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all geography including DSDP/ODP Sites and Legs
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Baosbheinn Mountain
Rock-slope failure at Baosbheinn, Wester Ross, NW Scotland: age and interpretation
Debris flow activity and gully propagation: Glen Docherty, Wester Ross
Rockfall talus slopes and associated talus-foot features in the glaciated uplands of Great Britain and Ireland: periglacial, paraglacial or composite landforms?
Abstract The traditional interpretation of talus slopes and talus-foot landforms in the glaciated uplands of Great Britain and Ireland has been that they are periglacial landforms associated with freeze–thaw activity and permafrost. Since about 1990 some reassessment of this widely held view has occurred, and paraglacial rockfall and rock-slope failure are now considered to have played a significant role in the development of some talus landforms; in certain cases a wholly paraglacial origin is advocated. In order to determine formative processes, critical site-specific evidence (morphological and sedimentological) needs to be obtained. This will enable models of the deglacial–post-glacial evolution of these landscapes to be proposed and allow the palaeoenvironmental significance of the landforms to be established. Distinguishing between a periglacial and paraglacial origin might be assisted by application of cosmogenic isotope surface-exposure dating, which may demonstrate a Holocene age for a particular landform and thus rule out a permafrost-related origin. However, there will be instances where the application of dating will not differentiate, as in the case of Late Glacial landforms that could be either periglacial or paraglacial. It is likely that equifinality applies with respect to these landforms and a composite origin for them should also be considered. This latter issue is one that has not previously been given much consideration, probably because of the inherent difficulties in recognizing the products of different processes in landforms for which exposures of their constituent materials are rare.
Paraglacial rock slope failure as an agent of glacial trough widening
Abstract Rock slope failure (RSF) generates the largest single erosional events in the glacial–paraglacial land system, leaving numerous obvious cavities and less obviously weakened valley walls. Its contribution to trough widening in a mountain range has not previously been systematically quantified. Map-based measures of RSF ‘depth of bite’ are applied to five sample areas in the Scottish Highlands, and a comparator area in north Norway, all in metasediments structurally conducive to mass deformation and block sliding. Problems in applying map-based measures include bedrock cavities remaining partially occupied by failed debris or subsequent infill, and multiple planes of reference. The most practical measure is of maximum recess depth on any single contour ( D MAX ). This is a standardizable single-point indicator of visible impact, not a measure of actual cavity depth, nor an average applying to the whole RSF. In four of the five areas, average D MAX is consistent at 40–45 m. RSF breadth averages 270–600 m over the five areas. RSF affects 9% and 14% of total valley wall length in the two densest RSF areas, rising to 47% and 52% on two specific valley sides. The depth:breadth ratio in areas dominated by slope deformation can be twice that in areas of translational sliding. An evolutionary model of glacial–paraglacial cycling proposes a ‘zone of paraglacial relaxation’ in which RSF is intense in early cycles as fluvial profiles adjust to ice discharge, diminishing with maturity as trough walls become stress-hardened, and reviving in response to neotectonic and glaciological perturbations, notably ice piracy via transfluent breaching. However, a major unknown is the efficacy of glacial exploitation of RSFs: if it takes several cycles to evacuate debris and pare back cavity angles, cumulative RSF impact is lessened. Glacial–paraglacial cycling is a classic positive feedback loop, promoting valley widening beyond the parabolic norm. Preferential exploitation of structure by RSF promotes asymmetrical trough profiles. RSF acts both as a scarp retreat process, and as a slope reduction counterpoint to glacial slope steepening. In landscape evolution, it is a powerful agent in destruction of paleic relief, notably around watersheds that are undergoing breaching by transfluent ice, where trough development and widening is still vigorous.
