Evidence Of Abrasion Research Articles

Impact of climate on landscape form, sediment transfer and the sedimentary record

AbstractThe relationship between climate, landscape connectivity and sediment export from mountain ranges is key to understanding the propagation of erosion signals downstream into sedimentary basins. We explore the role of connectivity in modulating the composition of sediment exported from the Frontal Cordillera of the south‐central Argentine Andes by comparing three adjacent and apparently similar semi‐glaciated catchment‐fan systems within the context of an along‐strike precipitation gradient. We first identify that the bedrock exposed in the upper, previously glaciated reaches of the cordillera is under‐represented in the lithological composition of gravels on each of three alluvial fans. There is little evidence for abrasion or preferential weathering of sediment sourced from the upper cordillera, suggesting that the observed bias can only be explained by sediment storage in these glacially widened and flattened valleys of the upper cordillera (as revealed by channel steepness mapping). A detailed analysis of the morphology of sedimentary deposits within the catchments reveals catchment‐wide trends in either main valley incision or aggradation, linked to differences in hillslope–channel connectivity and precipitation. We observe that drier catchments have poor hillslope–channel connectivity and that gravels exported from dry catchments have a lithological composition depleted in clasts sourced from the upper cordillera. Conversely, the catchment with the highest maximum precipitation rate exhibits a high degree of connectivity between its sediment sources and the main river network, leading to the export of a greater proportion of upper cordillera gravel as well as a greater volume of sand.Finally, given a clear spatial correlation between the resistance of bedrock to erosion, mountain range elevation and its covariant, precipitation, we highlight how connectivity in these semi‐glaciated landscapes can be preconditioned by the spatial distribution of bedrock lithology.These findings give insight into the extent to which sedimentary archives record source erosion patterns through time.

Thousands of Potholes in the Mekong River and Giant Pedestal Rock from North-eastern Thailand: Introduction to a Future Geological Heritage Site

A site with thousands of potholes, locally named “Samphunbok (SPB),” and a giant pedestal (mushroom) rock possess a very high geological value. The potholes are being formed on the riverbed of the Mekong River; whereas the giant pedestal rock is the largest on the Khorat Plateau, and is located in Ubon Ratchathani province, north-eastern Thailand. Based on detailed geological, geomorphological, and sedimentological perspectives, this paper presents the geological history of both erosional features for the future promotion of the Global Geopark Network. The giant pedestal rock is likely to be a remnant of degradation and weathering after a regional tectonic event. It is sculptured in the fracture zone of thick sandstone beds after uplifting and folding. No evidence of abrasion by wind was found. The formation of potholes is limited inside the Mekong River bankfull elevation. The appearance of potholes in different levels of the sandstone beds infers their formation in relation to the vertical evolution of the Mekong River. All geological data shown in this paper confirm the rareness of potholes and the giant pedestal rock in terms of how they were sculptured and preserved through time. These erosional features at SPB are discussed and introduced in this paper as a global geological heritage site.

Scelidosaurus harrisonii from the Early Jurassic of Dorset, England: cranial anatomy

