Rockfall Debris Research Articles

Landsliding in an urban area

The city of Wollongong is nestled within a narrow coastal plain approximately 70 km south of Sydney in the state of New South Wales (NSW), Australia (Fig. 1 ). Over the last 150 years of settlement the population of the Wollongong area, extending from Helensburgh in the north to Windang in the south has increased to about 200 000 people. The coastal plain is triangular in shape with a coastal length of 25 km. It is widest in the south at Yallah (14 km) and narrows sharply towards the north, disappearing north of Thirroul. The coastal plain is bounded to the north, west and south by an erosional escarpment ranging in height from 300 m up to 500 m. Fig. 1 Location plan. The escarpment consists of slopes with moderate to steep inclinations with several intermediate benches and cliff lines. Spectacular cliffs of Hawkesbury Sandstone (Middle Triassic) cap the escarpment and there is dense vegetation over most of the escarpment below these cliffs (Fig. 2 ). Fig. 2 Escarpment with large rock fall debris flow near Dombarton railway siding, 1974. Photo by Young. The main road link to Sydney is the F6 freeway that traverses the escarpment via Mount Ousley Road. There are …

Sediments and landforms at Kvı́árjökull, southeast Iceland: a reappraisal of the glaciated valley landsystem

Abstract Eyles identified Kviarjokull, an outlet of the icecap Oraefajokull, southeast Iceland, as an exemplar of the glaciated valley landsystem, emphasising rockfall debris supply, passive transport and reworking of a thick cover of supraglacial morainic till. In this paper, we take a fresh look at the sediments and landforms of Kviarjokull, focusing on clast form analysis to reconstruct primary transport pathways. New data demonstrate the importance of active transport of debris derived from the glacier bed. Substantial portions of the supraglacial debris cover can be traced back to englacial channel fill deposits or anomalously thick exposures of basal ice. These two types of sediments also make up much of Kviarjokull's large Neoglacial moraine rampart. We attribute the abundance of these non-rockfall sediments to the presence of a terminal overdeepening, and the switch from predominantly subglacial to predominantly englacial drainage that this induces. This hydrological switch enhances retention of debris within ice transport, preventing it from being washed away and making it available to feed ice-marginal moraine formation. The complexity of sediment transport processes observed at Kviarjokull and the contrast between it and alpine glaciers undermines any single concept of the glaciated valley landsystem; accordingly, we propose Kviarjokull as the type example of a distinct subclass, defined by its moderate relief, high-debris turnover, and drainage behaviour characteristic of an overdeepened basin.

Errata: Neogene fjordal sedimentation on the western margin of the Lambert Graben, east Antarctica

The Lambert Graben is occupied by the world’s largest fjord system, through which flows the Lambert Glacier, the Amery Ice Shelf and their tributaries. Along the western margin of the graben, in the northern Prince Charles Mountains, remnants of uplifted Miocene and Pliocene strata of the glacigenic fjordal Pagodroma Group total more than 800 m in thickness. These sediments provide evidence for a dynamic East Antarctic ice sheet during the Neogene Period. Each of the four Pagodroma Group formations defined from this region rests unconformably on either Proterozoic or Permo-Triassic rocks. The unconformity surfaces represent parts of the walls and floors of Neogene fjords. For these surfaces to have been eroded, the ice must have been grounded out as far as the continental shelf in Prydz Bay. The Pagodroma Group was deposited by wet-based glaciers discharging into a fjordal setting and includes lithofacies that are quite different from those produced by modern Antarctic ice masses. The principal lithofacies are massive diamicts and soulder gravels, deposited both close to a calving, grounded glacier terminus and from icebergs. The few stratified diamicts are the product of more distal iceberg sedimentation. An ice-transported gravel lithofacies includes rockfall debris derived from palaeofjord walls and mixed with subglacially derived diamicts. Some lithofacies contain evidence of subaquatic slumping and gravity flowage. Volumetrically minor lithofacies include laminites, with some exposures exhibiting large ice-rafted clasts. The laminites represent less proximal, mainly ice-free fjordal sediments, resulting either from tidal-current sorting of suspended sediment originating from subaquatic glaciofluvial discharge, or from turbidity currents derived from unstable subaquatically deposited glacigenic sediment. The Pagodroma Group provides a record of multiple glaciation by dynamic, sliding glaciers carrying large amounts of both basal and supraglacial debris. The closest modern analogues, in terms of the thermal and dynamic characteristics of the Neogene Lambert Glacier, appear to be the fast-flowing tidewater glaciers of East Greenland. These glaciers originate from the interior ice sheet and discharge large volumes of icebergs; the resulting lithofacies are predominantly diamicts.

