Scree Research Articles

Rockfall Kinematics from Massive Rock Cliffs: Outlier Boulders and Flyrock from Whitney Portal, California, Rockfalls

ABSTRACT Geologic conditions and topographic setting are among the most critical factors for assessing rockfall hazards. However, other subtle features of rockfall motion may also govern the runout of rockfall debris, particularly for those sourced from massive cliffs where debris can have substantial momentum during transport. Rocks may undergo collisions with trees and talus boulders, with the latter potentially generating flyrock—launched rock pieces resulting from boulder collisions that follow distinctively different paths than the majority of debris. Collectively, these intricacies of rockfall kinematics may substantially govern the hazards expected from rockfall to both persons and infrastructure located beneath steep cliffs. Here, we investigate the kinematics, including outlier boulder and flyrock trajectories, of seismically triggered rockfalls on 24 June 2020 that damaged campground facilities near Whitney Portal, CA, a heavily used outdoor recreation gateway to the Sierra Nevada mountains. Our results, obtained in part by rockfall runout model simulations, indicate that outlier boulder trajectories resulted from opportunities provided by less steep terrain beyond the talus edge. The influence of trees, initially thought to have served a protective capacity in attenuating rockfall energy, appears to have been negligible for the large boulder volumes (>50 m3) mobilized, although they did potentially deflect the trajectory of flyrock debris. Rockfall outlier boulders from the event were comparable in volume and runout distance to prehistoric boulders located beyond the talus slope, thereby providing some level of confidence in the use of a single rockfall shadow angle for estimating future rockfall hazards at the site.

The Thickness of Talus Deposits in the Periglacial Area of SW Spitsbergen (Fugleberget Mountainside) in the Light of Slope Development Theories

Periglacial slopes are susceptible to recent climate change. The rate of morphogenetic processes depends on numerous factors. The most important of these is the warming of the air and ground, increased precipitation (extreme rainfall in particular), and the rate of snow cover decay. The dynamics of these processes may effectively modify contemporary slope development models. The paper shows the structure of selected talus slopes on a Fugleberget mountainside, based on field observations and radar (GPR) soundings. The results are then compared to classical slope models. The radar survey in April and May 2014 used a RAMAC CU II Malå GeoScience system equipped with a 30 MHz RTA antenna (Rough Terrain Antenna). Five GPR profiles of different lengths were obtained along the talus axes, transversally on Fugleberget and partly on the Hansbreen lateral moraine. According to the radar soundings, the maximum thickness of the debris deposits is 25–30 m. The thickness of the weathered material increases towards the talus cone’s terminal part, and debris deposits overlap marine sediments. The talus slopes’ morphometry shows that their current forms differ from standard slope models, which may be due to the significant acceleration of geomorphic processes resulting from climate change, including rapid warming in the last four decades.

Morphometric quantification of cockpit karst in a Jamaican bauxite mining concession

A digital surface model (DSM) is used to characterize the landscape and topical karst landforms within a bauxite mining lease in north central Jamaica. Five case study sites are selected for detailed geomorphic analysis and cartographic visualization using a GIS. In Jamaica, tropical cockpit karst landscapes are composed of several landforms: cockpits, glades, corridors, saddles, and talus slopes. Evidence for the existence and quantification of these landforms is provided in this paper. Study sites have a mean basin area of 96,514 m2, an average highest elevation of 541 m, mean lowest point of 444 m, with an average enclosed depression depth of 97 m. Depth and slope values within the study sites are consistent with previously published literature for cockpit karst landscapes. The literature also supports the findings in this study which are steep and convex slopes surrounding cockpit floors.

Геоинформационный анализ пространственной изменчивости геосистемы острова Райкоке (Курильские острова)

