaciated alpine rockwalls. This helps to explain why paleo Holocene deglaciated rockwall erosion rates tend to exceed recent rates in the European Alps.

Inferring rockfall frequency-magnitude relationships and talus accumulation times from lichenometric study of talus deposits, Glenwood Canyon, CO, USA

Rockfall is a frequent hazard in mountainous areas of the world. Rockfall hazard assessment and mitigation design typically require design volumes of blocks and design frequencies of events. However, constraining the frequency of rockfall is challenging due to limited data, limited timespans of data collection, and difficulty of estimating dates for past rockfalls. This paper presents a study of rockfall frequency from natural rock slopes in Glenwood Canyon, Colorado using lichenometry measurements of Lecanora novomexicana and Lecidea atrobrunnea lichens from 1070 boulders from six talus deposits. The lichen ages are used to develop frequency-magnitude relationships for rockfall, to calculate recurrence intervals for selected volumes, and to estimate the time required to develop the observed talus deposits. Power laws fit to frequency-magnitude distributions of lichen ages yield b parameters ranging from −0.61 to −1.04. Based on these distributions, recurrence intervals for selected rockfall magnitudes include: sub-annual to annual for 0.1 m3, a few years to decades for 1 m3, and several decades for 10 m3 rockfalls. These estimated recurrence intervals appear to agree with observed signs of rockfall activity from the studied slopes and with rockfall data from the limited historical record for Glenwood Canyon. Mapped lichen ages indicate no preferential spatial distribution of rockfalls, implying the majority of rockfall events are small relative to the scale of the deposit. A comparison of the estimated volumetric rockfall accumulation rates with estimated total talus deposit volumes suggests that each of the studied deposits has a depositional history extending back longer than the beginning of the Holocene.

Depositional history of a talus cone in an arid intermontane basin in Central Asia: An interdisciplinary study at the Late Pleistocene–Late Holocene Obishir‐I site, Kyrgyzstan

AbstractCliff‐related talus accumulations are often highly affected by post‐depositional processes, and the sedimentological characteristics are poorly documented, especially in arid settings. In the southern margin of the Fergana Valley, Kyrgyzstan, the Obishir‐I is an archaeological site of the Epipaleolithic Obishirian industry, located within a talus cone. Archaeological excavations have allowed us to study the internal structure, chronology, and depositional history of the cone as well as the relationship between the slope processes and the archaeological assemblage. We applied a multiproxy approach, including sedimentological studies supported by the basic geochemistry of sediments, luminescence dating, paleoecology of fossil mollusks, and archaeological analyses. The Obishir‐I rockfall talus accumulated from the Last Glacial Maximum to the early part of the Middle Holocene. The talus onlaps the bedrock at the foot of a limestone cliff, and its chronology is coherent, spanning from ca. 19–8 ka B.P. The material is an unconsolidated, poorly sorted mixture of angular rock fragments accumulated by rockfall and fine‐grained materials accumulated by aeolian processes, illuviation, and in situ weathering. The sequence bears an archaeological collection attributed to two settlement phases: unidentified Upper Paleolithic and the Obishirian. Both assemblages are affected by post‐depositional relocation within the slope.

Origin, structure and geochemistry of a rock glacier near Don Juan Pond, Wright Valley, Antarctica

AbstractThe South Fork of Wright Valley contains one of the largest rock glaciers in the McMurdo Dry Valleys, Antarctica, stretching 7 km from the eastern boundary of the Labyrinth and terminating at Don Juan Pond (DJP). Here, we use results from ground-penetrating radar (GPR), qualitative field observations, soil leaching analyses and X-ray diffraction analyses to investigate rock glacier development. The absence of significant clean ice in GPR data, paired with observations of talus and interstitial ice influx from the valley walls, support rock glacier formation via talus accumulation. A quartz-dominated subsurface composition and discontinuous, well-developed desert pavements suggest initial rock glacier formation occurred before the late Quaternary. Major ion data from soil leaching analyses show higher salt concentrations in the rock glacier and talus samples that are close to hypersaline DJP. These observations suggest that DJP acts as a local salt source to the rock glacier, as well as the surrounding talus slopes that host water track systems that deliver solutes back into the lake, suggesting a local feedback system. Finally, the lack of lacustrine sedimentation on the rock glacier is inconsistent with the advance of a glacially dammed lake into South Fork during the Last Glacial Maximum.

