Angular Boulders Research Articles

Inselbergs shaped by collapse: considerations on the structural control on granitic scarps

Inselbergs characterised by fracturing features correspond to the geomorphological manifestation of break-up of the rock mass and collapse. The steep slopes partially covered by angular boulders scattered chaotically reveal, in evolutionary terms, the predominance of the morphostructural controls, given by the presence of veins/dikes, sheeting joints, and vertical fractures. These structural planes in turn create anisotropies guiding fracturing directions and rockfall. In this work, we propose a model of geomorphological evolution of granitic inselbergs subjected to a structural propensity to fracturing. For this purpose, we analysed the Pedra da Galinha Choca inselberg (PGC) and Pedra Faladeira (Quixadá, NE of Brazil) inselbergs, whose morphologies attest to mechanical disintegration processes. Data extraction and interpretation were based on fieldwork and UAV photogrammetry, which provided products such as high-resolution models, orthomosaic, and DEM. Networks of multidirectional fractures were observed to promote an arrangement of interlocking blocks, allowing the maintenance of steep slopes (>60°), in which solutional features tend to be incipient due to the structural instability promoting collapse. Thus, the main features on the hillslopes are cavities like collapse tafone (CT) and large collapsed boulders (LCB). As a result of this structural setting, the inselberg displays sharp outlines, steep slopes, and wide talu deposits.

Lithofacies and Depositional Environments of the Garubathan Alluvial Fan, North Bengal, India

Abstract The sedimentology of the Garubathan alluvial fan of north Bengal foothills is a product of Himalayan neotectonics and monsoon climate. The active neo-tectonics of the Main Central Thrust (MCT), Ramgarh Thrust (RT) and Ghis Transverse Fault (GTF) produce a huge amount of angular clasts in the upper reaches of the river Chel and Mal. During the wet season, either sediment flow by gravity or turbulent stream flow or both, move the clasts to the downhill. The tectonic zone of Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT) are relative passive thrusts than the MCT, RT and GTF in the study area. A very low angle slope is observed in the area of MBT and MFT of the study area. So, the downhill deposition occurred in MBT and MFT zones of the study area. The river Chel and Mal are perinial rivers. During dry periods (pre-monsoon and post-monsoon), in the absence of sediment flow by gravity or turbulent stream flow, the low stream competency deposit the fine sediments. To identify the depositional environments three, seven and six lithofacies varieties were identified in the upper, middle and lower fan surfaces, respectively. The non-cohesive sediment flows by gravity deposited large angular boulders, cobbles and pebbles in the upper fan area. These deposits are either chaotic, unsorted, un-imbricated or graded and imbricated. Deposition of sand is scanty. Both, sediment flow by gravity and fluvial processes deposited angular to round clasts with matrix in the middle fan area. In this area the deposits are reverse graded, reverse to normally graded, laminated and cross-bedded. The high facies diversity of this area indicates highly variable nature of the depositional environments. In the lower fan area, presence of both clast-supported chaotic and matrix-supported laminated or cross-bedded layers suggests a complex nature of the depositional environment of sediment flow by gravity and fluvial processes.

Modulating the lateral migration of a gravel bed channel using the coarse tail of the bed material distribution

SummaryMost stream channels around the world have been, to some extent, laterally constrained by non‐erodible banks constructed of concrete or large angular boulders (called riprap). These lateral constraints generally have a negative impact on aquatic ecosystems by reducing stream channel complexity, and limiting the natural disturbance processes that are critical for renewing the physical habitat. In this paper, we test a novel approach to limiting stream channel instability by exploiting the stabilizing effects of the coarsest grains in the bed material distribution, which we call geomorphic river engineering. This approach modulates the rate of bank erosion rather than halting it altogether. It also exploits the geomorphic feedback between bank erosion, channel width, and mean boundary shear stress during a flood event to limit the potential for vertical incision into the stream bed. The basic premise of the geomorphic river engineering approach is to place the coarse sediment on the floodplain adjacent to the channel margins, and rely on bank erosion by the river to recruit the sediment, allowing the river to remain the architect of its own configuration. Our results suggest that the recruitment of a modest volume of coarse sediment to a coarse, gravel bed stream during a flood could potentially dramatically reduce the amount of lateral migration that would otherwise occur, and that the degree of stabilization could be modified by changing the volume of coarse sediment per unit area placed on the floodplain. More generally, our results confirm the fundamental role that the coarse tail of the bed material distribution plays in dictating the channel dynamics of gravel bed streams with cohesionless banks, and indicate the potential for future river stabilization projects to focus on the engineering that can be done by rivers, rather than to them.

