Slopewash Deposits Research Articles

Luminescence ages of sediments from the margin of the penultimate glaciation in the north-eastern East European plain

In this study we apply optically stimulated luminescence and infra-red stimulated luminescence to the sediments from the interfluve dividing three river basins: the Pechora, the Kama and the Severnaya Dvina, hoping to gather new age data on glaciofluvial sediment. We also studied the valley of the Berezovka, a medium-scale river cutting through this interfluve. We obtained a chronology from 17 quartz and 15 feldspar ages. Sedimentological analyses and luminescence dating show that most of the sediments examined here are water-transported (gully alluvium, slopewash deposits), and their ages mark a period of accumulation during MIS 2. Some of these sediments were reworked by aeolian processes; these took place over a time span consistent with such processes in other parts of the East European Plain (19-12 ka). The Berezovka river experienced incision before ∼22 ka and another erosion event with extremely high floods at 15–16 ka. Its terrace formed in MIS 5, constraining the minimum age of the valley. In the bottom part of the sections, much older, possible MIS 8, glaciofluvial sediments were found; the apparent absence of MIS 6 sediments may argue for moving the MIS 6 ice sheet limit further to the north of the study area.

Analysis of February 2023 Thatri Landslide in Doda, Jammu and Kashmir: Insights from Field Observations, Geotechnical Parameters, and GPR Survey

Abstract The present study deals with the field observations and results obtained from the Geotechnical investigations and Ground Penetrating Radar survey carried out on Thatri landslide. Thatri township is situated on the left bank of the Chenab River along NH-244, 43 km from district headquarter Doda, Jammu and Kashmir. The landslide occurred on 2nd February, 2023 damaged 21 buildings, affected about 150 people and created situation like Joshimath tragedy. Detailed field investigations on lithology, sub-surface structure determined by GPR investigations and geotechnical parameters of the soil revealed that the leading edge of the slide caused most damage due to cracks developed in the slope wash deposits followed by subsidence and down slope movement of a portion of the township (Nai Basti). The study revealed that percolation of water from soak pits, sewerage, and rain water into the terrace deposits comprising of clays, silt, sand and gravels was the major triggering factors.

Aeolian dust accretion outpaces erosion in the formation of Mediterranean alpine soils. New evidence from the periglacial zone of Mount Olympus, Greece

AbstractSoil formation in Mediterranean periglacial karst landscapes remains poorly understood as the interplay between local and allochthonous sources of parent materials, and mineral alteration and pedogenesis, as dominant post‐depositional processes, depends on a variety of climatic and environmental factors. Herein, we investigate the balance between erosion and aeolian dust accretion in the formation of an alpine soil profile in the periglacial zone of Mount Olympus in Greece. We applied a wide range of analytical methods to 23 samples, from a soil profile developed in a glaciokarstic plateau, from colluvial sediment horizons interbedded in postglacial scree slopes of different maturity and formation age and from modern Sahara dust samples deposited on the snowpack. Colluvial sediment horizons exhibit high concentrations of carbonate gravel and calcite‐rich sand and represent the local erosion products. The soil B horizon developed on the glaciokarstic plateau, contains high amounts of fine earth and is rich in quartz, mica, plagioclase, clays and Fe–Ti oxides. Based on physical and textural characteristics, the soil profile is partitioned in a surficial weathered Bw and an illuvial Bt horizon that overlies the local regolith composed of fragmented glacial till and slope wash deposits. Radiogenic isotope systematics, textural and mineralogical analysis show that the contribution of aeolian (Sahara and locally sourced) dust to the development of the soil B horizon ranges between 50% and 65%. Cryoturbation results in fine earth translocation from Bw to the Bt horizon, whereas weak pedogenetic modifications of detrital (aeolian and bedrock‐derived) minerals result in magnetic mineral weathering and secondary clay formation. Our findings reveal that aeolian dust accretion from Saharan and lcal sources is the dominant process in providing alpine soil parent material and that cryoturbation, weak pedogenesis and clay mineral alteration occur within the Mediterranean periglacial zone of Mount Olympus.