Volcanic eruptions significantly transform natural geosystems, which is most characteristic of components such as terrain, flora and fauna. The theoretical significance of this research lies in acquisition of new knowledge that contributes to a deeper understanding of response of natural environment to consequences of volcanic eruptions of various types and power. The practical significance is due to possibility of using the obtained results in substantiating assessment of volcanic hazard territories and compiling corresponding maps. Eruptions of the Raikoke volcano are rare, but all recorded facts of activity are characterized as strong, and significantly changed appearance of Raikoke Island. The research used satellite images from Sentinel-2 and Landsat satellites. Spatial dynamics of coastline was studied on the basis of synthesized images (NIR-SWIR-RED). In the research of vegetation transformation, time series of normalized relative vegetation index (NDVI) was used. We analysed the time course of changes in the area of Raikoke Island, the maximum and average NDVI values within its limits from 1972 to 2021, and also compiled a schematic map of the dynamics of the components of the geosystem of Raikoke Island. Grass and grass-shrub vegetation covered about 80 % of total area of island in 2018. The areas devoid of vegetation cover are confined to large mobile talus slopes, beach, marginal parts of lava flows around perimeter of the island, as well as large block mass collapse of crater floor. As of 2021, the closed vegetation cover occupies no more than 0.03 % of the island’s area and is fragmentally located in central part of southern slope, lower parts of eastern, northern and western slopes. An analysis of eruptive activity of the Raikoke volcano and the time course of NDVI suggests that it will take about 100 years for vegetation cover occupy an area comparable to area before eruption of 2019.

Low-altitude periglacial activity in southeastern Australia during the late Pleistocene

AbstractOnly a small area of the Australian mainland was glaciated during the Pleistocene, whereas periglacial deposits are far more common, indicating that cold environments were extensive and a major influence on landscape evolution. Here we identify representative low-elevation examples of scree slopes and frost action, together with fans and valley fills, indicating pronounced erosion cycles during the Pleistocene. To date the deposits, we explore approaches using radiocarbon, optically stimulated luminescence, and profile dating using the cosmogenic nuclide10Be. The radiocarbon and optical ages show that screes, alluvial valley fill, and fans were deposited between 66–13 ka during the coldest part of the last glacial cycle, and within the previous glacial cycle. Exposure dating indicates further landscape erosion cycles back to the mid Pleistocene. Together, the deposits indicate the frost cracking limit was ~1300 m lower at 680 ± 10 m and mean winter temperature was 8.2 ± 0.5°C colder than present. Periglacial conditions probably affected much of southeastern Australia. The treeless and dry conditions resulted in widespread erosion and increased run off. Combined with increased snow storage within catchments, rivers were paradoxically larger, with high seasonal discharge and sediment loads.

Chronostratigraphy of talus flatirons and piedmont alluvium along the Book Cliffs, Utah – Testing models of dryland escarpment evolution

Research on the evolution of dryland escarpments and what drives and controls their erosional retreat have been limited by a lack of chronologically constrained records. Prior investigations of escarpments in the southwestern U.S. and globally garner two endmember conceptual models -- one focused on bottom-up baselevel drivers and autogenic variations in piedmont and toeslope erosion processes, the other focused on the role of climate as a top-down forcing mechanism on sediment production and transport from cliffs. We test these conceptual models along a section of the Book Cliffs in central Utah, where climate has varied strongly and baselevel has fallen significantly over the late Quaternary.Four generations of talus flatirons and piedmont terraces are preserved in the study area we date them by optically stimulated luminescence and 10Be-exposure techniques. OSL depositional ages cluster into four generations of deposits with mean ages and mean standard errors of 117.8 ± 6.7 ka, 83.4 ± 3.3 ka, 52.4 ± 1.8 ka, and 5.6 ± 0.3 ka. 10Be-exposure results from talus boulders protruding from flatirons confirm that the youngest generation of talus is Holocene, with recent rock-fall boulders still being received on the landform top. Older flatiron generations produce incoherent, minimum exposure ages reflecting an increasing dominance of in situ boulder weathering and a steady-state denudation rate of ∼45 mm/ky after tens of thousands of years. Based on the chronostratgraphy and correlation of talus and piedmont deposits, we interpret climate as the primary control on escarpment evolution. Depositional ages do not correspond to glacial epochs as historically presumed, but instead to episodes of intermediate and unstable or variable Pleistocene climate during Marine Isotope Stage-3 and 5 a/b.Results support the significance of high climate variability at millennial timescales in driving both sediment production by mass wasting from the cliff as well as sediment storage and reworking along the piedmont. In contrast, wetter climates of glacial epochs apparently caused greater erosion and sediment transport to trunk drainages. Our chronology also reveals a systematic decrease in deposit age with increasing distance up the piedmont from the regional baselevel of the Price River, consistent with transient fluvial incision and terrace abandonment. The Book Cliffs record notably does not correlate with similarly constrained records in Spain or the contrasting record from the Negev, yet they all share the feature of enhanced talus deposition and cliff retreat corresponding to climate instability rather than in response to steady glacial-climate conditions. Thus, we outline a model of escarpment evolution and retreat governed largely by top-down climate-forcing of sediment production and transport during climate variability, with local changes in baselevel and erosional processes on the piedmont being a modifying, secondary factor.