Characterizing the catastrophic 2017 Mud Creek landslide, California, using repeat structure-from-motion (SfM) photogrammetry

Along the rugged coast of Big Sur, California, the Mud Creek landslide failed catastrophically on May 20, 2017, and destroyed over 400 m of scenic California State Highway 1. We collected structure-from-motion (SfM) photogrammetry data using airborne platforms that, when combined with existing airborne lidar data, revealed that the area exhibited significant topographic change and displacement before, during, and after the catastrophic failure. Before the catastrophic failure, we document two areas of elevated change in the zone of depletion, which aligned with the double-peaked head scarp produced by the catastrophic failure. The catastrophic failure extended from 337-m elevation to at least 8 m below sea level, was 490 m wide, displaced ~ 3 million m3 of earth and rock, and deposited landslide debris at least 175 m seaward of the original shoreline. The failure was not a complete slope-clearing event, however, and several upslope and lateral regions that did not slip into the ocean exhibited significant displacement and topographic change during the days and months after the catastrophic failure. Additionally, we use the post-slide data to quantify several other processes, including the time-varying rates of talus accumulation and coastal erosion of the landslide toe. We conclude that repeat SfM surveys from aerial imagery can provide valuable information about landslide evolution and the potential for deep-seated landslide hazards—especially in the lead up to catastrophic failure—if photos are collected and processed regularly.

After the ice: Lateglacial and Holocene landforms and landscape evolution in Scotland

ABSTRACTDuring Lateglacial cold periods, permafrost developed throughout Scotland, sediment-mantled slopes were extensively modified by solifluction and other forms of periglacial mass movement, large-scale sorted patterned ground formed on plateaux, and enhanced rockfall resulted in talus accumulation below cliffs. Most rock-slope failures occurred within five millennia following ice-sheet deglaciation, with many probably triggered by uplift-induced earthquakes; numerous debris-free scarps represent sites where Lateglacial rockslide debris was excavated by glaciers during the Younger Dryas Stade (∼12.9–11.7 ka). Sandar and outwash fans deposited by glacial rivers during ice-sheet retreat were incised to form high-level terraces. Under the cool temperate but relatively stable climate of the Holocene, solifluction and patterned ground formation continued to operate on high ground, though accumulation of high-level aeolian deposits on most mountains was terminated by erosion during the Little Ice Age. Drift-mantled slopes and talus slopes have been extensively eroded by translational failures and debris flows, the latter depositing debris cones on upland valley floors. The incidence of Holocene rockslides has been much lower than during the Lateglacial period. Dating of alluvial deposits and low Holocene terraces suggests no consistent pattern of Holocene floodplain evolution: incision has apparently dominated in the Highlands, aggradation in the lowlands, and floodplains in the Southern Uplands have asynchronous histories of incision and aggradation. Studies of floodplain behaviour over the past 200–300 years suggest that though major floods rework the floodplains of braided and piedmont rivers, there is no tendency towards net floodplain aggradation or incision. Most valley-side alluvial fans accumulated episodically in the last 4000 years, many in response to lowering of hillslope stability by woodland clearance. For many postglacial landsystems, disentangling the effects of declining paraglacial sediment supply, climate change and local influences (extreme rainstorm events or anthropogenic impacts) remains challenging.

Quantification of controls on regional rockfall activity and talus deposition, Kananaskis, Canadian Rockies

Rockfall is a significant geomorphic process in many mountainous regions that also poses a notable natural hazard risk. Most previous studies of rockfall erosion have investigated the mechanics and rates of local rockwall retreat and talus deposition, with only a few investigations of rockfall and/or associated talus considering larger spatial scales (i.e., drainage basin, mountain range). The purpose of the current research is to investigate the areal extent of rockfall-talus and controlling factors of its distribution over regional spatial scales (of order 102km2) in Kananaskis, Canadian Rockies to inform our understanding of its significance in mountain development. To achieve this goal, a large talus inventory is collected and analyzed for 11 steep tributaries of the Kananaskis River, Canadian Rockies. Talus accumulations associated with rockfall provide evidence about the nature and rates of rockfall activity that supplies sediment to these deposits and are the focus of the present study. To quantify the controls of rockfall-talus activity in this region, we analyze the association of talus deposits with structural geology, glacial topography, and temperature-related weathering (i.e., frost cracking). A total of 324 talus polygons covering a surface area of 28.51km2 are delineated within the 11 study basins, with the number of talus polygons in each study basin ranging from 1 to 73. Analysis of the talus inventory shows that cirques and glacially sculpted valleys are locations of notable talus accumulation in Kananaskis, with other locations of significant talus deposition being associated with thrust faults. We also found that the upper elevations at which talus deposits are typically found are the general range of elevations experiencing a notable number of days in the frost cracking window when this window is defined as −3 to −15°C; no such association is found for the frost cracking window of −3 to −8°C. Estimates of average erosion rates for all study basins combined are between 0.15mmy−1 (lower estimate) to 3.1mmy−1 (upper estimate). Rockfall activity is expected to have been most active for the several millennia following deglaciation (during the paraglacial period) when hillslopes were oversteepened.