A multi-proxy record of hurricanes, tsunami, and post-disturbance ecosystem changes from coastal southern Baja California

Tsunamis and hurricanes are two earth surface processes that can dramatically impact coastal landforms and ecosystems. This study uses a combination of palynological, grain-size, X-ray fluorescence, and loss-on-ignition analyses, short-lived isotopic and radiocarbon dating, and statistical analysis to differentiate the tsunami and hurricane deposits, establish a Late-Holocene record of extreme events, and document the landscape and vegetation transformation in response to disturbance events and environmental changes from a small coastal lagoon in Baja California, Mexico. Prior to ~530 cal yr BP, Playa Los Cocos was occupied by a short-hydroperiod tidal marsh bounded by desert vegetation on the surrounding hillslopes. At ~530 cal yr BP, a tsunami created a backbarrier lagoon and introduced mangrove propagules from other coastal localities, and the lagoonal environment and substrates also provided suitable habitats for red mangroves to proliferate. Once established, red mangrove populations rapidly expanded until ~180 cal yr BP, when modern human activities diminished the mangrove forest in our study area. Overall, the multi-proxy dataset revealed four hurricane events at ~770, ~600, ~280, and ~0 cal yr BP, and one tsunami event at ~530 cal yr BP. The hurricane deposits were preserved in the form of fluvial and slope-wash deposits characterized by low organic and water contents, low concentration of marine elements, and high concentration of terrestrial elements. The tsunami run-up deposits are characterized by abundant broken and intact sea shells, high content of carbonate and marine elements, low concentration of terrestrial elements, and sharp basal contact with the underlying sediments. The tsunami backwash deposits are characterized by a mixed physical and chemical signature resembling both marine and terrestrial sediments. Results also suggest that both hurricanes and tsunamis can help propagule dispersal and create suitable coastal habitats favorable for the spread and proliferation of mangroves in a desert coastal environment.

Sediment Budget and Sediment Trap efficiency of Baglihar Hydroelectric project Reservoir – a calibrated model for prediction of longevity of the Dam

The field investigation of the reservoir area of Baglihar Hydropower project shows that the sediment budget to the reservoir is controlled by fragile rock type like shales, sandstones, phyllites and slates, soil characteristics, steep hill slopes, rainfall and landslides. The rocks are highly weathered, fissile and micaceous in nature and very sensitive to water absorption. The analysed sediments are characterised by dominance of sands, silts and clays with lower values of plasticity (14.3PL), liquidity (23.5 LL), cohesion (118) and shear strength (202 Kpa). The slope wash deposits are highly susceptible to landslides and slope failures and directly contribute to the sediment budget in the reservoir. In addition tributaries of Chenab River also bring sediments in the reservoir from the catchment area. The empirical relationship for estimating the long-term reservoir trap efficiency for large storage based on correlation between the relative reservoir size and trap efficiency was simulated in 3D model which shows that the annual sediment trap efficiency of the Baglihar reservoir is of 0.39%. The extrapolation of the calculated values shows that the total sediment load shall increase by 11% in the next 30 years and 20% in the next 50 years and correspondingly 40% in the next 100 years that shall induce corresponding decrease in the reservoir volume over the time. By applying flushing schemes, life span of the reservoir can be extended. It is estimated that after 100 years the reservoir shall lose ~35.6% storage volume. On further extrapolation, the trap efficiency will decrease from 25.5% after 30 years to 23% after 100 years. The estimated trap efficiency of Baglihar reservoir is 60%, which is greater than that based on numerical results, showing a significant overestimation.