Petrography, geochemistry and depositional model of Ispikan Conglomerate, Makran Accretionary Prism, Southwest Pakistan

Sedimentology, petrography, and geochemistry of the Ispikan Conglomerate of southwest Makran have been studied to establish its stratigraphic position, age, provenance, and depositional environment. Very thin to massive beds, poor sorting, no fabric, and poor grading are the common features. It is composed of mostly reworked, medium- to coarse-grained pebbly sandstone, siltstone, and discontinuous lenses of matrix- and clast-supported conglomerate. The sandstone is composed of angular to sub-angular, poorly sorted, immature grains having Q72F13L15 as average composition. Pebbles are of mostly metamorphic and acidic igneous origin. Large angular boulders (~1.5m) of sandstone indicate quick debris flow conditions. Geochemical discriminants suggest the derivation of Ispikan sediments from an active continental margin. The Nb and Zr/Th and Ba/Y values indicate the continental island arc setting, whereas a high K/Rb ratio points to the derivation of detritus from an acid and intermediate source. Ispikan Conglomerate is interpreted as an Eocene olistostrome formed after a localized submarine debris flow triggered by slope failure.

Late Pleistocene Boulder Slumps Eroded from a Basalt Shoreline at El Confital Beach on Gran Canaria (Canary Islands, Spain)

This study examines the role of North Atlantic storms degrading a Late Pleistocene rocky shoreline formed by basaltic rocks overlying hyaloclastite rocks on a small volcanic peninsula connected to Gran Canaria in the central region of the Canary Archipelago. A conglomerate dominated by large, ellipsoidal to angular boulders eroded from an adjacent basalt flow was canvassed at six stations distributed along 800 m of the modern shore at El Confital, on the outskirts of Las Palmas de Gran Canaria. A total of 166 individual basalt cobbles and boulders were systematically measured in three dimensions, providing the database for analyses of variations in clast shape and size. The goal of this study was to apply mathematical equations elaborated after Nott (2003) and subsequent refinements in order to estimate individual wave heights necessary to lift basalt blocks from the layered and joint-bound sea cliffs at El Confital. On average, wave heights in the order of 4.2 to 4.5 m are calculated as having impacted the Late Pleistocene rocky coastline at El Confital, although the largest boulders in excess of 2 m in diameter would have required larger waves for extraction. A review of the fossil marine biota associated with the boulder beds confirms a littoral to very shallow water setting correlated in time with Marine Isotope Stage 5e (Eemian Stage) approximately 125,000 years ago. The historical record of major storms in the regions of the Canary and Azorean islands indicates that events of hurricane strength were likely to have struck El Confital in earlier times. Due to its high scientific value, the outcrop area featured in this study is included in the Spanish Inventory of Geosites and must be properly protected and managed to ensure conservation against the impact of climate change foreseen in coming years.

Glacial ripping: geomorphological evidence from Sweden for a new process of glacial erosion

ABSTRACT In low relief Precambrian gneiss terrain in eastern Sweden, abraded bedrock surfaces were ripped apart by the Fennoscandian Ice Sheet. The resultant boulder spreads are covers of large, angular boulders, many with glacial transport distances of 1–100 m. Boulder spreads occur alongside partly disintegrated roches moutonnées and associated fracture caves, and are associated with disrupted bedrock, which shows extensive fracture dilation in the near surface. These features are distributed in ice-flow parallel belts up to 10 km wide and extend over distances of >500 km. Our hypothesis is that the assemblage results from (1) hydraulic jacking and bedrock disruption, (2) subglacial ripping and (3) displacement, transport and final deposition of boulders. Soft sediment fills indicate jacking and dilation of pre-existing bedrock fractures by groundwater overpressure below the ice sheet. Overpressure reduces frictional resistance along fractures. Where ice traction overcomes this resistance, the rock mass strength is exceeded, resulting in disintegration of rock surfaces and ripping apart into separate blocks. Further movement and deposition create boulder spreads and moraines. Short boulder transport distances and high angularity indicate that glacial ripping operated late in the last deglaciation. The depths of rock mobilized in boulder spreads are estimated as 1–4 m. This compares with 0.6–1.6 m depths of erosion during the last glaciation derived from cosmogenic nuclide inventories of samples from bedrock surfaces without evidence of disruption. Glacially disrupted and ripped bedrock is also made ready for removal by future ice sheets. Hence glacial ripping is a highly effective process of glacial erosion.