Scottish landform examples 44: the Dirc Mhòr meltwater gorge

ABSTRACTThe Dirc Mhòr gorge in the central Grampian Highlands is 1.1 km long, ∼250 m wide and up to 110 m deep, and is incised in granite bedrock. It is the largest of several bedrock gorges that are incised into spurs, cols or plateau margins in the area, and its present catchment area is negligible. The floor of the gorge is covered by coarse rockfall debris, and the sides of the gorge are mantled by talus accumulations; surface water flow is absent. The gorge is inferred to have evolved over several glacial-interglacial cycles, with subglacial meltwater erosion deepening the gorge under successive ice sheets, and paraglacial rockwall collapse widening the gorge during intervening interglacial periods. Subglacial erosion may also have contributed to gorge enlargement as it widened.

Alpine glaciers variation and stress strain effect due to deglaciation

Paraglacial geomorphology is the study of earth-surface processes, sediments, landforms, landsystems and landscapes that are directly conditioned by former glaciation and deglaciation. The withdrawal of glacier ice exposes landscapes that are in an unstable or metastable state, and consequently liable to modification, erosion and sediment release at rates greatly exceeding background denudation rates. This paper (1) reviews research on paraglacial processes, landforms and landscape change in a range of geomorphological settings; (2) explores the importance of paraglacial landscape modification and sediment recycling as a component of alternating glacial/nonglacial landscape evolution; (3) assesses the nature and significance of paraglacial facies in Quaternary stratigraphic sequences; and (4) develops a general model of the sequence of paraglacial landscape modification and the changing nature of paraglacial landsystems.Six paraglacial landsystems are identified: rock slopes, drift-mantled slopes, glacier forelands, and alluvial, lacustrine and coastal systems. Each contains a wide range of paraglacial landforms and sediment facies. Collectively these landforms and sediments (e.g. talus accumulations, debris cones, alluvial fans, valley fills, deltas and coastal barrier structures) can be conceptualised as storage components of an interrupted sediment cascade with four primary sources (rockwalls, drift-mantled slopes, valley-floor glacigenic deposits and coastal glacigenic deposits) and four terminal sediment sinks (alluvial valley-fill deposits, lacustrine deposits, coastal/nearshore deposits and shelf/offshore deposits). Paraglacial sediment stores and sinks may form major sources of readily erodible sediment during the early stages of glacial cycles, leading to high rates of sediment transport during periods of glacier or ice-sheet expansion. Probably because of the limited preservation potential of paraglacial sediments that were subsequently over-run by glacier ice, identification of paraglacial facies in both terrestrial and marine settings has been almost exclusively limited to sequences that post-date the Last Glacial Maximum.The unifying concept of paraglacial geomorphology is that of glacially conditioned sediment availability. Relaxation of landscape elements to nonglacial conditions operates over timescales of 101–>104 years, and is conditioned by both process and spatial scale. Rate of sediment reworking can be described by an exhaustion model. In the case of primary reworking of glacigenic sediment, the rate of reworking declines approximately exponentially through time, though extrinsic perturbation may rejuvenate paraglacial sediment flux long after termination of the initial period of paraglacial adjustment. Landscape-scale (particularly alluvial and coastal) systems may exhibit intrinsically complex responses due to reworking of secondary paraglacial sediment stores. The long relaxation time of such systems implies that many areas deglaciated in the Late Pleistocene or Early Holocene have still not fully adjusted (in terms of sediment supply) to nonglacial conditions.