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.

Analysis of Stability of Concealed Cataclinal Slopes with Talus Deposits Accumulated at Their Toes with Respect to Biplanar Failure

Talus deposits naturally form to a certain height in the toes of slopes because of the accumulation of collapsed material and small landslides. However, the stability of concealed cataclinal slopes with talus deposits accumulated at their toes (referred to as TDAT-CC slopes) with respect to biplanar failure is rarely studied. In this work, the typical geological profiles associated with TDAT-CC slopes are summarized and generalized and their failure modes are classified. A qualitative analysis of their stability is also carried out. The biplanar failure process is investigated by analyzing the development of plastic failure and displacement deformation using Universal Distinct Element Code. The analysis reveals the penetration process occurring in the biplanar sliding planes and clarifies the compression–shear failure processes taking place in such slopes. A new method for determining the stability of TDAT-CC slopes and predicting the failure surface formed is proposed based on limit-equilibrium theory and a model consisting of sliding mass blocks. Finally, the proposed method is used to calculate the stability of TDAT-CC slopes with deposits of different shapes and strengths and the results compared with the results of numerical simulations. The results obtained using the two methods are generally consistent. The talus deposits at the toe of the slope play the role of a toe buttress and help improve the slope’s stability. The safety factor of the slope increases as the inclination of the toe deposits becomes gentler; it also increases as the height of the toe deposits increases. The method proposed offers a sound basis for investigating biplanar failure in TDAT-CC slopes.

Tracking the behavior of rocky coastal cliffs in northwestern Spain

Rocky coastal regions are often inaccessible due to steep slopes and high relief. Remotely sensed data can, therefore, be useful, but they often have low spatial and temporal resolution and, in the case of airborne LiDAR, if not publically available, are costly to obtain. This paper reports on the use of high-resolution images from unmanned aerial vehicles (UAVs) and Structure-from-Motion (SfM) photogrammetric techniques, supplemented by a series of orthophotos and aerial LiDAR, to examine changes in rocky coastal cliffs from 2002 to 2018. The study was conducted over an 800 m-long, orthogneiss-dominated coastal section in northwestern Galicia, Spain. Cliff changes are due, primarily, to rockfalls, resulting from weathering and wave undercutting, which cause talus deposits to accumulate at the cliff foot. These deposits provide temporary protection to the cliff from wave action, until destabilized and removed by wave erosion and shallow landslides. Cliff recession rates are affected by changing conditions within a cycle and are dependent, in part, on when a survey is conducted. The data suggest that rates of cliff recession are increasing in this region and that the plan shape of the coast, which consists of headlands and bays, is continuing to evolve. Most coastal landslides in this region help to transport and dispose of talus fallen from the cliff. In contrast to landslides that remove intact materials from the cliff face, and are, therefore, primary erosional mechanisms, talus landslides are triggered by wave erosion and probably simultaneously, by storm wave spray and splash, rather than by heavy rainfall.

Population and vulnerability assessment of high value medicinal plants in the Alpine regions of western Himalaya