AbstractScelidosaurus harrisonii is an early (Late Sinemurian) armoured ornithischian dinosaur whose remains have, to date, only been recovered from a restricted location on the south coast of Dorset (Charmouth), England. This dinosaur has been known since 1859, but only on the basis of a partial description found in two articles published in the early 1860s by Richard Owen. The original material, discovered in 1858, comprised the majority of the skull and its associated postcranial skeleton, and represents the first ever, more or less complete dinosaur discovered. In addition to the original material, a number of further discoveries have been made at Charmouth; these latter supplement the information that can be gleaned from the original specimen. This article describes the skull of Scelidosaurus.The external surface of individual skull bones in ontogenetically relatively mature individuals displays exostoses, a patina of fibrous or granular-textured bone that anchored an external shielding of keratinous scales. There is a small, edentulous rostral beak, behind which is found a row of five heterodont premaxillary teeth. There is a minimum of 22 maxillary teeth and 27 dentary teeth in jaws of the largest well-preserved individuals known to date. Both dentitions (upper and lower) are bowed medially and are sinuous longitudinally. Maxillary and dentary crowns are tilted lingually on their roots, trapezoidal in outline and have crenellate (coarsely denticulate) margins. Adjacent crowns of teeth have mesiodistally (anteroposteriorly) expanded bases that overlap slightly and are consequently arranged en echelon. The dentitions are flanked by deep cheek pouches. Tooth abrasion is usually discontinuous along the dentition. In one individual nearly all teeth seem to be fully emerged and there is little evidence of abrasion. There is no physical evidence of a predentary, but the presence of this (typically ornithischian) element may be inferred from the structure of the symphyseal region of the dentary. The external narial and antorbital fenestrae are comparatively small, whereas the orbit and temporal fenestrae are large and open. A sclerotic ring was undoubtedly present and supported the eyeball, but it is too poorly preserved to allow it to be reconstructed with accuracy. A prominent supraorbital brow ridge overhangs the orbit. There are three osteoderms: palpebral, middle supraorbital and posterior supraorbital, sutured to the dorsal margin of the orbit. The occiput provides an area for attachment of a pair of curved, keratin-sheathed, osteodermal horns.Epistyloid bones project from the ventrolateral region of the braincase; their distal ends flank the anterolateral region of the neck. Rugose facets on either side of the basioccipital are suggested to have provided attachment sites for the epistyloid bones. Internally, the skull has a deeply vaulted snout and the nasal chambers are roofed by what are here named epivomer bones that appear to have been sutured to the dorsolateral edges of the vomers. Unusually, among dinosaurs generally, an epipterygoid is preserved attached to the dorsolateral surface of the pterygoid; there is no obvious point of articulation for the epipterygoid against the lateral wall of the braincase. A deep pit on the posterior surface of the quadrate of an immature specimen is suggestive of the existence of a remnant of cranial pneumatism. This pit becomes occluded in larger, more mature specimens.

An intertropical opossum (Mammalia, Marsupialia, Didelphidae) from the late Middle–Late Pleistocene of austral South America

The record of an opossum is reported here coming from a Middle–Late Pleistocene fluvial sedimentary sequence outcropping in the austral Chaco geomorphological region, Santa Fe province. Despite this family has different living forms widely distributed in South American plains, with great tolerance to diverse habitats conditions, the Pleistocene occurrence of didelphoids is very scarce and only limited to Holocene deposits sensu lato from austral pampean plains. The specimen found represents at the moment the only Pleistocene didelphoid opossum from Argentina with an accurate stratigraphical context. In a taphonomic framework, MFA-Pv 1582 shows no evidence of abrasion produced by transport, probably resulting of a very short time-transport. This intertropical form of Didelphis genus from Pleistocene sequences displays undoubtedly novel and sustancial paleoenvironmental and ecomorphological conditions to test in the austral Chaco region. Although the specimen shares a generalized didelphoid form, some morphologies in the humerus are closer with arboreal habits. In this context, added to the sedimentological and stratigraphical framework, it’s possible to suggest that subtropical conditions with some woodlands and wetlands dominate the provenance region of the fossil material, over a late Middle–Late Pleistocene interglacial period.

The Late Jurassic Cleveland-Lloyd Dinosaur Quarry as a Drought-Induced Assemblage

Abstract A comprehensive taphonomic analysis has yielded a novel interpretation for one of the most famous dinosaur quarries in the world. The Cleveland-Lloyd Dinosaur Quarry (CLDQ) traditionally has been interpreted as an attritional predator trap. This scenario is based largely on a remarkable 3:1 predator:prey ratio, dominated by the remains of the theropod Allosaurus fragilis. This study addresses the taphonomy of CLDQ by combining analyses of fossils and entombing sediments along with putative modern analogues. Thousands of bones have been excavated from CLDQ, representing at least 70 individual dinosaurs from a minimum of nine genera. The fossils occur in a 1-m-thick fine-grained calcareous mudstone interpreted as a floodplain ephemeral-pond deposit. The bones show minimal carnivore modification and surface weathering, whereas approximately 1/3 of the elements studied possess pre-depositional fractures and evidence of abrasion. The vast majority of elements are found horizontal to subhorizontal, wit...