Geochronology, biostratigraphy, and changing shell morphology in the land snail subgenus Poecilozonites during the Quaternary of Bermuda

Shells from over 280 collections representing all known forms of subgenus Poecilozonites (Zonitidae) from Bermuda have been ranked in stratigraphic order and relative-age aminostratigraphy. The samples were first grouped by their source lithostratigraphic unit, with additional refinements being made based on the epimerization ratio d-alloisoleucine/ l-isoleucine or A/I from nearly 400 representative individual shells of each of the forms. A plot of A/I frequency defines several modes or “aminozones”. The aminozones correlate strongly with defined lithostratigraphy, with some minor variation resulting from less optimally preserved shells. The succession of land snails is ranked over chronological range from the middle and early Pleistocene, with greatest resolution since 500 ka. This > 500 ka age-range and density of sampling is among the most extensive for any organism on an oceanic island. Morphological analysis of Poecilozonites demonstrates oscillation and repetition of morphotypes over multiple sea-level cycles. Shells from the small island(s) that formed during interglacial times are generally small and show little variation in form. During glacial periods, when Bermuda reached maximal size, shells are much larger. Extreme variation in shell size and shape occurs at interglacial–glacial transitions in as little as 2–4 ka. Because the subgenus has been endemic to Bermuda for at least ca. 1000 ka, such dramatic and rapid shifts in shell morphology cannot be the result of recolonization from outside sources. An intact and undisturbed 130-ka-long biostratigraphic record of Poecilozonites is contained in a single, vertical 2 m-thick talus accumulation in Admirals Cave that has been dated at high resolution by three independent techniques. The bio- and chronostratigraphy show that snails were very large at the close of glacial stage marine isotope stage (MIS) 6, small during interglacial MIS 5, re-evolved identical large size during the last glaciation (MIS 4-2), and changed abruptly to the small paedomorphic modern form at the onset of the Holocene (MIS 2-1) about 10–12 ka ago. When assessed with care, shells of Poecilozonites may be used as guide fossils for rapid field identification of lithostratigraphic units in surface deposits and as chronological indicators in cave and fissure fills when used in combination with vertebrate remains.

Influence of rock mass strength on the erosion rate of alpine cliffs

AbstractCollapse of cliff faces by rockfall is a primary mode of bedrock erosion in alpine environments and exerts a first‐order control on the morphologic development of these landscapes. In this work we investigate the influence of rock mass strength on the retreat rate of alpine cliffs. To quantify rockwall competence we employed the Slope Mass Rating (SMR) geomechanical strength index, a metric that combines numerous factors contributing to the strength of a rock mass. The magnitude of cliff retreat was calculated by estimating the volume of talus at the toe of each rockwall and projecting that material back on to the cliff face, while accounting for the loss of production area as talus buries the base of the wall. Selecting sites within basins swept clean by advancing Last Glacial Maximum (LGM) glaciers allowed us to estimate the time period over which talus accumulation occurred (i.e. the production time). Dividing the magnitude of normal cliff retreat by the production time, we calculated recession rates for each site. Our study area included a portion of the Sierra Nevada between Yosemite National Park and Lake Tahoe. Rockwall recession rates determined for 40 alpine cliffs in this region range from 0·02 to 1·22 mm/year, with an average value of 0·28 mm/year. We found good correlation between rockwall recession rate and SMR which is best characterized by an exponential decrease in erosion rate with increasing rock mass strength. Analysis of the individual components of the SMR reveals that joint orientation (with respect to the cliff face) is the most important parameter affecting the rockwall erosion rate. The complete SMR score, however, best synthesizes the lithologic variables that contribute to the strength and erodibility of these rock slopes. Our data reveal no strong independent correlations between rockwall retreat rate and topographic attributes such as elevation, aspect, or slope angle. Copyright © 2009 John Wiley & Sons, Ltd.