The Late Quaternary Evolution of the Upper Reaches of Fluvial Systems in the Southern East European Plain

Networks of dry valleys (or balkas) and hollows in the upper reaches of fluvial basins in extraglacial areas in the Penultimate Glaciation (Marine Isotope Stage 6—MIS 6) regions of the East European Plain demonstrate clear incision/aggradation rhythms corresponding to global glacial/interglacial climate cycles. The first phase of each incision/aggradation rhythm began after the global glacial maximum and was characterized by a cool and humid climate, permafrost and sparse vegetation, when high surface runoff and active linear erosion formed a dense network of gullies. The second phase occurred at the glacial–interglacial transition and the subsequent interglacial period with its warm and humid climate and dense vegetation. This phase was distinguished by the partial filling of fluvial forms with slopewash deposits, the transformation of gullies into dry valleys (balkas) and the subsequent stabilization of fluvial forms marked by the formation of mature soils on the sides and bottoms of balkas. The third phase of the rapid accumulation of balkas developed during the cold and dry part of the next glacial epoch, resulting in the balkas becoming shallow hollows filled in with sediments. The last full incision/aggradation rhythm occurred in the late MIS 6 to mid-MIS 2. The erosion network formed during the late MIS 6 was almost completely filled by mid-MIS 2, and its manifestation in the modern topography is limited to a network of shallow hollows in the upper parts of the fluvial systems. The modern (incomplete) incision/aggradation rhythm began in the late MIS 2 and caused the formation of the modern erosion landscape in the upper reaches of fluvial systems. This rhythm is now in the stabilization phase, and the main accumulation phase of this rhythm is still far in the future.

Chronostratigraphic interpretation of intermediate layer formation cycles based on OSL-dates from intercalated slope wash sediments

Stratified periglacial sediments extensively cover the slopes of central European low mountain areas. Periglacial cover beds and loess-like sediments as most abundant strata influence many near-surface processes, forming an important component of the Earth's critical zone.The present study focusses on the analysis of a complex sequence of periglacial slope deposits with multiple intermediate layers and intercalated slope wash sediments in order to determine and date periods of intermediate layer formation.Since luminescence dating often provides inconsistent data for cover beds, there is need to reconsider the numerical dating strategy. This study hence places stronger emphasis on numerical dating of interbedded, sufficiently bleached slope wash sediments. The gained OSL-ages are more reliable and consistent and thus, provide a temporal framework for relative dating of intermediate layer formation cycles. In combination with paleosol relics the numerical ages allowed a chronostratigraphic interpretation of Late Pleistocene paleoenvironmental dynamics and slope evolution.Our findings not only prove multiple phases of intermediate layer formation in the study area during the last glacial period but also enabled the correlation of single intermediate layers with specific time spans, including Early and Middle Weichselian as well as the Late Glacial. Remnants of the Eemian soil in the studied sequence even indicate a Saalian age for the lowermost intermediate layer.The present study shows that luminescence dating of intercalated slope wash deposits instead of cover beds is a promising numerical dating strategy for the chronostratigraphic interpretation of cover bed sequences. The findings further illustrate that complex, fanned out periglacial sediment sequences form despite their scarcity important paleoenvironmental archives for Late Pleistocene periglacial landscape dynamics in central European low mountain areas.

Volcanological study of the middle Miocene Okiduse Volcanic Group, Woodlark Island (Muyuw), eastern Papua

