Rockfall Talus Research Articles

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.

Geomorphology of the upper Kalguty Basin, Ukok Plateau, Russian Altai mountains

ABSTRACT As part of an archaeological research project, we investigate the geomorphology of the cold and arid study area by combining field mapping with use of orthoimages and DEM. To the north of a broad trunk valley floor, gentle slopes continuously vegetated culminate around 3000 m, while to the south steep slopes reaching to 3500 m are deeply incised and covered mainly by regolith. The basin has been intensively glaciated as evidenced by the till covering the gentler slopes, several morainic complexes, kame terraces and roches moutonnées. The present morphodynamics of this permafrost-affected basin is mainly periglacial, with blockfields, solifluction lobes, patterned ground and rock glaciers on gentle slopes to high-elevated flat terrain, and block slopes and rockfall talus on steeper slopes. Large alluvial fans at the outlet of the steep lateral valleys constrain the anastomosing channel of the Kalguty river. Active braided channels in those valleys evidence seasonal high peak discharge.

Temporal variations in rockfall and rock-wall retreat rates in a deglaciated valley over the past 11 k.y.

Abstract This study addresses the temporal variations in rockfall activity in the 5.2 km2 calcareous cliffs of the deglaciated Lauterbrunnen Valley, Switzerland. We did this using 19 campaigns of repeated terrestrial laser scans (TLS) over 5.2 yr, power-law predicted behavior from extrapolation of the TLS-derived frequency-magnitude relationship, and estimates of long-time-scale (∼11 k.y.) activity based on the volume of preserved postglacial rockfall talus. Results from the short-time-scale observations indicate no statistically significant difference between TLS observations averaging over 1.5 versus 5.2 yr. Rock-wall retreat rates in both cases are 0.03–0.08 mm/yr. In contrast, the power-law predicted rock-wall retreat rates are 0.14–0.22 mm/yr, and long-term rates from talus volumes are 0.27–0.38 mm/yr. These results suggest (1) short (1.5 yr) TLS inventories of rockfalls provide (within uncertainties) similar frequency-magnitude relationships as longer (5.2 yr) inventories, thereby suggesting short observation periods may be sufficient for hazard characterization from TLS, and (2) higher rock-wall retreat rates over long time scales (Holocene averaged) may reflect debuttressing and stress relaxation effects after glacial retreat, and/or enhanced rockfall activity under periglacial (climatic) conditions.

Processes and facies relationships in a Lower(?) Devonian rocky shoreline depositional environment, East Lime Creek Conglomerate, south‐western Colorado, USA

AbstractRocky shorelines are relatively common features along modern coastlines, but few have been recognized in the geological record. The hard substrates of rocky shorelines telescope the width of offshore marine environments, thus the diagnostic deposits observed in such settings today have a low preservation potential due to small accommodation space and high‐energy conditions. This study recognized previously overlooked, laterally extensive Lower(?) Devonian rocky shoreline deposits in the San Juan Mountains of south‐western Colorado. The newly defined lithostratigraphic unit, the East Lime Creek Conglomerate (ELCC), is 0–23 m thick, unconformably overlying Proterozoic crystalline rocks and unconformably overlain by the Upper Devonian Ignacio Formation and/or Elbert Formation. The unit mostly consists of clast‐supported cobble‐boulder conglomerate with rounded quartzite clasts up to 1.4 m in length interbedded with thin sandstone layers and lenses. Sandstones in the ELCC are distinguished from unconformably overlying Upper Devonian sedimentary rocks because they have sericite cements. Most importantly, there are buttressing relationships between the ELCC and underlying Proterozoic crystalline rocks interpreted as palaeo‐sea cliffs, palaeo‐wave‐cut platforms and palaeo‐tombolos. A proposed rocky shoreline facies model includes headlands with upper shoreface‐beachface tabular cobble‐boulder gravels sourced from rock fall talus, nearshore subaqueous debris‐flow deposits and intervening pocket beaches with imbricated, stratified pebble‐cobble gravel sheets. Palaeocurrent data (n = 338) from clast long‐axis orientations, imbrication and cross‐bedding indicate south‐to‐north transport roughly onshore‐offshore to a coastline consisting of alternating rocky headlands and pocket beaches. This Lower(?) Devonian unit documents a previously unrecognized episode in the geological history of south‐western Colorado.