Talus structure and evolution on sandstone mountains in NW Scotland

Twelve gully-wall sections excavated in the upper rectilinear slope of three talus accumulations in NW Scotland reveal a common stratigraphy, similar to that previously described elsewhere in upland Britain: a lower unit of in situ rockfall debris and an upper unit of debris-flow and/or slopewash deposits, often overlying or intercalated with buried organic soils. The upper unit indicates downslope reworking of sediments derived from the top of the talus. Radiocarbon dating of buried organic soils implies that the onset of talus reworking was asynchronous, and occurred before 6.6—7.0 cal. kyr BP at one site (roughly 5.0 kyr after deglaciation) but after c. 2.7 cal. kyr BP at another. A general model for talus evolution in upland Britain is proposed, based on the geomorphological, stratigraphic and dating evidence of this and earlier studies. This comprises: (1) initial accumulation of rockfall debris within a few millennia following deglaciation; (2) near-cessation of rockfall accumulation and development of gullies near the talus crest, with concomitant redistribution of eroded talus sediments on the upper slope by debris flows and slopewash; (3) downslope extension of gullies with associated erosion of both talus and overlying debris-flow and slopewash deposits, and redistribution of these sediments as slope-foot debris cones. This sequence can be explained as an intrinsic response to diminution of paraglacial rockfall inputs, and does not necessarily imply any change in climate, frequency of extreme rainstorm events or other extrinsic effects.

Clast fabric and mass wasting on Asteroid 25143-Itokawa: Correlation with talus and other periglacial features on Earth

Regolith on Itokawa, a 300-meter diameter near-Earth asteroid, first imaged by the Hayabusa Spacecraft in 2005, provides a new database of information on surface features and different space-form (i.e. landform) assemblages. The satellite imagery was taken from distances of near-surface and 2 km to 63 m, the latter giving image resolutions of 6 mm/pixel. Small regolith-covered asteroids produce rubble-covered surface mantles, resident on slopes ranging from 27° relative to an artificial horizon, and distributed non-uniformly across the surface. Low gravity acceleration on Itokawa means that clast movement is largely the result of global vibrations caused by impacts producing seismic shaking. Gravel-network orientations, previously correlated with imbricated stream gravels on Earth by the Hayabusa Scientific Team (Miyamoto, H., Yano, H., Scheeres, D.J., Abe, S., Barnaouin-Jha, O., Cheng, A.F., Demura, H., Gaskell, R.W., Hirata, N., Ishiguro, M., Michikami, T., Nakamura, A.M., Hakamura, R., Saito, J., Sasaki, S., 2007. Regolith migration and sorting on asteroid Itokawa. Science. 316, 1011–1014, Fig. 2F), are here partly correlated with talus and stone-banked lobe accumulations on Earth. We present ‘sliderock’ accumulations on Itokawa correlated with talus accumulations on Earth from several high, middle and low latitude Cryic localities. The presence of debris-flow accumulated clastic material indicates the presence of permafrost and issuance of meltwater at some time in the past, presumably linked to an orbital or solar radiation perturbation. The available data indicate the presence of water, at least on a punctuated time scale; along with the presence of Fe, the combination of the two suggesting weathering processes and a dynamic surface geology on what was previously considered to be a dry, barren, lifeless world.

Glassy fragmental rocks of Macquarie Island (Southern Ocean): Mechanism of formation and deposition

Glassy fragmental rocks are interlayered with pillow basalt and tabular basalt on Macquarie Island (54°30′ S, 158°54′ E). These facies formed along the Proto-Macquarie Spreading Ridge between 6 and 12 Ma and have since been uplifted and exposed on the apex of the Macquarie Ridge Complex. Through a combination of field and microscopic analyses, we investigate the submarine production, transportation, deposition and lithification of basalt and sideromelane clasts within a spreading-ridge environment. The findings of this study indicate that these glassy grains form predominantly by cooling-contraction granulation of pillow lava rinds while crystalline basalt clasts are derived from the fragmentation of pillows along concentric and radial cooling joints. Hyaloclastite breccias consist of crystalline volcanic clasts in a matrix of glassy fragments, and are termed pillow-fragment breccias when clasts identifiable as pillows account for > 25% of the cobble-sized fraction. This glassy fragmental sediment was transported predominantly by short-lived grain flows and deposited as a result of syn-eruptive talus accumulation. The above interpretations culminate in the production of a depositional model: these glassy fragmental rocks formed on the slopes of pillow cones following gravitational collapse of a destabilised cone flank along the Proto-Macquarie Spreading Ridge. Scanning electron microscopy reveals that palagonite alteration rims on glassy grains lithify the sediment. The findings may be used as an analogue for the formation of glassy fragmental rocks along past and present mid-oceanic ridges.