ABSTRACTWoodlark Island (Muyuw) is located in a tectonically complex region, one of the few places on Earth where continental breakup is occurring ahead of seafloor spreading. Rifting commenced in the late Miocene (8.8–6 Ma) and is associated with the westward-propagating Woodlark Basin Spreading Centre. The island comprises approximately 850 km2 of raised Pleistocene coral reef and associated sediments with a central, moderately elevated range underlain by the middle Miocene calc-alkaline to shoshonitic Okiduse Volcanic Group (new name). It provides an exposure of upper Cenozoic geology in close proximity to the spreading centre. The Okiduse Volcanic Group is host to most of the island's historical gold and silver production and recently defined mineral resources totalling 1.75 Moz gold. This study uses facies analysis of pyroclastic deposits to develop a detailed geological map of the Okiduse Volcanic Group, with a revision and reinterpretation of the unit. Facies associations suggest that two major volcanic centres erupted synchronously during the middle Miocene (14–12 Ma), referred to as the Watou Mountain Eruptive Centre (new name) and the Uvarakoi Caldera (new name). The mafic–intermediate Watou Mountain Eruptive Centre formed during frequent small eruptions of widely varying style. Strombolian, subplinian, vulcanian and dome-related explosive eruptions occurred, alternating with extrusion of block and ash flow deposits and lava domes. Pyroclastic deposits were rapidly reworked from the steep cone, and were redeposited in a series of coalescing aprons surrounding the volcano. The felsic Uvarakoi Caldera formed during a series of violent explosive eruptions by rapid removal of magma from the underlying chamber, followed by collapse. Plinian and possibly phreatoplinian eruptions, as a result of magma–water mixing in the surface environment, resulted in widely dispersed, highly fragmented tuff deposits. The caldera was modified by widespread erosion following eruptions, resulting in fluvial, laharic and slope-wash deposits. This study highlights lithological controls (porosity and permeability) by various units within the Okiduse Volcanic Group on ore deposition.

Slope deposits as an indicator of anthropopressure in the light of research in Central Poland

Results of research on Neo-Holocene slope covers uncovered at archaeological sites in Central Poland are described in this paper. Geoarchaeological investigations have been undertaken in the Ner River basin. Distinctive archaeological relicts of human activity document periods of intensive activity of human societies. Artefacts included within deposits play a very important role in correct recognition of the chronology of covers and ecofacts for reconstructions of environmental evolution. The origin and development of slope covers have been correlated with anthropogenic changes of the natural environment. Two main phases of acceleration of slope processes have been recognised, dated to the Middle Bronze Age and to the Late Roman Period. Slope deposits have been classified as deluvia (deluvial, slope wash deposits or colluvial deposits). Their features are most typical for “deluvial sands” and, if they contain more humic matter, for “soil deluvia”. The deluvial covers have been deposited as an effect of moderately intensive slope wash processes. Accumulation of slope deposits has resulted from natural processes initiated by human impact during periods of intensive settlement and economic activity. The slope cover sediments are the source for recognition of intensity of human impact and of economic activity of ancient societies, and for distinct phases of hiatuses of settlement development. The possibility of archaeological and palaeogeographical interpretations based on research on slope covers at archaeological sites is presented.

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.

Stratigraphy of near-valley head quaternary deposits and evidence of climate-driven slope-channel processes in southern Brazilian highlands

Abstract During the past 40 years colluvial and alluvial deposits have been used in Brazil as good indicators of regional landscape sensitivity to Quaternary environmental changes. In spite of the low resolution of most of the continental sedimentary record, geomorphology and sedimentology may favor palaeoenvironmental interpretation when supported by independent proxy data. This paper presents results obtained from pedostratigraphic sequences, in near-valley head sites of southern Brazilian highlands, based on geomorphologic, sedimentologic, micromorphologic, isotopic and palynologic data. Results point to environmental changes, with ages that coincide with Marine Isotopic Stages (MIS) 5b; 3; 2 and 1. During the late Pleistocene, although under temperatures and precipitation lower than today, the local record points to relatively wet local environments, where shallow soil-water saturated zones contributed to erosion and sedimentation during periods of climatic change, as during the transition between MIS 2 and MIS 1. Late Pleistocene events with ages that coincide with the Northern Hemisphere Younger Dryas are also depicted. During the mid Holocene, slope-wash deposits suggest a climate drier than today, probably under the influence of seasonally contrasted precipitation regimes. The predominance of overland flow-related sedimentary deposits suggests an excess of precipitation over evaporation that influenced local palaeohydrology. This environmental condition seems to be recurrent and explains how slope morphology had influenced pedogenesis and sedimentation in the study area. Due to relative sensitiveness, resilience and short source-to-sink sedimentary pathways, near-valley head sites deserve further attention in Quaternary studies in the humid tropics.