Catastrophic mass movement of 1998 monsoons at Malpa in Kali Valley, Kumaun Himalaya (India)

A devastating landslide on 18 August 1998 near Malpa Village in Kali Valley of Higher Kumaun Himalaya killed 221 persons. The landslide was a complex rock fall–debris flow. The mass movement generated around one million cubic metres of debris and partially blocked the Kali River, Malpa Gad (a tributary of Kali) being blocked completely. The rock mass failed primarily due to the near vertical slopes hanging over the valley along joints, the formation of structural wedges along the free face, the sheared rock mass due to the close proximity of major tectonic planes, and the enhanced pore–water pressure due to prolonged heavy precipitation in the preceding days. The mesoscopic shear zone, exhibiting ramp and flat structure in quartzites, shows a southward thrust movement that might have generated shear stress in the rocks. The slide clearly demonstrates the distressed state of the rock mass in the Himalayan region due to the ongoing northward drift of the Indian plate.

Neogene fjordal sedimentation on the western margin of the Lambert Graben, East Antarctica

The Lambert Graben is occupied by the world’s largest fjord system, through which flows the Lambert Glacier, the Amery Ice Shelf and their tributaries. Along the western margin of the graben, in the northern Prince Charles Mountains, remnants of uplifted Miocene and Pliocene strata of the glacigenic fjordal Pagodroma Group total more than 800 m in thickness. These sediments provide evidence for a dynamic East Antarctic ice sheet during the Neogene Period. Each of the four Pagodroma Group formations defined from this region rests unconformably on either Proterozoic or Permo-Triassic rocks. The unconformity surfaces represent parts of the walls and floors of Neogene fjords. For these surfaces to have been eroded, the ice must have been grounded out as far as the continental shelf in Prydz Bay. The Pagodroma Group was deposited by wet-based glaciers discharging into a fjordal setting and includes lithofacies that are quite different from those produced by modern Antarctic ice masses. The principal lithofacies are massive diamicts and soulder gravels, deposited both close to a calving, grounded glacier terminus and from icebergs. The few stratified diamicts are the product of more distal iceberg sedimentation. An ice-transported gravel lithofacies includes rockfall debris derived from palaeofjord walls and mixed with subglacially derived diamicts. Some lithofacies contain evidence of subaquatic slumping and gravity flowage. Volumetrically minor lithofacies include laminites, with some exposures exhibiting large ice-rafted clasts. The laminites represent less proximal, mainly ice-free fjordal sediments, resulting either from tidal-current sorting of suspended sediment originating from subaquatic glaciofluvial discharge, or from turbidity currents derived from unstable subaquatically deposited glacigenic sediment. The Pagodroma Group provides a record of multiple glaciation by dynamic, sliding glaciers carrying large amounts of both basal and supraglacial debris. The closest modern analogues, in terms of the thermal and dynamic characteristics of the Neogene Lambert Glacier, appear to be the fast-flowing tidewater glaciers of East Greenland. These glaciers originate from the interior ice sheet and discharge large volumes of icebergs; the resulting lithofacies are predominantly diamicts.