After the Ice: Holocene Geomorphic Activity in the Scottish Highlands

A rich variety of processes have modified the glacial landscape of the Scottish Highlands since the last glaciers disappeared 11,500 years ago. Many of these processes can be described as paraglacial (glacially-conditioned): retreat of glacier ice has resulted in exposure of metastable sediment sources (rock slopes, drift-mantled slopes and valley-floor glacigenic deposits) that have been reworked over a range of timescales. Much of the reworked sediment has been deposited in paraglacial sediment stores (talus accumulations, debris cones, alluvial fans, valley fills, river terraces and deltas). The trajectory of paraglacial sediment transfer can be approximated by an exhaustion model. A corollary of this model is that all sediment stores eventually undergo a change from net accumulation to net erosion, reflecting the non-renewable nature of sediment sources. This concept helps to explain why most talus accumulations, debris cones and alluvial fans in the Highlands are now relict, and why many floodplains appear to have experienced an initial period of aggradation followed by later river incision and terrace development. Since deglaciation, rock slopes have experienced paraglacial stress release due to differential deglacial unloading. Many rock slopes have remained stable or experienced gradual adjustment through intermittent rockfall. At over 600 sites, however, large-scale rock-slope failure has occurred, either in the form of catastrophic failure with complete or arrested debris runout, or as major slope deformations. Modification of drift-mantled slopes has been dominated by rainstorm-generated debris flows activity, which have stripped sediment from slopes and redeposited it in debris cones. Radiocarbon dating of buried organic horizons shows that debris flows have occurred intermittently throughout the Holocene. Snow avalanches have played only a localized role in paraglacial sediment transfer. The Holocene alluvial history of the Highlands is poorly understood. Some alluvial fans formed during a few exceptional flood events, and there is evidence that postglacial floodplain aggradation peaked in the late Holocene and was succeeded by floodplain incision and terrace development, possibly reflecting upstream reduction in paraglacial sediment supply. Other processes have operated independently of glacial conditioning. The effects of wind action are dominant in the coastal zone, forming dunefields through aeolian reworking of beach sand, and on plateaux in the form of deflation surfaces and sand sheets. Extensive erosion of plateau-top sands and aeolisols occurred in the period AD 1550–1700, a period of exceptional storminess. Small-scale periglacial processes have also been active on high ground throughout the Holocene, including granular weathering of exposed rock surfaces, the development of small-scale frost-sorted patterned ground on unvegetated terrain, and slow downslope movement of solifluction sheets, solifluction lobes and ploughing boulders.

Morphostratigraphic principles in glacier reconstruction -a perspective from the British Younger Dryas

Glacier reconstruction enables the calculation of palaeoglaciological and palaeoclimatic variables such as the equilibrium-line altitude and palaeo-precipitation values. Such data are important for our understanding of past atmosphere-cryosphere interactions and as input variables to constrain numerical models effectively. Numerical dating is crucial to constrain the age of glacial events, but, due to absence of dateable material and/or contamination problems, ice masses can frequently be constrained satisfactorily only in a few locations. Thus, extrapolation and interpolation of geomorphological evidence is required to establish the extent of glaciers at a given time for the whole ice mass. Using examples from areas in Britain that were last covered by glaciers during the Younger Dryas, geomorphological approaches are reviewed and potential pitfalls highlighted. A multiproxy morphostratigraphic approach that utilizes clear landsystem contrasts inside and outside dated glacial limits is developed and tested in an area where numerical dates are sparse. Landform assemblages suitable in this respect are the type and frequency of moraines, river terrace sequences, glaciofluvial landforms, raised beaches, the upslope terminations of sediment cover (‘drift limits’), periglacial trimlines and periglacial features such as blockfields, solifluction lobes and thick talus accumulations. It is concluded that, if multiple lines of geomorphological evidence converge, these can be used to confine the extent of past glaciers in a given area and to guide dating programmes.