Authigenic Ti-Bearing Crystals in a Precambrian Clay from Buenos Aires Province, Argentina

AbstractThe Precambrian Villa Mónica Formation clay was analyzed using petrographic and scanning electron microscopy, X-ray diffraction and chemical analysis, with the aim of characterizing the Ti-bearing phases present and determining their possible origin and/or source. Two Ti-bearing minerals were found to be present, rutile and anatase. The crystals are needles between 5 and 15 µm in size, with no evidence of abrasion or corrosion. They are commonly found in association with the (001) faces of illite flakes or in the pores between the flakes. Their disposition is similar in the three quarries studied. No zonation or differential settling due to differences in specific gravity was observed. In addition, chemical analyses indicated a positive correlation between TiO2 and K2O suggesting that both oxides behave similarly after the deposition of the sediment. The TiO2 content in the bulk fraction ranges from 0.8 to 1.98 wt.%, values that are similar to those exhibited in other clay deposits from different ages and geneses.All observations are consistent with an in situ origin of these Ti-bearing phases during post-depositional processes that included recrystallization of existing minerals and crystallization of new phases. The original detrital minerals such as biotite, ilmenite as well as detrital illite, were the primary sources of the TiO2 and of the Fe oxides that coat the clay. Understanding the origin and the reaction mechanisms involved during the post-depositional alteration of the Villa Mónica Formation suggests that the Ti-bearing phases in different sediment types were formed by similar mechanisms involving redox processes at low to medium temperatures, even in Precambrian sediments where the presence of rutile could lead to an incorrect assumption of high temperatures involved.

Volcanic ash bands in the Frontier Mountain and Lichen Hills blue-ice fields, northern Victoria Land

Dust bands in the blue-ice of the Frontier Mountain meteorite trap (northern Victoria Land, Antarctica) were previously reported as upthrust basal debris. Four of them have now been sampled at Frontier Mountain and Lichen Hills. The absence of local rocks and sedimentary fragments, the ubiquitous abundant volcanic glass with no evidence for abrasion, the igneous minerals, the chemical compositions of glass and minerals and the bulk chemical compositions indicate that they are volcanic ash bands (tephra) and not glacial debris. Although hardly distinguishable in the field, the different volcanic ash bands are discriminated using mineralogical and chemical data, as well as particle size, abundance and vesicularity of glass. Chronological constraints, particle size and chemical compositions localize the source for the Frontier Mountain and Lichen Hills tephra within the recent activity of the Mount Melbourne Volcanic Province in northern Victoria Land; possible emission centres are the Pleiades (40 ± 50 ka to 3 ± 14 ka) and/or Mount Rittmann (3.97 Ma to present).

Evidence for abrasion and enhanced growth of Ulva lactuca L. in the presence of colliery waste particles

Previous studies have highlighted a reduction in occurrence and biomass of Ulva lactuca L. (Chlorophyceae) on shores where inputs of colliery waste occur. It was postulated that this was owing to an abrasive effect of colliery waste on macroalgal fronds. To test this, individual U. lactuca plants were exposed to colliery waste (three different grain size categories: < 500 μm, 500–2000 μm, and 0–2000 μm) in both shaken (turbulent) and still conditions in the laboratory. Over an 8-day period, U. lactuca plants lost weight when colliery waste was present and gained weight when no colliery waste was present. Weight loss was most pronounced in U. lactuca exposed to colliery waste of 500–2000 μm grain sizes in shaken conditions (− 18.8% ± 4.65 SE, n = 4). However, greatest weight gain (+ 26.8% ± 6.26 SE, n = 4) also occurred in the presence of colliery waste (0–2000 μm), but in still conditions. Weight gain was also observed after 60 days in still conditions in the presence of colliery waste (grain sizes 0–2000 μm); U. lactuca showed significant growth (+ 69% ± 67 SE of starting weight, n = 5) compared with controls (− 51% ± 41 SE, n = 5). These results suggest that ‘large’ grains of colliery waste act as a physical abrading agent on macroalgae when in turbulent conditions, and may be responsible for lowering of species richness of macroalgae where colliery waste inputs occur. However, by contrast, colliery waste in still conditions promotes the growth of U. lactuca, suggesting that, for example, rock pool flora may benefit from its presence

Early Silurian Virgiana Pentamerid Brachiopod Communities of Anticosti Island, Quebec