Declining population of medicinal plants in the wild is a major conservation issue. Researchers across the globe have advocated assessments of their populations for effective management prioritization. We, therefore, assessed populations of 15 high valued medicinal plant species of the Himalaya and carried out their rapid vulnerability assessment (RVA). Seven different habitats viz., gentle slopes (GS), moist areas (MA), rocky areas (RA), scree slopes (SS), shrub gaps (SG), under canopy (UC) and undulating meadows (UM) identified based on their physiognomy and landform were surveyed and sampled across six alpine sites in the state of Himachal Pradesh using quadrats. We found that distribution and population of different species varied significantly across habitats. Overall, Trillium govanianum reported the highest density (3.83 individuals/m2) and frequency (58.33%) while the lowest frequency (11.86%) was noted for Picrorhiza kurrooa. Amongst the 7 habitats, the highest species richness (13) and diversity (H′=2.118) were recorded in the GS and UM, respectively. As opposed to other species which occurred in multiple habitats, T. govanianum was restricted to UC only. The RVA score of the species ranged between 20 (reported by Heracleum candicans) to 33 (observed for Aconitum heterophyllum and Fritillaria roylei, each). Based on the conservation status, habitats of occurrence and RVA, A. heterophyllum, Dactylorhiza hatagirea, F. roylei, P. kurrooa, Sinopodophyllum hexandrum and T. govanianum have been identified as the reddest of the red. Since medicinal plants are habitat specific, thus heterogeneous landscape are key to in-situ medicinal plant conservation.

Quantitative assessment of the complexity of talus slope morphodynamics using multi-temporal data from terrestrial laser scanning (Tatra Mts., Poland)

From 2011 to 2019 high-mountain talus slopes in the Tatra Mts. were observed using terrestrial laser scanning (TLS) measurements. The digital-elevation models (DEM-s) derived from these measurements permitted the quantification of mass wasting processes with an accuracy of a few centimetres, and the lichenometric measurements allowed the detection of past activity of the talus slopes. The analysis focused on the mechanism and local conditions of talus slope response to hydrometeorological impulses in the context of climate change. The results showed that changes in talus slopes are essentially effects of debris flows. Significant differences in spatiotemporal changes between two adjacent talus cones were found to be related to their catchment lithologies. The spatial extent of talus slope change during the period of TLS monitoring is confined to debris flows tracks, and debris flows do not affect the entire talus cones as was the case in the past. Quantitative analysis of slope surface changes demonstrates that their final effect is building up of the talus slope, even if the debris flows moved part of the debris sediments out. Variability in the volume of displaced debris ranges from several dozen to almost 17,000 m3. A single input of loose debris from a massive rockfall dominated the effectiveness of debris flows and demonstrated the significance of the cooperation of mass wasting processes in talus slope morphodynamics as well as confirming the importance of the availability of loose debris for debris flows. This input influences slope morphodynamics, at least during the 8 years of observations, even if heavy rainfall occurred.

Directional arrangement of phyllite fragments in phyllite talus slope at the eastern margin of the Tibetan Plateau

Directional arrangement of phyllite fragments in phyllite talus slope at the eastern margin of the Tibetan Plateau

Assessing the ecological value of dynamic mountain geomorphosites

Abstract. Geoheritage is a component of geodiversity constituted by all the elements of geodiversity recognized by society for their particular values. The definition of these values, including the importance of geoheritage for biodiversity, plays a key role in the process of heritage recognition and geoconservation policymaking. In mountain environments, dynamic geomorphosites have a strong influence on plant diversity because the active geomorphological processes responsible for their formation act as renovators for habitats of pioneer species. In this paper, we propose criteria to assess the ecological value of dynamic mountain geomorphosites. We show that the interest of plant communities (species richness and presence of rare or protected species) and the influence of geomorphological processes on plant communities (disturbances, surface movement and soil) are fundamental criteria for assessing the ecological value in an exhaustive and objective way and that the question of the scale (local and national scales) is also a crucial parameter. We then illustrate this methodological proposal by evaluating the ecological value of three dynamic geomorphosites and a talus slope in the western Swiss Alps.

Contrasted effects of climate change on low-altitude relict Pinus uncinata stands in the Northern French Alps

This paper reports on climate-induced growth changes in relict, low-altitude mountain pines (Pinus uncinata Mill. ex. Mirb.) from two refugia with cold microclimates located in the Northern French Alps. The P. uncinata stands analyzed grow at the lower bound of their ecological limit and are thus thought to be sensitive indicators of ongoing climate change. Using dendroecological approaches, we compare tree-ring growth at two closely spaced low-altitude stands in the Chartreuse massif (French Alps): La Plagne and Cirque de Bresson. La Plagne is a N-NW-exposed, ventilated slope with cold air circulating in the scree during summer, and the presence of sporadic permafrost as well as ground overcooling, whereas Cirque de Bresson is located on a small, S-exposed fan with sporadic avalanche activity. At both sites, growth responses of P. uncinata to changes in twentieth and twenty-first centuries temperature and precipitation conditions were investigated by means of moving correlation analyses. At Cirque de Bresson, a significant and rapid decline in tree-ring widths has been observed since the early 1990s. We attribute this decline to (i) increasing air temperatures at the beginning of the growing season (May–June) as well as to (ii) a decrease in soil water potential. At La Plagne, we do not detect any significant trend between the higher summer temperatures and tree growth, presumably as a result of the circulation of cold air in the scree slope, which is thought to maintain fresh and humid soil conditions and therefore favor tree growth. These forest stands provide prime examples on how dendroecology can contribute to the study of the dynamics and local variability of tree growth and climate change in relict forest populations with high ecological and conservation values.