Vztah mezi výskytem balvanů v ledovcových sedimentech a charakterem preglaciálního podloží v předpolí Rychlebských hor a ve Zlatohorské vrchovině

In Rychleby Mountains foreland and Zlaté Hory Highlands exists relationship between frequency of boulders in glacigenic sediments and character of preglacial bedrock. On the soft bedrock (Neogene sediments, kaolinic granitoid) occurs lodgement of boulders. Glacial sediments on the soft bedrock are therefore characterized by high concentration of boulders. On the hard bedrock (non-kaolinic fresh granitoid, metamorphites) there is no lodgement or the lodgement is sporadic. Glacial sediments on the hard bedrock have therefore lower concentration of boulders. On suitable places the glacier detached from the hard bedrock angular boulders which the glacier deposited in the subglacial mobile deformable bed. Loose local boulders from cracked bedrock or eroded glactectonites and tills into glacifluvial sediments also occurred. In the sediments are occurring small boulders (the size is just above the most coarse gravel) of rocks from Rychleby Mts. and Zlaté Hory Highland which the glacier took over from the preglacial colluvial, alluvial and fluvial sediments.

Impact of Neotectonism in the Discussion of Geomorphological Processes as a Feedback System: North Bengal Foothills, West Bengal

The ongoing research paper is concerned about the sensitive issue like neotectonism and its impact on the landscape development. It is a detail and micro level study of North Bengal Himalayan foothills where the effect of tectonism is precisely observed. The term, neotectonism was introduced by Obruchen (1948) to summarize the active tectonic processes including the last major tectonic configuration change and establishment of modern stress. The North Bengal Himalayan foothill are very much concerned about the neotectonism activities at last to present geological periods. It is the one of the active unstable area of NE India where faults/thrusts and presence of synformal axis are predominantly controlling the geomorphological landscape development. This frontal region of an active orogenic beltlikethe NE Himalaya is characterized by complex terrace morphology with fluvial aggradation, degradationandstrath terraces. The detail study is mainly about the lineaments, drainage sequences, thrusting zone and synformal depressed zone of the area. Near the Neora-Mal river valley, close to to the mountain front a thick supported boulder bed with huge angular boulders of gneisses are exposed near Neora valley of Samsing area. Concerning the fault zone area of Matiali scarp, it is very clearly observed the extended part of Garubathan thrust (MBT) under the Matiali formation near the upper part of Kurtiriver. But, in the middle part of the river, where the synformation axis is present, river terraces are magnificiently developed over two sides of R. Kurti near Matiali hat. Here, two terraces are well virtualized but the upper most flat terrain is designated as fan surface which is made of mainly by the sediments of Matialiformation. In this region, slope of the fan surface is southerly 2.1˚. In south of the Matiali scarp on the banks of R. Neora, Kurti, Murti raised terraces are distinctly observed as T1 and T2.There kinds of features have developed due to the active tectonic activities from last Pleistocene to recent Holocene. As an evidence , we can consider the recent seismic study with in meizoseismal zone of both the 1934 Bihar-Nepal and 1950 Assam earthquakes but last surface rupture with a recorded displacement of 14m of Chalsafault related to the historical earthquake in Nepal around 1100 AD (Kumar et.al. 2011). It is a positive feedback system where neotectonism and geomorphological landscape are well related and in a classical model of fault propagation; geomorphic evolution through the structure/process is present in respect of time and space.