Late-Pleistocene and mid-Holocene environmental changes in highland valley head areas of Santa Catarina state, Southern Brazil

Low resolution of the continental sedimentary record is a common source of skepticism about the application of geomorphology and sedimentology to Quaternary studies. In spite of this, when supported by independent proxy data, geomorphology and sedimentology may favor palaeo-hydrologic interpretation. This paper associates geomorphologic, stratigraphic, sedimentologic, isotopic, palynologic and geochronologic data. The research was conducted in valley head sites in southern Brazilian highlands, under mild subtropical climate. The results point to environmental changes, the ages of which coincide to Marine Isotopic Stages (MIS) 5b, 3, 2 and 1. Although late-Pleistocene temperatures and precipitation were lower than those of today, the study valley heads seem to have sustained locally wetter environments, which fed shallow soil–water saturated zones. These saturated zones are believed to have expanded during transitions between stadial and interstadial states, contributing to hillslope erosion and sedimentation regardless of the sign of the climatic change. The interior organization of holocenic slope-wash deposits suggests that the mid-Holocene climate was drier than today’s and was under the influence of seasonally contrasting precipitation regimes. The predominance of overland flow-related sedimentary records suggests that an excess of precipitation over evaporation influenced local palaeo-hydrology. This palaeo-hydrologic condition seems to have been recurrent and also explains the alternating periods of pedogenesis and sedimentation.

Unravelling the evolution of an Alpine to post-glacially active fault in the Swiss Alps

The Gemmi fault is a prominent NW–SE striking lineament that crosses the Gemmi Pass in the central Swiss Alps. A multidisciplinary investigation of this structure that included geological mapping, joint profiling, cathodoluminescence and scanning electron microscopy, stable isotope measurements, luminescence- and U-TH-dating, 3D ground penetrating radar (GPR) surveying and trenching reveals a history of fault movements from the Miocene to the Holocene. The main fault zone comprises a 0.5–3 m thick calcite cataclasite formed during several cycles of veining and brittle deformation. Displaced Cretaceous rock layers show an apparent dextral slip of ∼10 m along the fault. A detailed study of a small sediment-filled depression that crosses the fault provides evidence for a post-glacial reactivation of the fault. A trench excavated across the fault exposed a Late-Glacial-age loess layer and late Holocene colluvial-like slope-wash deposits that showed evidence for fault displacement of a few centimeters, indicating a recent strike-slip reactivation of the fault. Focal mechanisms of recent instrumentally recorded earthquakes are consistent with our findings that show that the fault at the Gemmi Pass, together with other parallel faults in this area, may be reactivated in today's stress field. Taking together all the observations of its ancient and recent activity, the Gemmi fault can be viewed as a window through geological space and time.

Chrono-toposequences of soils on the river terraces in Transylvania (Romania)

A sequence of surface soil profiles and buried soils described on the 4th, 3rd, 2nd and 1st terraces of different rivers in Transylvania is presented as part of an attempt to perform a paleopedologic interpretation of the dominant soil types of this area. Excepting the first terrace, the described soils were formed from Pleistocene slopewash deposits of Würm, or possibly of Riss age, which overlay alluvial terrace deposits. On the basis of fossil animal remains, periglacial cryostructures, prehistoric artifacts (Coţofeni culture of the transition period from the Neolithic to the Bronze Age) and Scythian graves an approximate age of different soil horizons, as well as the polycyclic nature of soils was established. There is no essential difference between a Würmian buried soil horizon and the argic Bt horizon of the surface soils; moreover, in some cases, the buried paleosols become the Bt horizon itself of the surface soils, through the thinning of the overlying deposits. Even if the possibility of formation of the argic Bt horizons during the postglacial until the end of Neolithic, cannot be excluded, from the presented situations it seems that these horizons in the Transylvanian Basin are Würmian relics, formed in the periglacial interstadials. In most of the cases Bt horizon has nothing in common genetically with the topsoil.