Morphological and Morphometrical Analyses Reveal the Avalanche Influence over the Talus Cones in the Rybi Potok Valley, Tatra Mountains

AbstractIntense physical weathering engenders formation of scree cones at the foot of slopes dissected by couloirs. Combination of several geomorphic processes operating within the slope results in formation of cones of a polygenetic character (talus-alluvial cones, rockfall talus cones, or – in particular cases, talus-alluvial-avalanche cones). This study was aimed at determination of morphometric parameters characteristic for particular morphogenetic types of the cones in the Rybi Potok Valley in the High Tatra Mountains. For the purpose of the research, knowledge about the terrain and available cartographic materials (geological, geomorphological and topographical maps, LiDAR data) were employed. In the studied valley, 24 cones were selected and for each of them, morphogenetic types and morphometric parameters (surface, minimal and maximal height, roughness coefficient, Topographic Position Index) were determined. The correlations between different parameters enabled establishment and determination of the combination of morphometric parameters typical for the particular types of the cones. The impact of snow avalanches on one of the cones (at the mouth of Żleb Żandarmerii) was identified. This cone has morphometric parameters dissimilar to those of the other cones in the studied valley. It is the only cone in the valley which was classified as the avalanche cone.

Rockfall talus slopes and associated talus-foot features in the glaciated uplands of Great Britain and Ireland: periglacial, paraglacial or composite landforms?

Abstract The traditional interpretation of talus slopes and talus-foot landforms in the glaciated uplands of Great Britain and Ireland has been that they are periglacial landforms associated with freeze–thaw activity and permafrost. Since about 1990 some reassessment of this widely held view has occurred, and paraglacial rockfall and rock-slope failure are now considered to have played a significant role in the development of some talus landforms; in certain cases a wholly paraglacial origin is advocated. In order to determine formative processes, critical site-specific evidence (morphological and sedimentological) needs to be obtained. This will enable models of the deglacial–post-glacial evolution of these landscapes to be proposed and allow the palaeoenvironmental significance of the landforms to be established. Distinguishing between a periglacial and paraglacial origin might be assisted by application of cosmogenic isotope surface-exposure dating, which may demonstrate a Holocene age for a particular landform and thus rule out a permafrost-related origin. However, there will be instances where the application of dating will not differentiate, as in the case of Late Glacial landforms that could be either periglacial or paraglacial. It is likely that equifinality applies with respect to these landforms and a composite origin for them should also be considered. This latter issue is one that has not previously been given much consideration, probably because of the inherent difficulties in recognizing the products of different processes in landforms for which exposures of their constituent materials are rare.