Schmidt‐hammer exposure‐age dating (SHD) of snow‐avalanche impact ramparts in southern Norway: approaches, results and implications for landform age, dynamics and development

AbstractSchmidt‐hammer exposure‐age dating (SHD) was applied to the problem of dating the diachronous surfaces of five distal river‐bank boulder ramparts deposited by snow avalanches plunging into the Jostedøla and Sprongdøla rivers in the Jostedalsbreen region of southern Norway. Approaches to local high‐precision linear age calibration, which controlled in different ways for boulder roundness, were developed. The mean age (SHDmean) and the maximum age (SHDmax) of surface boulders were estimated for whole ramparts, crests and distal fringes. Interpretation was further assisted by reference to R‐value distributions. SHDmean ages (with 95% confidence intervals) ranged from 520 ± 270 years to 5375 ± 965 years, whereas SHDmax ages (expected to be exceeded by <5% of surface boulders) ranged from 675 to 9065 years. SHD ages from the Jostedøla ramparts tended to be older than those associated with the Sprongdøla, rampart crests were younger than the respective distal fringes, and use of relatively rounded boulders yielded more consistent SHD ages than angular boulders. The SHDmean ages indicate differences in recent levels of snow‐avalanche activity between ramparts and provide insights into rampart dynamics as boulders are deposited on rampart crests and, in smaller numbers, on the distal fringes. SHDmax ages provide minimum age estimates of rampart age (i.e. the time elapsed since the ramparts began to form) and suggest that at least some of the ramparts have been developing since the early Holocene. Copyright © 2015 John Wiley & Sons, Ltd.

Geomorphology and weathering characteristics of erratic boulder trains on Tierra del Fuego, southernmost South America: Implications for dating of glacial deposits

Erratic boulder trains (EBTs) are a useful glacial geomorphological feature because they reveal former ice flow trajectories and can be targeted for cosmogenic nuclide exposure dating. However, understanding how they are transported and deposited is important because this has implications for palaeoglaciological reconstructions and the pre-exposure and/or erosion of the boulders. In this study, we review previous work on EBTs, which indicates that they may form subglacially or supraglacially but that large angular boulders transported long distances generally reflect supraglacial transport. We then report detailed observations of EBTs from Tierra del Fuego, southernmost South America, where their characteristics provide a useful framework for the interpretation of previously published cosmogenic nuclide exposure dates. We present the first comprehensive map of the EBTs and analyse their spatial distribution, size, and physical appearance. Results suggest that they were produced by one or more supraglacial rock avalanches in the Cordillera Darwin and were then transported supraglacially for 100s of kilometres before being deposited. Rock surface weathering analysis shows no significant difference in the weathering characteristics of a sequence of EBTs, previously hypothesized to be of significantly different age (i.e., different glacial cycles). We interpret this to indicate that the EBTs are much closer in age than previous work has implied. This emphasises the importance of understanding EBT formation when using them for cosmogenic nuclide exposure dating.

Lightning as a geomorphic agent: Reply to comments by Sumner

Sumner's comments are mainly concerned with the climatic interpretation of angular boulders and associated landforms. This goes beyond the aims and scope of our paper (Knight and Grab, 2014), which are to examine the role of lightning as a geomorphic agent on mountain summits. We critically address three issues raised by Sumner: (1) that we have misinterpreted the literature that we cited in our paper; (2) that the role of lightning during the last glacial maximum can be evaluated; and (3) that studies of processes, geomorphology and palaeoclimate in the Drakensberg, southern Africa, do not make use of circular reasoning. We reiterate that lightning strikes are a significant geomorphic agent in generating angular debris on low-latitude mountain summits, and therefore to make uncritical climatic inferences based on the presence of ‘frost shattered debris’ is erroneous and should be avoided.

Internal structure and permafrost characteristics of the rock glaciers of southern carpathians (romania) assessed by geoelectrical soundings and thermal monitoring