Abstract Periglacial environments are characterized by cold-climate non-glacial conditions and ground freezing. The coldest periglacial environments in Pleistocene Britain were underlain by permafrost (ground that remains at or below 0°C for two years or more), while many glaciated areas experienced paraglacial modification as the landscape adjusted to non-glacial conditions. The growth and melt of ground ice, supplemented by temperature-induced ground deformation, leads to periglacial disturbance and drives the periglacial debris system. Ice segregation can fracture porous bedrock and sediment, and produce an ice-rich brecciated layer in the upper metres of permafrost. This layer is vulnerable to melting and thaw consolidation, which can release debris into the active layer and, in undrained conditions, result in elevated porewater pressures and sediment deformation. Thus, an important difference arises between ground that is frost-susceptible, and hence prone to ice segregation, and ground that is not. Mass-movement, fluvial and aeolian processes operating under periglacial conditions have also contributed to reworking sediment under cold-climate conditions and the evolution of periglacial landscapes. A fundamental distinction exists between lowland landscapes, which have evolved under periglacial conditions throughout much of the Quaternary, and upland periglacial landscapes, which have largely evolved over the past c. 19 ka following retreat and downwastage of the last British–Irish Ice Sheet. Periglacial landsystems provide a conceptual framework to interpret the imprint of periglacial processes on the British landscape, and to predict the engineering properties of the ground. Landsystems are distinguished according to topography, relief and the presence or absence of a sediment mantle. Four landsystems characterize both lowland and upland periglacial terrains: plateau landsystems, sediment-mantled hillslope landsystems, rock-slope landsystems, and slope-foot landsystems. Two additional landsystems are also identified in lowland terrains, where thick sequences of periglacial deposits are common: valley landsystems and buried landsystems. Finally, submerged landsystems (which may contain more than one of the above) exist on the continental shelf offshore of Great Britain. Individual landsystems contain a rich variety of periglacial, permafrost and paraglacial landforms, sediments and sedimentary structures. Key periglacial lowland landsystems are summarized using ground models for limestone plateau-clay-vale terrain and caprock-mudstone valley terrain. Upland periglacial landsystems are synthesized through ground models of relict and active periglacial landforms, supplemented by maps of upland periglacial features developed on bedrock of differing lithology.
Geomorphological framework: glacial and periglacial sediments, structures and landforms
Abstract The development of the conceptual ground model (CGM) is a critical component of any desk study or ground engineering project planning process. A key task of the engineering geologist is to develop the CGM in order to predict the occurrence of known terrain units, elements and facets within a given landsystem, and to communicate the lateral and vertical variability of engineering rocks and soils found within that system. This chapter details the significant ground components of glacial and periglacial landsystems within a geomorphological framework describing the sediments, structures and landforms that could reasonably be expected to be encountered in these terrains. Examples are provided of both modern and relict glacial and periglacial landforms, their mode of formation and their field recognition. Glaciogenic and periglacial sediments are described both in terms of their sedimentological and formal engineering description. The chapter provides a suggested naming nomenclature for these sediments that can be used within a BS 5930 description. An extensive photoglossary is presented as a field aide memoir, enabling the engineering geologist to identify these features once on site.
Abstract With its rich lithological variation, upland, lowland and coastal settings, and past climatic changes, the UK presents a wide variety of landslide features that can pose significant hazards to people, construction and infrastructure, or simply add to landscape character and conservation value of an area. This chapter describes and defines the nature and extent of this landsliding; the causes, effects and geological controls on failure; and their mitigation and stabilization. A risk-based approach to landslide management is outlined with qualitative and semi-quantitative methodologies described. Numerous case studies are presented exemplifying landslide and slope stability hazards in the UK.
Abstract The following chapter is intended to provide a comprehensive field description of the Tòrridonian, replacing that given in the Geological Survey’s NW Highlands memoir of 1907, and citing all relevant literature. Stratotypes and palaeocurrents are described, along with the section lines used to construct the regional stratigraphic sections (Figs 4&23), but detailed consideration of topics such as geochemistry, diagenesis, sediment source areas, palaeomagnetism and basin tectonics is contained in Chapters 2–5. for convenience the rocks are described under thirty-three compact subareas, most of which are shown on Plate 2. This plate also locates all figured maps and sections. The Directory starts with Cape Wrath and continues with sub-areas progressively farther south.