Rockfall hazard and risk assessment in the Yosemite Valley, California, USA

Abstract. Rock slides and rock falls are the most frequent types of slope movements in Yosemite National Park, California. In historical time (1857–2002) 392 rock falls and rock slides have been documented in the valley, and some of them have been mapped in detail. We present the results of an attempt to assess rock fall hazards in the Yosemite Valley. Spatial and temporal aspects of rock falls hazard are considered. A detailed inventory of slope movements covering the 145-year period from 1857 to 2002 is used to determine the frequency-volume statistics of rock falls and to estimate the annual frequency of rock falls, providing the temporal component of rock fall hazard. The extent of the areas potentially subject to rock fall hazards in the Yosemite Valley were obtained using STONE, a physically-based rock fall simulation computer program. The software computes 3-dimensional rock fall trajectories starting from a digital elevation model (DEM), the location of rock fall release points, and maps of the dynamic rolling friction coefficient and of the coefficients of normal and tangential energy restitution. For each DEM cell the software calculates the number of rock falls passing through the cell, the maximum rock fall velocity and the maximum flying height. For the Yosemite Valley, a DEM with a ground resolution of 10 × 10 m was prepared using topographic contour lines from the U.S. Geological Survey 1:24 000-scale maps. Rock fall release points were identified as DEM cells having a slope steeper than 60°, an assumption based on the location of historical rock falls. Maps of the normal and tangential energy restitution coefficients and of the rolling friction coefficient were produced from a surficial geologic map. The availability of historical rock falls mapped in detail allowed us to check the computer program performance and to calibrate the model parameters. Visual and statistical comparison of the model results with the mapped rock falls confirmed the accuracy of the model. The model results are compared with a previous map of rockfall talus and with a geomorphic assessment of rock fall hazard based on potential energy referred to as a shadow angle approach, recently completed for the Yosemite Valley. The model results are then used to identify the roads and trails more subject to rock fall hazard. Of the 166.5 km of roads and trails in the Yosemite Valley 31.2% were found to be potentially subject to rock fall hazard, of which 14% are subject to very high hazard.

Geochemistry and origin of chloritoid schist from the Alpi Apuane, Italy: evidence of a prevailing lateritic signature

Chloritoid schist layers frequently occur as the uppermost part of the Rhaetian Brecce di Seravezza Formation, interpreted as rockfall talus metabreccias intercalated between Norian and Liassic marbles, in the Alpi Apuane Tuscany, Italy. The chloritoid schist is characterised by mm-sized chloritoid porphyroblasts (20–50 % modal range) and a matrix made up of chlorite, muscovite, pyrophyllite, chloritoid, hematite, calcite, quartz and epidote. Most chloritoid porphyroblasts show cores with abundant hematite inclusions. Many pieces of duricrust, pisoid and composite pebbles come from older lateritic bauxite deposits of supposed Kasimovian to Sakmarian age. Their uneven to well rounded shapes indicate variable duration of transport and multistage reworking. In comparison with many bauxite and metabauxites, Apuan chloritoid schist shows: great enrichment in SiO 2 , K 2 O, Na 2 O, a slight increase in CaO, MgO, remarkable depletion in Al 2 O 3 and Fe 2 O 3 , a slight decrease in TiO 2 and P 2 O 5 , strong depletion in Cu, Pb, Zn, Bi, and As, unexpected Ni, Co and Ba enrichments; slight to strong depletion for Ga, Cr, Nb, Zr, and V reflecting variable dilution of primary bauxite detritus. The REE spider diagram shows enrichment factors of 2–3 for LREE, of 1–2 for HREE. Strong enrichment to strong depletion in Sm, Eu, Gd, and Tb is correlated with maximum and minimum contents of duricrust fragments respectively. Four detrital components were distinguished. Two of them represent about 90–95 % of the whole protolith: 1. Fe 2 O 3tot -Al 2 O 3 -SiO 2 -rich lateritic bauxite detritus; 2. illite-rich desert soil particles with high K 2 O, SiO 2 , Al 2 O 3 contents. The calculated relative proportions are: component 1): 61 %, component 2): 39 %. Minor detrital components are given by: 1) Ba-rich mineralised rock fragments, a witness to Late Triassic hydrothermal circuits; 2) detritus supplied by Na, Ca, Mg-rich salts abundant in hot desert soils. The very low TiO 2 and very high SiO 2 contents of the calculated lateritic bauxite component strongly suggest a granite to rhyolite protolith for the primary bauxite deposit, the ancestor of the Apuan lateritic bauxite component.