Six rock glaciers in the outhern arpathians have been investigated by means of geoelectrical soundings in order to detect their internal stratigraphy and the existence of frozen sediments. In the case of three relict rock glaciers, the electrical resistivity measurements indicated a typical internal structure. Low resistivity values (<10-kΩm) which are typical of unfrozen fine‐grained materials were obtained, but high resistivity values (25–240-kΩm) measured in the ietroasa, Ieşu and ietrele rock glaciers denote the presence of sediments cemented by interstitial ice and ice lenses. Based on the moderate resistivity values, the ice content is probably low to medium in the upper portion of these rock glaciers, that is, above 2040 m.At two sites (ietroasa and ăiuga rock glaciers), ground surface temperature evolution was monitored using digital dataloggers. Mean annual ground surface temperature and regime throughout the winter were extracted from the recordings and confirmed the probability of permafrost occurrence in ietroasa rock glacier. In the Ieşu and ietrele rock glaciers, measurements of bottom temperatures of the winter snow cover were performed in March 2012. Considering the thick active layer, the reduced ice content and the presence of scarce vegetation on their surface it could be assumed that the permafrost exists in marginal conditions in the outhern arpathians. The ground ice in permafrost is produced by the groundwater freezing or by snow banks buried by coarse angular boulders following large rockfalls.

Inland fields of dispersed cobbles and boulders as evidence for a tsunami on Anegada, British Virgin Islands

Marine overwash from the north a few centuries ago transported hundreds of angular cobbles and boulders tens to hundreds of meters southward from limestone outcrops in the interior of Anegada, 140 km east–northeast of Puerto Rico. We examined two of several cobble and boulder fields as part of an effort to interpret whether the overwash resulted from a tsunami or a storm in a location where both events are known to occur. One of the cobble and boulder field extends 200 m southward from limestone outcrops that are 300 m inland from the island’s north shore. The other field extends 100 m southward from a limestone knoll located 800 m from the nearest shore. In the two fields, we measured the size, orientation, and spatial distribution of a total of 161 clasts and determined their stratigraphic positions with respect to an overwash sand and shell sheet deposit. In both fields, we found the spacing between clasts increased southward and that clast long-axis orientations are consistent with a transport trending north–south. Almost half the clasts are partially buried in a landward thinning and fining overwash sand and none were found embedded in the shelly mud of a pre-overwash marine pond. The two cobble and boulder fields resemble modern tsunami deposits in which dispersed clasts extend inland as a single layer. The fields contrast with coarse clast storm deposits that often form wedge-shaped shore-parallel ridges. These comparisons suggest that the overwash resulted from a tsunami and not from a storm.

Pronival rampart formation in relation to snow-avalanche activity and Schmidt-hammer exposure-age dating (SHD): Three case studies from southern Norway

Pronival (protalus) ramparts at three sites in southern Norway—Smørbotn, Alnesreset and Nystølsnovi—are assessed in relation to evidence of snow-avalanche activity, and dated using Schmidt-hammer exposure-age dating (SHD). The position of each rampart suggests frequent snow avalanches played a major role in rampart formation. The pronival ramparts at Smørbotn and Nystølsnovi are characterized by a high proportion of unweathered, very angular clasts, many clean boulders (without any lichen or moss cover), steep distal slope angles, and a disturbed ground cover of vegetation with characteristic alpine pioneer plant species on crests and proximal slopes. High Schmidt-hammer R-values (mean 56.2–58.5) yielded SHD ages of <2900years and <1550years, respectively. These ramparts are currently active and have probably developed since the early Holocene under environmental conditions similar to those at present. Pronival ramparts at Alnesreset are characterized by highly weathered, mainly angular boulders (virtually none of which is clean), a mature, undisturbed vegetation cover, and lower distal slope angles. Mean R-values of 39.9–40.3 from these ramparts yielded SHD ages within the range 9450 to 14,650years (with 95% certainty), indicating formation during the Younger Dryas Stadial and relict status for most of the Holocene. Our results: (1) show that rockfall is not the only primary process involved in the formation of pronival ramparts; (2) broaden the range of landforms included in snow-avalanche geomorphology; (3) demonstrate the feasibility of dating pronival ramparts; and (4) indicate their potential in palaeoenvironmental reconstruction.