Snowline depression in the tropics during the Last Glaciation

Five primary methods have been used to reconstruct Pleistocene snowlines or equilibrium-line altitudes (ELAs) in the tropics (23.5°N–23.5°S) during the last glaciation, but each has inherent errors that limit the accuracy of the results. Additional potential errors in determining ELA depression involve estimates of modern snowline altitude, dating resolution, topographic reconstruction of former glaciers, orographic effects, the presence of rockfall debris on glaciers, and calculation of regional ELA gradients. Eustatic sea-level lowering during the last glaciation is an additional factor influencing estimates of ELA depression (ΔELA). In cases where modern snowline lies above a mountain summit, only a minimum value for ΔELA can be obtained. At 12 tropical sites in Africa, the Americas (to 10°S latitude), and Pacific islands, estimates of average ΔELA range from 440 to 1400 m, but most fall in the range of 800–1000 m (mean ±1 σ=900±135 m). In a regional study of ELA depression in the southern tropical Andes (8–22°S), an average ΔELA of ca 920±250 m has been reported. Based on the assumption that glacier mass balance was controlled solely by ablation-season temperature, and assuming a full-glacial temperature lapse rate of −6°C/km, depression of mean annual temperature in glaciated alpine areas was ca 5.4±0.8°C. If adjusted for a sea-level fall of −120 m at the glacial maximum, this value is reduced to 4.7±0.8°C. The figure is based on the (unlikely) assumption that accumulation on alpine glaciers has been invariant; nevertheless, it is similar to values of temperature depresson (5–6.4°C) for the last glaciation obtained from various terrestrial sites, but contrasts with tropical sea-surface temperature estimates that are only 1–3°C cooler than present.

Rockfall activity from an alpine cliff during thawing periods

Rockfall activity during thawing periods was investigated by collecting rockfall debris fallen on the snow-covered talus slope in a cirque in the Japanese Alps. Near-surface rock temperature was also monitored on the cirque wall. Maximum rockfall activity occurs on average about 10 days after the meltout of the cirque wall. The intensive activity is rarely associated with precipitation events or diurnal frost cycles on the rockface. A thermal conduction model suggests that rockfalls at this site occur most frequently when the seasonal thawing front penetrates to a depth of about 1 m in the bedrock. While the freeze–thaw penetration controls the maximum dimensions of detachable rock mass, joint spacing on the rockface affects the size distribution of rockfall debris. Boulder falls resulting from seasonal frost weathering are considered to be the most important process responsible for the contemporary modification of the cirque wall. The rockfall volume during a thawing period is usually 1–3 m 3, which is equivalent to the rate of cirque wall retreat of the order of 0.01 mm year −1. However, big boulder falls that occur once per decade can increase the retreat rate significantly.

Reassessment of supposed early 'Little Ice Age' and older Neoglacial moraines in the Sandane area of western Norway: a reply to Matthews, Nesje and Dahl

Further evidence for an early 'Little Ice Age' in the Sandane area, southern Norway, is presented in support of the lichen data of Evans et al. (1994) and in response to the reassessment of Matthews et al. (1996). The geomorphological evidence for three phases of moraine deposition is clarified and relative age dating based upon Schmidt hammer R-values and soil development is presented. These data, in combination with the lichenometry reported earlier, indicate that the moraines of the Jardalen/Fagerdalen cirque in particular are not all of the same age (pre 'Little Ice Age' as suggested by Matthews et al. (1996) but instead date either to post-Preboreal/Younger Dryas times (Stage 1 moraines) or to the 'Little Ice Age' (Stage 2 and 3 moraines). The gradual increase in lichen diameters, R-values and soil depths from the youngest 'Little Ice Age' (Stage 3) moraine to the outermost and oldest Stage 2 moraine lends further support to the suggestion that the 'Little Ice Age' in western Nor way may have begun much earlier than previously documented in the literature. Problems in using the Schmidt hammer for relative age dating in mountainous terrain, where the moraines being dated are often con structed from old glacial deposits and fresh rock fall debris, are also dis cussed.