Morphological, taphonomical, and sedimentological data from lower Llandovery fossils and strata of Anticosti Island demonstrate that the Virgiana community occupied a wide range of substrate settings from near or above fair-weather wave base to well below normal storm wave base. Globally the large-shelled pentamerid brachiopod genus Virgiana had a relatively short temporal duration during the early Llandovery (Early Silurian) and a wide geographical distribution in shallow carbonate basins of the paleocontinents of Laurentia (North America) and Siberia. In most areas, shells of Virgiana are found in post-mortem assemblages associated with ripple-marked and cross-stratified carbonate sands, indicating that they occupied carbonate banks and shoals above fair-weather wave base, and were subject to reworking during storms. Most species of Virgiana developed a costate shell with thickened ventral umbo and flattened apex, which probably helped the organisms stay in an incurved beak-down feeding position in current-swept environments, by weighting and stabilizing the shellposterior. Virgiana shells of the Becscie Formation of Anticosti Island are most commonly preserved as post-mortem assemblages in sharp-based, poorly graded packstones, as disarticulated, broken, nested, stacked, or imbricated valves. They are intimately associated with hummocky cross-stratified carbonate sandstones and massive to laminated mudstones, indicative of deposition by storm-induced current in mid-shelf settings, below fairweather wave base. While some of the shells may have been washed in from shallower, inner-shelf environments during storms, many articulated specimens show well-preserved growth lines, and little evidence of abrasion, indicating local origin. The rare occurrences of in situ shells suggest that Virgiana communities extended into depths of 30 m. A further departure from the norm occurs in the overlying late Rhuddanian Merrimack Formation of Anticosti Island, where strongly costate, articulated shells of Virgiana mayvillensis are relatively well preserved in calcareous and micritic mudstone and calsisiltite beds. This suggests that V. mayvillensis was able to survive in deeper-water, relatively low-energy environments, in midto outer-shelf settings well below normal storm wave base. On Anticosti, there are no reefs in either of the Virgiana-bearing formations.

Crushing strength ofPuriana pacifica(Ostracoda), an experimental approach to taphonomy

Specimens of the ostracode speciesPuriana pacificaBenson, 1959, from a modern assemblage from the Gulf of California can be sorted by visual inspection into five groups based on taphonomic condition. Characteristics used for sorting include: presence or absence of appendages; color and opacity of the valves; and evidence of abrasion, fragmentation, and boring. As measured by resistance to crushing force, members of the five groups show appreciable, progressive, postmortem weakening so that specimens in the poorest apparent condition are the weakest mechanically. The groups also differ in style of fracturing in response to crushing force. Fractures of valves of the freshest specimens are located quite consistently. Among specimens with progressively more damage, the location of fractures becomes less predictable. On valves of the most damaged specimens, typical fractures are short and likely to occur almost anywhere on the valve. These experimental results emphasize the importance of rapid burial in enhancing the likelihood of fossilization. Furthermore, they cast doubt on the ability of paleoecologists to use valves and carapaces of fossil ostracodes to interpret levels of wave and current energy in ancient environments because taphonomic weakening and subsequent destruction may effectively mask functional morphology.

Mechanism of wear of rotary-percussive drilling bits and the effect of rock type on wear

A rotary-percussive rock drill was tested for wear under dry conditions when drilling granite, dolerite and diorite. Granite caused more rapid wear of drill bits than either diorite or dolerite but there were significant differences between diorite and dolerite despite their similar geological composition. Microscopy of the worn drill bits revealed that wear occurs by micro-spalling for all rocks and macro-spalling for granite. No evidence of abrasion was found, contrary to previous findings, and the difference between various parts of the drill depended on the scale of spalling.

Early Pliocene horses from late Pleistocene fluvial deposits, Gulf Coastal Plain, south Texas

Isolated teeth and post-cranial elements of fossil vertebrates were recovered from sand and gravel pits in valley fill and terrace deposits along the Nueces River in San Patricio and Nueces Counties, Texas. A log from the valley fill deposit has been radiocarbon dated at 13,230 ± 110 BP. The fauna is mixed and comprises typical late Pleistocene taxa and relatively abundant remains of early Pliocene (latest Hemphillian) horses. The latter group includes Astrohippus albidens (Mooser), Nannippus spp., Neohipparion eurystyle (Cope), and a derived species of either Calippus or Pseudhipparion. Many of these specimens show little or no evidence of abrasion, in spite of the fact that they may have been transported at least 12–25 km. The source beds for these early Pliocene horses are unknown, but the fossils were probably eroded from older, updip sediments of the upper Goliad Formation during a low stand of sea level at the end of the Pleistocene and deposited during the late Wisconsinan.