The glacier–rock glacier mountain landsystem: an example from North Iceland

ABSTRACT A valley floor rock glacier in Nautárdalur, eastern Tröllaskagi, northern Iceland, has been investigated since the 1970s. Adjacent valleys are also examined as mountain landsystems. The methodology identifies ‘landform elements’ (features evident on slopes) on transects that can be identified and mapped from Google Earth imagery. Transects can be compared to show similarities and differences between slopes and valleys and can be used to trace debris and ice/snow flow paths within landsystems. The paper confirms that a glacier ice core is present throughout the Nautárdalur rock glacier. Traces of glacier activity can be found in some neighbouring small valleys but where rock glaciers are not present. Previous field observations, when coupled to current Google Earth imagery, indicate glacier ice formation and deformation over the past 200 years. The quantities and timing of weathered debris from headwall cliffs were sufficient to partly cover small glaciers, thus forming rock glaciers in some, but not all, valleys. The landsystem and transect approach thus indicate glacier origins for the rock glaciers identified. No evidence for ‘permafrost creep’ in scree slopes that might produce valley-side rock glaciers has been found.

On pikas and ticks

The pika (Ochotona princeps) is a lagomorph that typically lives in rocky areas such as talus slopes, at elevations of 8000 to 13,000 feet in the Rocky Mountains of western North America. As the climate warms, species from lower elevations can shift their range upwards, but most pika populations have nowhere to go because they already live at the highest elevations (eg J Mammal 2020; doi.org/10.1093/jmammal/gyaa110). Range shifts of species from lower elevations might also harm pikas. In October 2020, one of us (Olmstead) photographed this pika near Silver Lake in Big Cottonwood Canyon, in the Wasatch Range within Salt Lake County, Utah. Numerous engorged ticks (species unknown) are evident in the lesion on its throat, and others appear to be around its ear. Johanna Varner, a pika expert at Colorado Mesa University (Grand Junction, CO), remarked: “None of us [referring to herself and other authorities] have ever seen a pika with so many ticks on it: in fact, your pika had more ticks on its cheek and ear than I’ve seen on all pikas combined in my 10 years of watching them!” Various tick species have expanded their ranges as the climate has warmed. If these and other ectoparasites become more prevalent in alpine zones, then the health of alpine wildlife populations, such as those of the already vulnerable pika, could be adversely affected.

Mechanical and topographic factors influencing lava dome growth and collapse

Lava dome collapse hazards are intimately linked with their morphology and internal structure. We present new lava dome emplacement models that use calibrated rock strengths and allow material behaviour to be simulated for three distinct units: (1) a ductile, fluid core; (2) a solid upper carapace; and (3) disaggregated talus slopes. We first show that relative proportions of solid and disaggregated rock depend on rock strength, and that disaggregated talus piles can act as an unstable substrate and cause collapse, even in domes with a high rock strength. We then simulate sequential dome emplacement, demonstrating that renewed growth can destabilise otherwise stable pre-existing domes. This destabilisation is exacerbated if the pre-existing dome has been weakened following emplacement, e.g., through processes of hydrothermal alteration. Finally, we simulate dome growth within a crater and show how weakening of crater walls can engender sector collapse. A better understanding of dome growth and collapse is an important component of hazard mitigation at dome-forming volcanoes worldwide.