Structure and Debris Characteristics of Medial Moraines in Jotunheimen, Norway: Implications for Moraine Classification

AbstractFour medial moraines on three glaciers in Jotunheimen, Norway, are described. The structure and debris characteristics of the moraines allow three debris-transport routes to be identified: (1) discrete concentrated longitudinal septa containing subglacially comminuted debris; (2) diffuse longitudinal septa containing passively englacially transported debris; and (3) supraglacially transported spreads containing rock-fall debris entrained below the firn line. Two of the moraines, occurring down-glacier of nunataks, are nourished by a combination of (1) and (3). The remaining two moraines are nourished solely by englacially transported rock-fall debris (2). Transport routes (1) and (2) represent two distinct types of longitudinal englacial debris septum. The contribution of debris from more than one glacial transport path to single medial moraines indicates the value of a flexible, process-based approach towards moraine classification, with emphasis on the dynamic links between basin morphology, glacial debris entrainment, and moraine characteristics.

The characteristics and significance of some lateglacial protalus ramparts in upland Britain

AbstractArcuate and linear protalus ramparts of inferred Loch Lomond Stadial age are widespread in upland Britain outside the limits of contemporaneous glaciers. Survey and analysis of the morphology of nine ramparts suggests that these may have formed at the foot of slowly thickening snowbeds, with rockfall debris accumulating against their distal slopes, and that snowbed melting at the end of the stadial was uninterrupted by prolonged periods of stability or renewed growth. Rampart sediments consist of poorly‐sorted assemblages of clasts with a variable infill of fines. Clast size and shape are strongly influenced by lithology, but rampart clasts are characteristically more angular and ‘slabbier’ than those of similar lithology in ice‐marginal moraines. Ramparts may also be distinguished from moraines by their location, morphology, lack of a true matrix of fines and absence of erratics, and from talus‐foot rock glaciers in terms of their width and the absence of flow structures. The mapped distribution of rampart altitudes across the Scottish Highlands displays a marked regional trend, with the lowest features in the west and south and highest in the Cairngorms. This pattern mirrors that of reconstructed firn line altitudes of Loch Lomond Stadial glaciers, and is inferred to reflect a pronounced eastwards and northwards decline in snowfall during the stadial.

Intra-slope canyon morphology and its modification by rockfall processes, U.S. Atlantic continental margin

Side-scan sonar data have been acquired at three different range settings and scales for a section of the continental slope. The data allow new perspectives on canyon morphology at varying complementary resolutions. Both regional context and site-specific details are provided. Intra-slope canyons on the lower slope are associated with outcrops of Middle Eocene carbonate rocks. Canyon geometries are heavily influenced by structural patterns of intersecting joints. Drop core analyses of surface sediments suggest that intra-slope canyon formation is not a geologically recent event, but occurred before 50,000 yrs B.P. Within the canyons, rocky debris results from episodic rockfall activity from the steep canyon walls. Dating of canyon floor sediments with inclusions of rockfall debris at one location suggests the last discrete period of rockfall there began about 10,000 yrs B.P. and ended about 6000 yrs ago. The rockfall activity may have coincided with a period of increased submarine dissolution along the margin that accompanied the upslope migration of the Western Boundary Undercurrent. Elements of the lower slope terrain may reflect surface solutional degradation of the joint patterns associated with the Eocene outcrops over longer time periods.

Chronology and palaeoenvironment of levantine prehistoric sites as seen from sediment studies