TAPHONOMY OF THE NORTH AMERICAN DECAPODS

ABSTRACT The geological record of the decapod crustaceans is sparse and fragmentary. Biologic, taphonomic, and collection factors bias the record, enhancing or diminishing it. These biasing factors must be relatively well understood before maximum use can be made of paleontologic samples. Biologic factors that are important are population density and mobility, preservation potential of habitats, differential calcification, and molting. The geologic record of shrimp is exceedingly deficient; that of mud shrimp, crabs, and lobsters is better, but still sparse, fragmentary, and strongly biased. Taphonomic variables affecting the preservation and interpretation of decapods include burial conditions and positioning of the remains for buried-alive decapods, corpses, and molts. Remains react differently and retain evidence of burial conditions, whether immediate or delayed. Exhumed and reburied remains often present evidence of abrasion, being bored, or settled by epizoans. Burrows enhance preservation potential, and such trace fossils can be assigned to construction by decapods, but only occasionally to a specific taxon. Characteristic preservation positions of decapods include those buried alive (Traumatized Position), those buried as corpses (Normal Position), molts of crabs and lobsters (Open Molt Position), or disarticulated remains. Evidence to distinguish molts, corpses, and buried-alive crabs is often circumstantial or lacking. Diagenesis is directly related to burial conditions, rate and depth of burial, and the presence of scavengers and deposit feeders. A general mode of formation of phosphatic concretions is envisioned which involves death, burial, decomposition, infilling, pelletization and bioturbation by worms (concentrating phosphate in fecal pellets), and precipitation of phosphatic concretions. Benthic nutrient enrichment by plankton kill resulting in a rapid increase in detritus feeders may have initiated a rapid population increase in decapods able to exploit the new food resource. The decapod-worm system would form a taphonomic feedback system insuring and allowing exploitation as well as preservation over an interval of time. Other modes of preservation include little altered remains in fine clastics, preservation as allochems in limestone, preservation as nuclei in calcite nodules and concretions, and in hematite concretions. Collections of decapods tend to be small and widely scattered, but contain virtually all remains found in allied geologic and paleontologic work. Most species-level taxa are represented by only a few, fragmental specimens. Approximately 200 decapod taxa are known from the North American Cretaceous. Because of the usual scarcity and small, fragmentary sample size of most taxa, decapod-rich or decapod-dominated fossil assemblages are exceedingly important, since they alone preserve large numbers of diverse decapod taxa.

The ice-rock interface for the climap 18000 years b.p. and 120000 years b.p. experiments