Geomorphological information mapping of debris-covered ice landforms using Google Earth: An example from the Pico de Posets, Spanish Pyrenees

Geomorphological mapping records ‘snapshots’, making it difficult to investigate change and sensitivity in changing environments. The high resolution now available in Google Earth (GE) permits re-investigation of landscape features/landforms previously recorded in the literature or mapped. Transects across geomorphologically interesting locations can be provided with decimal degree WGS 84 locations and bearings via GE. Landform elements are identified, located and coded onto transects that summarise geomorphological information. Transects crossing rock cliffs, scree slopes, snowpatches, protalus ramparts, ‘vanishing’ glaciers, moraines and rock glaciers are investigated using GE on the Pico de Posets in the Spanish Pyrenees. Observations of geometry/form, materials and processes involved in ice and sediment movement are linked to the Randolph Glacier Inventory and map records. The transect mapping shows that accumulated weathered rock debris insulated a remnant Little Ice Age glacier that formed a lateral moraine and a contiguous rock glacier as the glacier wasted down. Landforms previously considered to indicate permafrost are shown not to do so. Glacigenic formation of moraines, protalus rampart and rock glaciers provides the simplest explanation for their origin.

Recurring taphonomic processes in the carnivoran‐dominated Late Miocene assemblages of Batallones‐3, Madrid Basin, Spain

Carnivoran-dominated fossil sites are scarce in the fossil record but provide precious information on the diversity and ecology of past carnivoran guilds. The Cerro de los Batallones sites host the oldest carnivoran-dominated assemblages, with the highest carnivoran abundances observed in the fossil record. Batallones-3 (Late Miocene, Madrid Basin, Spain) hosts three discrete, carnivoran-dominated fossiliferous levels deposited in a 15m-diameter, 4m-high pseudokarstic cavity with 1m-high talus cone located beneath the mouth of the cavity on the roof. Levels I, II and III are multitaxic multidominant assemblages, with the sabretooth cats Promegantereon ogygia and Machairodus aphanistus and the ursid Indarctos arctoides being the most abundant species. These carnivoran-dominated assemblages are autochthonous and show diagenetically fractured but well-preserved remains (complete, little to no weathering or abrasion). Root marks and manganese oxide precipitation are common and more abundant in Level III, due to modern pedogenic processes. There are also a few allochthonous, badly preserved (weathered and abraded) herbivore remains that were washed into the cavity. The taphonomic homogeneity of all three levels suggests recurring taphonomic and geologic processes throughout the accumulation of infill in the cave. The trap-like nature of the cave, unbroken and largely unweathered carnivoran bones suggest these predators intentionally jumped into the cave but were unable to escape.

Sediment size on talus slopes correlates with fracture spacing on bedrock cliffs: implications for predicting initial sediment size distributions on hillslopes

Abstract. The detachment of rock fragments from fractured bedrock on hillslopes creates sediment with an initial size distribution that sets the upper limits on particle size for all subsequent stages in the evolution of sediment in landscapes. We hypothesize that the initial size distribution should depend on the size distribution of latent sediment (i.e., fracture-bound blocks in unweathered bedrock) and weathering of blocks both before and during detachment (e.g., disintegration along crystal grain boundaries). However, the initial size distribution is difficult to measure because the interface across which sediment is produced is often shielded from view by overlying soil. Here we overcome this limitation by comparing fracture spacings measured from exposed bedrock on cliff faces with particle size distributions in adjacent talus deposits at 15 talus–cliff pairs spanning a wide range of climates and lithologies in California. Median fracture spacing and particle size vary by more than 10-fold and correlate strongly with lithology. Fracture spacing and talus size distributions are also closely correlated in central tendency, spread, and shape, with b-axis diameters showing the closest correspondence with fracture spacing at most sites. This suggests that weathering has not modified latent sediment either before or during detachment from the cliff face. In addition, talus at our sites has not undergone much weathering after deposition and is slightly coarser than the latent sizes because it contains unexploited fractures inherited from bedrock. We introduce a new conceptual framework for understanding the relative importance of latent size and weathering in setting initial sediment size distributions in mountain landscapes. In this framework, hillslopes exist on a spectrum defined by the ratio of two characteristic timescales: the residence time in saprolite and weathered bedrock and the time required to detach a particle of a characteristic size. At one end of the spectrum, where weathering residence times are negligible, the latent size distribution can be used to predict the initial size distribution. At the other end of the spectrum, where weathering residence times are long, the latent size distribution can be erased by weathering in the critical zone.