Registro de mudanças ambientais Pleistocênicas e Holocênicas em depósitos de cabeceira de vale: Campo Alegre, planalto norte catarinense (SC)

Low resolution of continental sedimentary record is a common source of skepticism about the application of geomorphology and sedimentology to Quaternary studies. However, when supported by independent proxy data, geomorphology and sedimentology favor palaeoenvironmental interpretation. This paper presents results obtained from pedostratigraphic sequences, in valley head sites of southern Brazilian highlands, based on geomorphological, micromorphological, sedimentological, isotopical and palynological data. Results point to environmental changes, which ages coincide with Marine Isotopic Stages (MIS) 5b; 3; 2 and 1. During the late Pleistocene, in spite of temperatures and precipitation lower than today, the local valley head record points to wetter local environments, where shallow soil-water saturated zones contributed to erosion and sedimentation during periods of climatic change. During the mid Holocene, slope wash deposits suggest a climate drier than today, under the influence of seasonally contrasted precipitation regimes and torrential overland flow on hillslopes. The predominance of overland flow-related sedimentary deposits suggests an excess of precipitation over evaporation influencing local palaeohydrology. This palaeohydrological condition seems to be recurrent and explains how slope morphology had influenced pedogenesis and sedimentation in the study area, in response to environmental changes.

REGISTRO DE MUDANÇAS AMBIENTAIS PLEISTOCÊNICAS E HOLOCÊNICAS EM DEPÓSITOS DE CABECEIRA DE VALE: CAMPO ALEGRE, PLANALTO NORTE CATARINENSE (SC)

Low resolution of continental sedimentary record is a common source of skepticism about the application of geomorphology and sedimentology to Quaternary studies. However, when supported by independent proxy data, geomorphology and sedimentology favor palaeoenvironmental interpretation. This paper presents results obtained from pedostratigraphic sequences, in valley head sites of southern Brazilian highlands, based on geomorphological, micromorphological, sedimentological, isotopical and palynological data. Results point to environmental changes, which ages coincide with Marine Isotopic Stages (MIS) 5b; 3; 2 and 1. During the late Pleistocene, in spite of temperatures and precipitation lower than today, the local valley head record points to wetter local environments, where shallow soil-water saturated zones contributed to erosion and sedimentation during periods of climatic change. During the mid Holocene, slope wash deposits suggest a climate drier than today, under the influence of seasonally contrasted precipitation regimes and torrential overland flow on hillslopes. The predominance of overland flow-related sedimentary deposits suggests an excess of precipitation over evaporation influencing local palaeohydrology. This palaeohydrological condition seems to be recurrent and explains how slope morphology had influenced pedogenesis and sedimentation in the study area, in response to environmental changes.

Episodic Late Quaternary slopewash deposition as recorded in colluvial aprons, Southeastern Wyoming

Colluvial aprons found along fluvial terraces of the Laramie River and Sybille Creek in southeastern Wyoming are interpreted as late Quaternary slopewash deposits. Each apron studied contained multiple buried soils, interpreted to indicate that slopewash activity was episodic and short-lived, and was followed by relatively long periods of landscape stability and soil development. Apron deposits were described and subsequently correlated based on their internal stratigraphy and their relative degree of soil development. Optical dating was used to chronologically constrain periods of slopewash deposition, and based on 17 optical ages taken from five aprons, aggradation occurred at ∼65–56, 16.0, 12.9, 11.8, 8.1, 7.3, 5.0, and 1.7 ka. Several of these events correspond with climatic transitions such as the termination of Heinrich Event 1, the onset and termination of the Younger Dryas, and the 8.2 ka event, suggesting that apron aggradation was driven by changes in climatic regime. Although either increased aridity or a change in precipitation regime could result in increased erosion of terrace scarps, apron aggradation events do not correlate with regional records of aridity. Instead, periods of increased precipitation intensity and/or frequency that occur during climatic transitions most likely drive apron aggradation events.