Surface morphological features of boulders on Asteroid 25143 Itokawa

Abstract On the sub-kilometer S-type asteroid, 25143 Itokawa, some boulders on rough terrains seem to be exposed without any powdery material covering. Based on surface morphological features, there are two major types of boulders: one has rounded edges and corners (rounded boulders), while the other has angular edges and corners (angular boulders). The surface features of the rounded boulders suggest that they have hardness heterogeneity and that some may be breccias. The angular boulders appear to be more resistant to impact disruption than the rounded ones, which may be due to a difference in lithology. The major constituents of Itokawa may be LL chondrite-like brecciated lithology (rounded boulders) along with a remarkable number of boulders suggesting that lithology is atypical among LL chondrites (angular boulders). Some of both types of boulders contain intersecting and stepped planar foliations. Comparison with meteorite ALH76009 suggests that the planar foliations may be marks where rocks were torn apart. As lithified breccias cannot be formed on present-day sub-kilometer-sized Itokawa, it is reasonable that boulders with various lithologies on Itokawa were formed on its large ancestor(s). The rubble-pile structure of Itokawa suggested by its low density (∼1.9 g/cm 3 ) indicates that boulders on Itokawa are reassembled fragments formed by catastrophic disruption of large ancestor(s).

Characterization of mortars used for the construction of Saithidai Heroon Podium (1st century AD) in ancient Messene, Peloponnesus, Greece

The Saithidai Heroon Podium is a Mausoleum dated to the Roman Period located in ancient Messene, Peloponnesus, Greece. Its outer walls were constructed with porous and massive limestone blocks with no mortar in the joints. The core of the podium was filled by rock chips to angular boulders of sandstone, limestone, sand, gravel and earthy groundmass material. The groundmass is mostly composed of quartz and calcite, with minor amounts of clay minerals. Small, irregular lime lumps also occur, composed of authigenic euhedral calcite crystal assemblages and rare sub-microscopic quartz fragments, or rod-like CaO/SiO 2-rich mineral assemblages developed in pore spaces on fine-grained authigenic calcite crystals. The mortar used for the filling material of the Podium was poorly homogenized semi-hydraulic lime derived from the calcination of pure limestone and siliceous limestone fragments, possibly the waste material from the finishing of the building blocks of the Podium walls.

Surface-exposure ages of Front Range moraines that may have formed during the Younger Dryas, 8.2 cal ka, and Little Ice Age events

Surface-exposure ( 10Be) ages have been obtained on boulders from three post-Pinedale end-moraine complexes in the Front Range, Colorado. Boulder rounding appears related to the cirque-to-moraine transport distance at each site with subrounded boulders being typical of the 2-km-long Chicago Lakes Glacier, subangular boulders being typical of the 1-km-long Butler Gulch Glacier, and angular boulders being typical of the few-hundred-m-long Isabelle Glacier. Surface-exposure ages of angular boulders from the Isabelle Glacier moraine, which formed during the Little Ice Age (LIA) according to previous lichenometric dating, indicate cosmogenic inheritance values ranging from 0 to ∼3.0 10Be ka. 1 1 Surface-exposure ages in this paper are labeled 10Be; radiocarbon ages are labeled 14C ka, calendar and calibrated radiocarbon ages are labeled cal ka, and layer-based ice-core ages are labeled ka. 14C ages, calibrated 14C ages, and ice core ages are given relative to AD 1950, whereas 10Be ages are given relative to the sampling date. Radiocarbon ages were calibrated using CALIB 5.01 and the INTCAL04 data base Stuiver et al. (2005). Ages estimated using CALIB 5.01 are shown in terms of their 1-sigma range. Subangular boulders from the Butler Gulch end moraine yielded surface-exposure ages ranging from 5 to 10.2 10Be ka. We suggest that this moraine was deposited during the 8.2 cal ka event, which has been associated with outburst floods from Lake Agassiz and Lake Ojibway, and that the large age range associated with the Butler Gulch end moraine is caused by cosmogenic shielding of and(or) spalling from boulders that have ages in the younger part of the range and by cosmogenic inheritance in boulders that have ages in the older part of the range. The surface-exposure ages of eight of nine subrounded boulders from the Chicago Lakes area fall within the 13.0–11.7 10Be ka age range, and appear to have been deposited during the Younger Dryas interval. The general lack of inheritance in the eight samples probably stems from the fact that only a few thousand years intervened between the retreat of the Pinedale glacier and the advance of the Chicago Lakes glacier; in addition, bedrock in the Chicago Lakes cirque area may have remained covered with snow and ice during that interval, thus partially shielding the bedrock from cosmogenic radiation.