The rock-avalanche of February 1995 at Claix (French Alps)

The sedimentology of a rock-avalanche involving ca. 7·10 3 m 3, that happened on February 1995 at Claix, the French Alps, is analysed here. The talus shows a proximal depression due to the impact of the debris, then a grooved area with few, mainly sandy silt deposits and a downslope accumulation of coarse debris that reaches up to 3 m in thickness. The material underwent mass movement as demonstrated by the steep distal front, the strong fabric strength and the thrust planes. The greatest part of the deposits consists of a clast-supported to partially openwork debris with angular clasts over a sheared matrix-supported basal layer. The largest blocks, often more than 1 m in length, are concentrated at the surface and the front. The overall sedimentological characteristics are similar to those of the small dry grain flows that occur usually on rockfall talus, except for the abundance of fine-grained particles that results from intense cataclasis during flow. It is suggested that the deposits of such medium-scale rock-avalanches, probably not uncommon in seismic regions, may have been confused in previous studies with other slope mechanisms such as debris flow and rockfall.

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.

Talus fabric in Tuckerman Ravine, New Hampshire: Evidence for a tongue-shaped rock glacier

Tuckerman Ravine is a glacial cirque located in the White Mountains of New Hampshire. The Ravine contains four talus accumulations formed through rockfall and toppling and characterized by large joint blocks with long axes of at least one meter. Previous research has classified one of the deposits (identified as Site 1 in the current study) variously as a moraine, a lobate rock glacier, and a protalus rampart. To resolve this controversy and provide a more reliable interpretation, block fabric analysis was performed at this and the other three talus sites. A bimodal fabric distribution was encountered at Site 1 and implies that the blocks at the base of the deposit collectively met some obstruction to movement. Such arrangement is not accounted for in simple talus accumulation models or in previous interpretations. The fabric data and its other characteristics support the classification of Site 1 as a relict tongue-shaped rock glacier. Fabric analysis indicates that the majority of blocks at Sites 2, 3, and 4 have a preferred orientation in the downslope direction and that these deposits represent rockfall talus which has not experienced post-depositional movement or activity. The mere presence of the talus deposits and their locations (which include the cirque headwall) do not support the reactivation of cirque glaciers in the White Mountains during the Holocene.

Comparing the characteristics of rockfall talus and snow avalanche landforms in an Alpine environment using a new methodological approach: Massif des Ecrins, French Alps

The geometry and sedimentology of 49 talus slope deposits have been analysed in the French Alps to show the effects of snow avalanche activity on their characteristics. Twenty slopes are heavily avalanche-modified, 15 are snow-avalanche boulder tongues and 14 are rockfall-talus slopes. Slope angle, slope segmentation, longitudinal and sorting indices provide criteria to differentiate these deposits. Although the transition from rockfall- to avalanche-dominated slopes is gradational, a “transitional talus” is identified with distinctive characteristics.

Sedimentation processes and facies on a semi-vegetated talus, Lousteau, southwestern France

A two-year survey of clast displacements on a small rockfall talus at Lousteau, southwestern France, shows that creep reaches up to 19 cm/month near the talus apex, whereas the distal segment remains stable. Movements are not distributed homogeneously but concentrate in distinct areas because of the deflection of clasts by vegetation patches. Sections across the talus reveal stratified deposits, thought to result from the lateral shifting of the tongues of very mobile debris. Poorly sorted gravel layers alternate with coarser debris corresponding to former pavements developed where sedimentation is reduced due to the protective effect of vegetation. Similar deposits have been found in larger, semi-vegetated taluses from the forest belt of the French Alps. Interactions between vegetation development and sedimentation possibly controlled by the climatic changes during postglacial times, may be responsible for their stratification. Copyright © 1999 John Wiley & Sons, Ltd.

A complex slope failure on Beinn nan Cnaimhseag, Assynt, Sutherland

A rare, multi‐phased form of slope failure is described from the Assynt area of NW Scotland. Although possessing some of the characteristics of a rock glacier, detailed analysis of the clastic deposits involved in the slope failure suggest that it is best interpreted as a complex landslide. After the downwastage of the Late Devensian ice sheet, a rockfall talus sheet developed on the southern slopes of Beinn nan Cnaimhseag. The removal of confining pressure imposed by the ice sheet, together with the presence of mylonised facies and fissuring due to tectonic movement along the Ben More thrust plane which cuts across the top of this hill, led to rock sag and collapse of the central part of the slope. Holocene rockfall talus cones have subsequently formed, and partly bury the upper slopes of the blocky apron of debris created by the landslide. Large boulders found on the upper surface of the blocky debris are due to later toppling processes from the free face below the summit of the hill.