AbstractMost numerical models of present ice-sheet dynamics predict basal thermal conditions for an assumed geothermal heat flux and measured ice thickness, surface temperature, and snow precipitation. These models are not ideally suited for reconstructing former ice sheets because what is known for present ice sheets is unknown for former ones, and vice versa. In particular, geothermal heat fluxes are immeasurable at an ice-sheet bed but can be measured after the ice sheet is gone, and the thermal conditions predicted at an ice-sheet bed can be inferred from the glacial-geological–topographic record after the ice sheet is gone. The Maine CLIMAP ice-sheet reconstruction model uses these inferred basal thermal conditions to compute ice thicknesses from basal shear stresses.Basal shear stress is assumed to reflect the degree of ice–bed coupling which, in turn, is assumed to reflect the amount and distribution of basal water under the ice sheet. Under the ice-sheet interior, basal water exists in a thin film of constant thickness covering the low places on the bed. This film expands for a melting bed and contracts for a freezing bed. Along the ice-sheet margin, basal water exists in narrow channels of varying thickness corresponding to troughs on the bed. These water channels become deeper for a melting bed and shallower for a freezing bed. In areas covered by the Laurentide and Scandinavian ice sheets, myriads of interconnected lakes in regions of greatest postglacial rebound are interpreted as evidence suggesting the interior basal water distribution, whereas eskers pointed toward terminal moraines and troughs across continental shelves are interpreted as evidence suggesting the basal water distribution toward the margins. Continental-shelf troughs were assumed to correspond to former ice streams, by analogy with observations in Greenland and Antarctica.Three modes of glacial erosion are considered to be responsible for the lakes, eskers, troughs, and associated topography. Quarrying is by a freeze-thaw mechanism which occurs where the melting-point isotherm intersects bedrock, so it is important only for freezing or melting beds because high places on the bed are frozen, low places are melted, and minor basal temperature fluctuations shift the isotherm separating them. Crushing results when rocks at the ice-bed interface are ground against each other and the bed by glacial sliding, so it occurs where the bed is melted and is most important when the entire bed is melted. Abrasion of bedrock occurs when rock cutting tools imbedded in the ice at the ice–rock interface are moved across the interface by glacial sliding, so it is also most important when the entire bed is melted. If basal melting continued after the entire bed is melted, abrasion-rates drop because the basal water layer thickens and drowns bedrock projections otherwise subjected to abrasion. Basal freezing reduces both crushing and abrasion-rates by coating quarried rocks with a sheath of relatively soft ice and transporting them upward from the ice–rock interface.An initially flat subglacial topography will develop depressions where glacial erosion is greatest and deposition is least, and ridges where the opposite conditions prevail. We interpret the central depressions represented today by Hudson Bay and the Gulf of Bothnia as caused by erosion on a melting bed under the Laurentide and Scandinavian ice sheets, respectively. The arc of lakes, gulfs, and shallow seas surrounding these depressions are interpreted as resulting from a freezing bed under the former ice sheets. The present watershed separating the depressions from the arcs marks the approximate former basal equilibrium line where the bed was melted. The Canadian and Baltic continental shields beyond these arcs are blanketed by material eroded from within the arcs, and represent areas having a frozen bed where evidence for abrasion is missing and a second zone having a melting bed where evidence for abrasion is present. This basic pattern was assumed to be imprinted on the bed during the steady-state period of maximum ice-sheet extent, and maintained in varying degrees during growth and shrinkage of these ice sheets.

The ice-rock interface for the climap 18000 years b.p. and 120000 years b.p. experiments

Abstract Most numerical models of present ice-sheet dynamics predict basal thermal conditions for an assumed geothermal heat flux and measured ice thickness, surface temperature, and snow precipitation. These models are not ideally suited for reconstructing former ice sheets because what is known for present ice sheets is unknown for former ones, and vice versa. In particular, geothermal heat fluxes are immeasurable at an ice-sheet bed but can be measured after the ice sheet is gone, and the thermal conditions predicted at an ice-sheet bed can be inferred from the glacial-geological–topographic record after the ice sheet is gone. The Maine CLIMAP ice-sheet reconstruction model uses these inferred basal thermal conditions to compute ice thicknesses from basal shear stresses. Basal shear stress is assumed to reflect the degree of ice–bed coupling which, in turn, is assumed to reflect the amount and distribution of basal water under the ice sheet. Under the ice-sheet interior, basal water exists in a thin film of constant thickness covering the low places on the bed. This film expands for a melting bed and contracts for a freezing bed. Along the ice-sheet margin, basal water exists in narrow channels of varying thickness corresponding to troughs on the bed. These water channels become deeper for a melting bed and shallower for a freezing bed. In areas covered by the Laurentide and Scandinavian ice sheets, myriads of interconnected lakes in regions of greatest postglacial rebound are interpreted as evidence suggesting the interior basal water distribution, whereas eskers pointed toward terminal moraines and troughs across continental shelves are interpreted as evidence suggesting the basal water distribution toward the margins. Continental-shelf troughs were assumed to correspond to former ice streams, by analogy with observations in Greenland and Antarctica. Three modes of glacial erosion are considered to be responsible for the lakes, eskers, troughs, and associated topography. Quarrying is by a freeze-thaw mechanism which occurs where the melting-point isotherm intersects bedrock, so it is important only for freezing or melting beds because high places on the bed are frozen, low places are melted, and minor basal temperature fluctuations shift the isotherm separating them. Crushing results when rocks at the ice-bed interface are ground against each other and the bed by glacial sliding, so it occurs where the bed is melted and is most important when the entire bed is melted. Abrasion of bedrock occurs when rock cutting tools imbedded in the ice at the ice–rock interface are moved across the interface by glacial sliding, so it is also most important when the entire bed is melted. If basal melting continued after the entire bed is melted, abrasion-rates drop because the basal water layer thickens and drowns bedrock projections otherwise subjected to abrasion. Basal freezing reduces both crushing and abrasion-rates by coating quarried rocks with a sheath of relatively soft ice and transporting them upward from the ice–rock interface. An initially flat subglacial topography will develop depressions where glacial erosion is greatest and deposition is least, and ridges where the opposite conditions prevail. We interpret the central depressions represented today by Hudson Bay and the Gulf of Bothnia as caused by erosion on a melting bed under the Laurentide and Scandinavian ice sheets, respectively. The arc of lakes, gulfs, and shallow seas surrounding these depressions are interpreted as resulting from a freezing bed under the former ice sheets. The present watershed separating the depressions from the arcs marks the approximate former basal equilibrium line where the bed was melted. The Canadian and Baltic continental shields beyond these arcs are blanketed by material eroded from within the arcs, and represent areas having a frozen bed where evidence for abrasion is missing and a second zone having a melting bed where evidence for abrasion is present. This basic pattern was assumed to be imprinted on the bed during the steady-state period of maximum ice-sheet extent, and maintained in varying degrees during growth and shrinkage of these ice sheets.