Analysis of sediments from the sites of et-Tabun, Jebel Qafza, and Sefunim in Israel, of Ksar 'Aqil in Lebanon, and of Yabrud Rockshelter I and Jerf 'Ajla in Syria leads to a reconstruction of the environments of deposition, periods of weathering and erosion, and relations to changing sea level during the times of occupation of these sites by prehistoric man. The overlapping sequences span the interval from part or all of the last interglaciation through the time of the last glaciation into historic time. Aeolians and from the last interglacial littoral zone and aeolian silt from more distant deserts dominate the site sediments until the early part of the last glaciation (especially at Tabun), whereas colluvial slope deposits, alluvium (Ksar 'Aqil), and angular rockfall debris are characteristic of the mid-last glaciation sediments in most sites, commonly with an admixture of reworked terra rossa soil sediment. These latter sediments seem to reflect a period or periods of greater available moisture or surface run-off. Prominent unconformities mark many sites at times coincident with the final middle palaeolithic (mousterian) occupations or in the interval between middle palaeolithic and upper palaeolithic occupations. It is not clear, in the absence of firm radiometric dates, whether these hiatuses should be correlated from site-to-site or whether they are site-specific. Freeze/thaw phenomena appear not to have played a significant role, if any at all, in the origin of sediments in coastal Levantine sites, although the middle palaeolithic of the Syrian Desert (Yabrud, Jerf 'Ajla) is contained in typical cryoclastic rubble. Finally, the reconstructed sedimentary environments are compared with the still-too-sparse palynological record for the Near East. Parallels of more humid and less humid climatic intervals throughout the past 60,000 to 70,000 years in both of these records reinforce the growing impression of a regionally fluctuating climate in the eastern Mediterranean region during the time of the last glaciation.

A rock glacier and moraine–ridge complex, Lyngen Peninsula, north Norway

A low-altitude rock glacier associated with two small come glaciers on the Lyngen Peninsula is described on the basis of aerial-photographic interpretation. It is argued that a rock glacier originated when rock-fall debris from the corrie headwall covered the gently-sloping lower part of the two coalesced glaciers during the 19th century. Continued glacier wastage resulted in the separation of the glacier from the rock glacier. The rock glacier has experienced markedly differential flow; one part, by 1954, had advanced as a narrow lobe beyond the Neoglacial moraine ridges into the valley bottom. Some theoretical implications of rock glaciers cored by glacier ice are considered: the relationship of debris accumulation to ice mass balance on such a thin corrie glacier appears to be critical, and if the ablation is effectively, reduced by a debris cover, then a rock glacier can form. It is proposed that such a rock glacier's ice core can be replenished by a much-reduced ice mass gain to the upper corrie glac...

Photo-Interpretation of two Types of Rock Glacier in the Colorado Front Range, U.S.A.

AbstractTwo types of rock glacier occur in the Colorado Front Range. Rock glaciers on the floors of modern cirques closely resemble the tongues of small valley glaciers. Because they contain cores of banded glacial ice and grade up-valley into lateral moraines, rock glaciers of this type are believed to represent the debris-covered tongues of former glaciers. Most consist of two or more superimposed lobes, bounded by longitudinal furrows, and resulting from independent ice advances. Despite their compound nature, the complexes now appear to be moving down-slope as single units. Two generations of “cirque-floor” rock glaciers, both tentatively dated as being of post-Pleistocene age, occur in the Front Range.Rock glaciers of an entirely different character occur beneath steep valley walls, where they are supplied with debris by avalanche couloirs. Interstitial ice, responsible for the movement of “valley-wall” rock glaciers, probably results from the metamorphism of snow buried beneath rock-fall debris or supplied by winter avalanching.

Photo-Interpretation of two Types of Rock Glacier in the Colorado Front Range, U.S.A.

Abstract Two types of rock glacier occur in the Colorado Front Range. Rock glaciers on the floors of modern cirques closely resemble the tongues of small valley glaciers. Because they contain cores of banded glacial ice and grade up-valley into lateral moraines, rock glaciers of this type are believed to represent the debris-covered tongues of former glaciers. Most consist of two or more superimposed lobes, bounded by longitudinal furrows, and resulting from independent ice advances. Despite their compound nature, the complexes now appear to be moving down-slope as single units. Two generations of “cirque-floor” rock glaciers, both tentatively dated as being of post-Pleistocene age, occur in the Front Range. Rock glaciers of an entirely different character occur beneath steep valley walls, where they are supplied with debris by avalanche couloirs. Interstitial ice, responsible for the movement of “valley-wall” rock glaciers, probably results from the metamorphism of snow buried beneath rock-fall debris or supplied by winter avalanching.