Breccia-hosted manganese-rich minerals of Alpi Apuane, Italy: A marine, redox-generated deposit

Mn-rich beds occur within the “Brecce di Seravezza” Formation (BSFm) intercalated between Norian metadolostone-Megalodontic marble and Hettangian marble in the Alpi Apuane, (Italy). The BSFm is a rockfall talus deposit that accumulates at the base of the footwall scarp of normal faults bordering extensional marine coastal basins. The type sequence of metabreccia ends with a bed of chloritoid schist interpreted as the metamorphic equivalent of lateritic soils. The four main types of Mn mineral assemblages are: (1) braunite, piemontite, and rare hausmannite; (2) piemontite, braunite, hollandite and minor rhodochrosite and kutnahorite; (3) braunite, hollandite and rare piemontite. The other minerals are: quartz, calcite, muscovite, phlogopite, baryte and minor hematite, rutile and apatite. During the Alpine orogeny pressures of 4–6 kb and temperatures of 350 °–380 °C were attained; Mn assemblages with hematite and phlogopite indicate ƒO 2 > 10 −8 for T= 350 °C and ƒO 2 ≥ 10 −6 when braunite appears. On the Mn-rich rocks Ti, Fe, Nb, Sn, Ta, Hf, Th, Ga, Rb, Pr, K, Na, Nd were identified as detrital in origin and are positively correlated with Al. U is positively correlated with Co, Pb, Bi, Mo. The rare earth element patterns normalized to seawater show a negative slope towards the heavy rare earth elements, with positive Ce anomalies in some samples, positive Ce and Eu anomalies in other samples and negative Ce and positive Eu anomalies in a third group. The Eu concentrations are nearly constant and the Eu anomalies reflect differences in the concentration of other rare earth elements; for Ce, a positive correlation with Al 2O 3 + SiO 2 indicate that a detrital signature prevails on that produced by duration of seawater exposure. But a significant good correlation of La, Tm, Lu with Mg could indicate, at least for these elements a seawater signature. Only in some samples, Ba, Eu, Sr show anomalously high concentrations, likely of hydrothermal origin. However, generally in all the chemical variation diagrams the BSFm samples fall outside the field of hydrothermal and hydrogenous deposits as defined in the literature for other Mn deposits. In addition, metabasites and metavolcaniclastic beds are absent from the BSFm sequence and BSFm coastal basins were for the most part separated from the open ocean and related streams, that must be excluded as possible Mn-sources. Owing to the arid climate of Late Triassic the runoff was negligible and the groundwater, oxidizing and alkaline, were unable to mobilize Mn. The BSFm basins are then the most suitable depositional environment for Mn. The water column was stagnant and stratified. Only where bottom of the basin was deeper than the oxic-anoxic boundary, reducing marine bottom waters percolated within the fractured basement and leached Mn from disseminated minerals of metamorphic rocks and, then, rising along fault and fractures, supplied Mn to the bottom waters. Afterwards, when tectonic uplift and sealevel drop brought the bottom into the oxic field, Mn precipitated and formed the Mn-rich matrix of the BSFm deposits.