UNUSUAL FORMS OF ABRASION OF TEETH: UNINTENTIONAL ‘FILING’ IN MICRONESIANS AND POLYNESIANS

ABRASION of teeth has been defined as 'the pathologic wearing away of tooth substance through some abnormal mechanical process' (Shafer et al., 1974). During a survey of some dental conditions amongst families of Tuvaluans (formerly Ellice) and Gilbertese living on Ocean Island (Banaba), Central Pacific (Sutton, 1966) two conditions were noticed on the labial surface of maxillary incisor teeth. The first was the occurrence of many 'double shovel-shaped' incisors, which have the two marginal ridges on the lingual surface which produce the form of the well-known 'shovel-shaped' incisor and two similar, but usually much less marked, marginal ridges on the label which form a 'shovel-shape' on that surface also. Two of these cases have been reported (Sutton, 1973) and the sparse literature on double shovel-shaped teeth has been reviewed (Sutton, 1977). The second condition on the labial surfaces the presence of unusual forms of abrasion is considered in this paper. Abrasion of labial enamel reduces the prominence of labial ridges, sometimes removing them (Plate I, A, B, C). Therefore, when studying double shovel-shaped teeth as a part of a dental survey of the Banabans (Sutton, 1970) and during five subsequent examinations of Gilbertese and Tuvaluans on Ocean Island between 1970 and 1976, each incisor was examined for evidence of abrasion, and the prevalence of this condition was recorded.

THE EFFECTS OF DIAGENESIS ON SOME HEAVY MINERALS FROM THE SANDSTONES OF THE MIDDLE LIMESTONE GROUP IN NORTHUMBERLAND

Summary The heavy minerals in the sandstones of the Middle Limestone Group in Northumberland include four ubiquitous species, namely garnet, rutile, tourmaline and zircon. With the aid of a scanning electron microscope, evidence of abrasion, corrosion and secondary growth on these grains was recognised. Garnet shows corrosion features only. Rutile is of both detrital and authigenic origin. Tourmaline displays abrasion and growth features, whilst zircon is a very stable constituent both mechanically and chemically. The form and surface textures of authigenic pyrite are also recorded.

Metallurgical appraisal of wear mechanisms and processes on high-speed-steel cutting tools

AbstractThe wear mechanisms and processes on high-speed-steel tools used to machine steels, cast iron and a nickel-based alloy under conditions encountered in practice have been investigated. Evidence is presented of five different wear processes: (a) superficial plastic deformation by shear at high temperature; (b) plastic deformation of the cutting edge; (c) wear based on diffusion; (d) attrition wear; (e) abrasion wear. All these wear processes operate under conditions of seizure between tool and work material, and frequently more than one process is taking place at the same time. The first three are dependent on temperature, and temperature distribution in the tool is of major importance. Cratering can be caused by (a) or by (c), the former being rapid and the latter relatively slow. Flank wear may be caused by diffusion (c) or by attrition (d). Evidence of abrasion (e) by Ti(C,N) particles in stabilized austenitic stainless steel is conclusive. In the range of materials tested, however, the contribut...