Determination of the internal structure of alpine talus deposits using different geophysical methods (Lechtaler Alps, Austria)

The total thickness and structure of alpine talus accumulations were investigated in a study area in the Northern Alps using ground-penetrating radar (GPR) completed by 2D-geoelectric (ERT) and seismic refraction soundings. The loose, steep talus cones of the Parzinn cirque are situated at an elevation of 2100–2300 m a.s.l. and end downslope at late-Glacial protalus ramparts. The surrounding rockwalls consist of “Hauptdolomit”, a brittle dolostone. In most of the GPR profiles, the bedrock surface was clearly recognizable from the radargrams. Total talus thicknesses of up to 45 m were estimated. The uppermost talus layer is characterized by surface-parallel radar reflection patterns, intermediate resistivities (4–10 kΩ m) and low seismic velocities (400–500 m/s). These deposits are interpreted as stratified debris, built up during the Holocene by rockfall deposits occasionally resorted by debris flows. Below this unit, a zone of more irregular radar reflection patterns and enhanced electrical resistivities follows. The combined results lead to the assumption of very coarse grained, paraglacial rockfall deposits, or to a core of basal moraine beneath the talus. Some zones of especially high resistivity (> 25 kΩ m) may be related to sporadic permafrost occurrence in the depressions between the talus and the moraine ridges. Striped radar structures are also recognizable in the moraine bodies. The stratified sediments give evidence for glaciofluvial transport or debris flows as an agent of protalus rampart formation. However, these features probably result from a combination of different processes including periglacial creep. GPR turned out to be a powerful tool for the determination of debris volumes and internal structures. However, supporting geoelectric and seismic investigations are advisable to validate the results and to facilitate and improve the interpretation. The results point to an average Holocene rockwall retreat of 500 to 800 mm/ka, which is roughly three times higher than recent backweathering rates.

The 5 April 2003 vulcanian paroxysmal explosion at Stromboli volcano (Italy) from field observations and thermal data

The 5 April 2003 paroxysmal explosion at Stromboli volcano was one of the strongest explosive events of the last century. It occurred while the effusive eruption, begun on 28 December 2002 and finished on 22 July 2003, was still on going and the summit craters of the volcano were obstructed. In this paper, we present a reconstruction of the sequence of events based on thermal and visual images collected from helicopter before, during and immediately after the paroxysm. One month before the blast, ash emission and temperature increase at the bottom of the summit craters were observed. An increasing amount of juvenile components in the emitted ash during March suggested that the magma level within the crater was rising accordingly. Hot degassing vents at the bottom of the summit craters were not persistent, and the craters remained almost entirely obstructed by talus accumulation until the paroxysm occurred. Three minutes before the explosion, we recorded a significant increase in temperature inside Crater 1, accompanied by a thicker gas plume. Thirty-two seconds before the blast, reddish ash was emitted from Crater 1. The paroxysm produced a vulcanian explosion that opened the feeder conduit, obstructed for over three months. The blast was accompanied by a shock wave recorded by the INGV seismic network at 07:13:37 GMT. Explosions with hot material started from Crater 1, and after 15 s propagated to Crater 3, about 100 m away. The velocity of ejecta was ∼80 m s − 1 , and increased when the eruptive plumes from both craters merged together during the vulcanian phase. An eruptive column rose 1 km above the top of the volcano, and explosions continued mainly at Crater 3. The paroxysm lasted about 9 min, with bombs up to 4 m wide falling on the village of Ginostra, on the west flank of the island, and destroying two houses. This event signalled the start of the declining phase of the effusive eruption, suggesting that the feeder conduit was returning to its former steady conditions, with open vents and continuous, mild strombolian activity.

Les pentes raides de la vallée de la rivière du nord – Étude de versants-échantillons

The author recently made a detailed investigation of the talus accumulations which mantle the steep slopes of the valley of the rivière du Nord, in the Laurentide hills, 45 miles north of Montréal, Québec. In postglacial times the lower section of the valley was occupied by an arm of the Champlain sea, and thick deposits of sand and clay were laid down in this estuary. The writer describes the various types of contacts which exist between the Champlain sea deposits and the talus accumulations — for example, certain talus slopes were completely covered by marine sediments and remain « fossilized » ; others have been stripped of their cover of unconsolidated materials and are consequently described as « exhumed ». The writer has also attempted to distinguish between those talus accumulations which antedate the Champlain incursion and those which postdate it. Most of the talus slopes appear to have formed immediately prior to or during the Champlain period, but a few have developed, at least in part, since that time. Frost-shattering (gelifraction) is the principal process contributing to the formation of these talus accumulations at the present time, although landslides also play an important role. The writer wonders whether these talus slopes of the valley of the rivière du Nord should not be classified as periglacial landforms.