Rates of periglacial processes in the Central Tianshan, China

The periglacial landforms of the Central Tianshan are mainly distributed above the lower boundary (3200 m ASL) of permafrost. In the eastern part of the region, there are 130 days per year when the daily average air temperature fluctuates around 0°C. The annual precipitation is 430 mm. Thus, periglacial landform development reflects the high frost-thawing frequency. By contrast, in the western part, there are only 90 days per year when the daily average air temperature fluctuates around 0°C; thus, periglacial landforms are those typical of areas with low frost-thawing fequencies, even though the annual precipitation reaches 827 mm. Based on observations from 1985 to 1989, schist (bedrock) cracks have a maximal dilatational value of 19 mm/a in the region, and gneiss cracks have a maximal dilatational value of 4 mm/a. The frost-heaving of diorite reaches 8–40 mm/a. The moving rate of rockfall talus is 1 m/a. On sunny slopes (195°SW), the talus debris has amounted to as much as 31.5 cm in the past 25 years, but on semi-shady slopes (65°NE), the debris accumulation is only 5 cm. The alluvial talus (scree) has a downward movement rate of 146 cm/a on south-facing slopes, and 73 cm/a on north-facing slopes. Block slopes have a downward movement rate of 96 mm/a on sunny or semi-sunny slopes and 72 mm/a on the shady or semi-shady slopes; the average rate is 81 mm/a in the study region. East of 86°E, the lobate rock glacier is the main type and its mean moving rate is about 6 cm/a, as deduced by debris discharge which has covered the highway over the past 25 years. Measurements from 1985 to 1986 show that the movement of lobate rock glaciers is from 1 to 49 cm/a. Rates at the higher elevation (15.5–49 cm/a) are faster than at the lower elevation. At the same elevation, the rates on the northward slopes (about 49 cm/a) are greater than on southward slopes (45 cm/a). According to lichenometry data, lobate rock glaciers formed between 3949 a BP to 180 a BP. West of 86°E, the main rock glacier type is the tongue-shaped rock glacier. The mean movement rate is 0.4 m/a. Gelifluction lobes and terraces have an average annual movement of 25 cm in the east. In the west, the average annual movement rate of gelifluction is 49 cm/a.

Transport, Sorting and Deposition Processes of Alpine Debris Slope Deposits in the Polish Tatra Mountains

This study is an inventory of different types of talus slopes found in the Polish Tatra mountains close to Zakopane. Several active and inactive types were found; rockfall talus, alluvial talus and complex slopes formed by different processes. The slope deposits are described in relation to the altitudinal zonation of climates and the active processes. The paper finally presents an hypothesis, based on our findings, for the Postglacial slope deposition in this part of the Tatra Mts.

Transport, Sorting and Deposition Processes of Alpine Debris Slope Deposits in the Polish Tatra Mountains

This study is an inventory of different types of talus slopes found in the Polish Tatra mountains close to Zakopane. Several active and inactive types were found; rockfall talus, alluvial talus and complex slopes formed by different processes. The slope deposits are described in relation to the altitudinal zonation of climates and the active processes. The paper finally presents an hypothesis, based on our findings, for the Postglacial slope deposition in this part of the Tatra Mts.

Alpine Mass Movement Forms (Noncatastrophic): Classification, Description, and Significance

A variety of noncatastrophic and distinctly slope, with the rare (in the alpine) block field, alpine mass movement forms may be classi- and block stream as subtypes. These mass fled under three basic terms: (1) talus, with movement forms reveal past and present local rockfall talus, alluvial talus, avalanche talus, and microclimates, rates of erosion, postglaavalanche boulder tongue, and protalus ram- cial valley wall, cliff, and ridge crest changes, part as subtypes; (2) rock glacier, subdivided and periglacial events during Neoglaciation. into tongue-shaped and lobate; and (3) block

Surface Stone Movement and Scree Formation

The accumulation of rockfall talus slopes has been studied for conditions in which stones are added relatively slowly, so that dynamic interactions between them may be ignored, and stone movements modeled "one-at-a-time." Laboratory scale models (1 m long) are compared with the results of mathematical and computer simulation. Mathematical analysis assumes that stone movement down the talus is equivalent to a "sliding-only" movement, and laboratory results support this assumption. Grain-size grading is explained in terms of the relative roughness of different material sizes to stones moving down the talus. Profile concavity is ascribed to the run-out of some material, corresponding to the tail of the frequency distribution for travel distances. Growth of the talus profile is modeled as a kinematic wave. The lower concavity obtained on both stochastic and scale models relative to deterministic models is thought to result from local accumulations of stones in the upslope parts of the talus which then produce small mass-movements at a constant slope.