The Muralla Pircada—an ancient Andean debris flow retention dam, Santa Rita B archaeological site, Chao Valley, Northern Perú

Debris flows caused by El Nino events, earthquakes, and glacial releases have affected northern Peru for centuries. The Muralla Pircada, a northeast-trending, 2.5 km long stone wall east of the Santa Rita B archaeological site (Moche-Chimu) in the Chao Valley, is field evidence that ancient Andeans recognized and, more importantly, attempted to mitigate the effects of debris flows. The Muralla is upstream from the site and is perpendicular to local drainages. It is 1–2 m high, up to 5 m wide, and is comprised of intentionally-placed, well-sorted, well-rounded, 20–30 cm cobbles and boulders from nearby streams. Long axes of the stones are gently inclined and parallel local drainage. Case-and-fill construction was used with smaller cobbles and pebbles used as fill. Pre-Muralla debris flows are indicated by meter-sized, angular boulders that were incorporated in-place into construction of the dam and are now exposed in breeches in the dam. Post-Muralla debris flows in the Chao Valley are indicated by meter-sized, angular boulders that now abut the retention dam.

Age and origin of upland block fields on melville peninsula, eastern canadian arctic

Block fields have developed on gently graded uplands of granite and gneiss on central and southern Melville Peninsula. The location of block fields is not controlled by elevation, but rather by areas covered by cold–based ice during the last glaciation. Block fields consist either of angular boulders, sorted circles 3–4 m across having blocky rims and central areas of weathered grus, concentrations of openwork boulders, or in the southeast, of immature bouldery till. The block fields are primarily relict features predating the last glaciation, produced by weakening of bedrock by weathering along joints, followed by frost heave, although some block fields have been modified by meltwater from regional glacier down–wasting. Frost processes are active in the modern environment but the extent of riven bedrock and the size of recent patterned ground forms on postglacial surfaces are insufficient to account for the forms in the block fields.

岩石氷河 氷河説と周氷河説

Recent progress in rock glacier studies is reviewed with some emphases on the competition between the glacial and periglacial hypotheses. Rock glaciers are tongue-shaped or lobate bodies composed of angular boulders that resembles a small glacier, usually accompanied by multiple transverse ridges resulting from a compressive flow. Rock glaciers are classified, in terms of the origin of surface materials, into talus and morainic rock glaciers, and in the light of the activity status, into active, inactive and fossil ones. The distribution of active rock glaciers are delimited by the regional glacier equilibrium line and lower limit of mountain permafrost.The internal structure of rock glaciers has been approached by direct observations and indirect geophysical soundings. In some rock glaciers, natural outcrops exhibit a massive ice body with debris bands beneath the surface boulder layer, which has encouraged the glacial hypothesis. Massive ice was also found in boreholes penetrating through a rock glacier permafrost in the Swiss Alps, despite being considered to originate from snow avalanche or refrozen meltwater. In fact, deformation occurred mostly in the frozen debris layer beneath the massive ice, indicating the periglacial origin of the rock glacier due to permafrost creep. Geophysical soundings, including seismic, geoelectric and gravimetric measurements, have provided useful information on the three-dimensional structure, stratigraphy and ice contents of rock glacier bodies, although authors preferring the glacial hypothesis tend to reject such indirect results. The origin of any rock glacier is thus equivocal without detailed analyses of internal stratigraphy and ice composition.Most of the active rock glaciers are moving at a speed of 101 cm yr-1, two orders of magnitude slower than 'ice' glaciers. The periglacial model attributes such a slow movement to permafrost creep. A possible consequence of this is that active rock glaciers usually have ages of several thousand years ; that inactive ones were activated repeatedly during the colder periods of the Holocene; and that fossil ones moved during the Late Glacial. In contrast, the glacial model explains that many rock glaciers originated from a debris-covered glacier during the Little Ice Age and have been loosing their ice content and speed rapidly with the 20th-Century warming.Only a few rock glaciers have been identified from Japanese mountains. Although locations favorable for active rock glaciers are restricted to the northern side of some high mountains, the mountain permafrost belt must have been wide enough to form a number of rock glaciers during some past cold periods. Subsequent permafrost melting would have fossilized these rock glaciers, some of which may have been misinterpreted as glacial moraines or protalus ramparts.