Structure, sedimentology and evolution of rockfall talus, Mynydd Du, south Wales

Gullies incised into relict rockfall talus slopes at Mynydd Du (south Wales) reveal stacked debris flow and slopewash deposits occasionally intercalated with buried peaty soils, reflecting episodic erosion and reworking of talus on the upper rectilinear slope. Radiocarbon-dating of the palaeosols indicates that reworking has been intermittently active since at least c . 4.8–44 cal ka bp. Particle-size analyses of bulk talus samples demonstrate that fine-grained ( c . 27% by weight of the Mynydd Du talus. Glacigenic and aeolian origins for this fine sediment are excluded, as is inwash of fines from the plateau above or in situ weathering of talus clasts. Flaking and granular weathering of the rockwall appear to be the most likely source of the fine sediment, the accumulation of which has probably proved instrumental in facilitating incision and reworking. Implications are considered for models of talus evolution, erosion histories of hillslope materials, and the role of small-scale weathering processes in rockwall retreat.

Holocene reworking of drift-mantled hillslopes in Glen Docherty, Northwest Highlands, Scotland

Sections exposed within a colluvial debris cone in Glen Docherty, Scotland, reveal in-situ palaeosols intercalated with debrisflow diamictons and horizons of slopewash deposits. Radiocarbon dating of the palaeo sols suggests that drift reworking at this site involved brief, intermittent episodes of debrisflow and slopewash activity since c. 6.5 cal. ka BP. At least four such phases can be recognized: c. 6.5–6.2, 5.6–5.3, 4.9–4.6 cal. ka BP and after c. 450 cal. BP. The palaeosols examined show no evidence of local burning of vegetation. A calibrated radiocarbon age for the onset of the most recent phase of reworking may be interpreted as reflecting woodland clearance and/or the influence of extreme rainfall events during the ‘Little Ice Age’ period of regional climatic deterioration. The combined chronostratigraphic and palaeoecological evidence points to discrete, local rainstorms that are seemingly unrelated to long-term climatic change as the most likely cause of enhanced slope reworking, at least prior to c. 450 cal. BP. If this interpretation is valid, it implies that the formation of Holocene debris cones elsewhere in upland Britain may have been due primarily to a small number of localized, extreme rainstorm events. Consequently, caution is necessary in using debrisflow chronostratigraphy alone as a means of reconstructing long-term climate patterns.

Ice‐wedge casts and relict polygonal patterned ground in North‐East Iowa, USA

AbstractSediment‐filled wedges and associated polygonal patterned ground occur in pre‐Illinoian sediments in north‐east Iowa. A total of 183 vertical to near‐vertical wedge‐shaped bodies of sand have been observed at 12 sites in Black Hawk County, Iowa, and 80 of them were examined in detail. The wedges average 105cm in width at their tops and about 190cm in depth. Infilling material is mostly sand, with pockets of silt, silty sand, and gravelly sand. Slump and subsidence features were also noted in some wedges. Loamy sediment about 1 m thick, with a stone line at its base, overlies the sediment‐filled wedges and usually descends slightly into their upper parts.Based on their characteristics, these sediment‐filled wedges are interpreted as ice‐wedge casts. Supportive palaeoenvironmental studies in Iowa and adjacent states indicate that tundra conditions probably existed in places between 21,000 and 16,500 years BP, the coldest part of late Wisconsinan time. Mean annual air temperatures at this time may have been at least 14°C colder than at present. The degradation of permafrost and formation of ice‐wedge casts must have occurred near the end of this episode of cold climate, which also promoted extreme erosion of the landscape in north‐east Iowa. Aeolian and slopewash deposits later buried the erosion surface and the ice‐wedge casts.