GROUNDWATER SAPPING AND VALLEY DEVELOPMENT IN THE HACKNESS HILLS,NORTH YORKSHIRE,ENGLAND     

DAVID J. NASH 《地球表面变化过程与地形》1996,21(9):781-795

This paper describes evidence for the role of groundwater sapping and seepage erosion processes in the development of valleys which cut the southern edge of the Hackness Hills plateau in North Yorkshire, England. The development of drainage in this region has previously been suggested to relate to erosion by Late Devensian sub-aerial glacial meltwater channels. The role of groundwater erosion is investigated through a combination of geomorphological studies, lithological logging and X-ray diffraction (XRD) analyses. The geology of the region consists of a series of permeable Middle and Upper Jurassic lithologies (the Corallian sequence and Lower Calcareous Grit) which overlie the impermeable Upper Oxford Clay. The rocks dip gently to the south at between 1° and 4° and are relatively unfolded. Valleys exhibit many characteristic features of groundwater sapping networks. They rise abruptly at the edge of the plateau with amphitheatre-like valley heads, alcoves in headwalls, steep bedrock side walls, flat floors, spring sites and seepage zones in many valley flanks. Lithological logging indicates that sites of groundwater emergence usually occur either at or slightly above the boundary of the Upper Oxford Clay and Lower Calcareous Grit. XRD analyses of bedrock samples indicate that seepage occurs within siltstones which contain no clay but a variable percentage of calcite. The cause of groundwater emergence is attributed to decreasing grain size and increasing calcite cementation within bedrock which combine to reduce permeability. Development of valleys in the Hackness Hills is suggested to have occurred by a combination of headward erosion by groundwater sapping processes operating in an up-dip direction superimposed onto a valley morphology shaped by surface fluvial erosion.  相似文献   

The formation of a geometrical ridge network by the surge-type glacier Kongsvegen,Svalbard     

MATTHEW R. BENNETT  MICHAEL J. HAMBREY  DAVID HUDDART  JEAN FRANOIS GHIENNE 《第四纪科学杂志》1996,11(6):437-449

Traditionally, geometrical ridge networks are interpreted as the product of the flow of subglacial sediment into open basal crevasses at the cessation of a glacier surge (‘crevasse-fill’ ridges). They are widely regarded as a characteristic landform of glacier surges. Understanding the range of processes by which these ridge networks form is therefore of importance in the recognition of palaeosurges within the landform record. The geometrical ridge network at the surge-type glacier Kongsvegen in Svalbard, does not form by crevasse filling. The networks consist of transverse and longitudinal ridges that can be seen forming at the current ice margin. The transverse ridges form as a result of the incorporation of basal debris along thrust planes within the ice. The thrusts were apparently formed during a glacier surge in 1948. Longitudinal ridges form through the meltout of elongated pods of debris, which on the glacier surface are subparallel to the ice foliation and pre-date the surge. This work adds to the range of landforms associated with glacier surges.  相似文献   

Mediterranean Quaternary River Environments     

DAVID SERRAT 《第四纪科学杂志》1996,11(5):433-433

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Fluid evolution and thermal structure in the rapidly exhuming gneiss complex of Namche Barwa–Gyala Peri, eastern Himalayan syntaxis     

D. CRAW  P. O. KOONS  P. K. ZEITLER  W. S. F. KIDD 《Journal of Metamorphic Geology》2005,23(9):829-845

High‐grade gneisses (amphibolite–granulite facies) of the Namche Barwa and Gyala Peri massifs, in the eastern Himalayan syntaxis, have been unroofed from metamorphic depths in the late Tertiary–Recent. Rapid exhumation (2–5 mm year^?1) has resulted in a pronounced shallow conductive thermal anomaly beneath the massifs and the intervening Tsangpo gorge. The position of the 300 °C isotherm has been estimated from fluid inclusions using CO₂–H₂O immiscibility phase equilibria to be between 2.5 and 6.2 km depth below surface. Hence, the near‐surface average thermal gradient exceeds 50 °C km^?1 beneath valleys, although the thermal gradient is relatively lower beneath the high mountains. The original metamorphic fluid in the gneisses was >90% CO₂. This fluid was displaced by incursion of brines from overlying marine sedimentary rocks that have since been largely removed by erosion. Brines can exceed 60 wt% dissolved salts, and include Ca, Na, K and Fe chlorides. These brines were remobilized during the earliest stages of uplift at >500 °C. During exhumation, incursion of abundant topography‐driven surface waters resulted in widespread fracture‐controlled hydrothermal activity and brine dilution down to the brittle–ductile transition. Boiling water was particularly common at shallow levels (<2.5 km) beneath the Yarlung Tsangpo valley, and numerous hot springs occur at the surface in this valley. Dry steam is not a major feature of the hydrothermal system in the eastern syntaxis (in contrast to the western syntaxis at Nanga Parbat), but some dry steam fluids may have developed locally.  相似文献   

Synchronous variability changes in Alpine temperature and tree-ring data over the past two centuries     

DAVID FRANK  ROBERT WILSON  JAN ESPER 《Boreas: An International Journal of Quaternary Research》2005,34(4):498-505

The understanding of extremes and their temporal distribution is useful in characterizing the behaviour of the climate system, and necessary for understanding their social and economic costs and risks. This task is analogous to the study of pointer years in dendrochronological investigations. Commonly used dendroclimatological methods, however, tend to result in an equalization of variance throughout the record by normalizing variability within moving windows. Here, we analyse a larger network of high-elevation temperature-sensitive tree sites from the European Alps processed to preserve the relative frequency and magnitude of extreme events. In so doing, temporal changes in year-to-year tree-ring width variability were found. These decadal length periods of increased or decreased likelihood of extremes coincide with variability measures from a long-instrumental summer temperature record representative of high-elevation conditions in the Alps. Intervention analysis, using an F-test to identify shifts in variance, on both the tree-ring and instrumental series, resulted in the identification of common transitional years. Based on a well-replicated network of sites reflecting common climatic variation, our study demonstrates that the annual growth rings of trees can be utilized to quantify past frequency and amplitude changes in extreme variability. Furthermore, the approach outlined is suited to address questions about the role of external forcing, ocean-atmosphere interactions, or synoptic scale changes in determining patterns of observed extremes prior to the instrumental period.  相似文献   

Palaeo-ice streaming in the central sector of the British—Irish Ice Sheet during the Last Glacial Maximum: evidence from the northern Irish Sea Basin     

DAVID H. ROBERTS  ROGER V. DACKOMBE  GEOFF S. P. THOMAS 《Boreas: An International Journal of Quaternary Research》2007,36(2):115-129

Late Devensian glacial sediments and landforms of the Isle of Man record the advance and deglacial signature of the central sector of the British-Irish Ice Sheet. Evidence from the area, gathered from striae, erratic trains and drift limits, show ice was routed over and around the island in two flow phases post-36 kyr BP. In the south of the island, streamlined depositional bedforms with low elongation ratios suggest low ice-flow velocities resulting from one or more of (i) the up-ice location of the island within a regional onset zone, (ii) flow retardation of ice interacting with the margins of the island and (iii) localized drainage of the deforming bed. The deglacial landform assemblage of lateral marginal sandurs and drainage diversions, coupled with a lack of dead-ice features, suggests ice did not downwaste in situ but retreated intact along the coastal margins as Manx Upland ice thinned. In the north of the island, however, the Bride Moraine complex indicates a change in deglacial ice-sheet dynamics, with temporary re-advance and marginal oscillation causing proglacial tectonism and thrusting of the glacial sediment pile, possibly during the Killard Point Stadial event (18.8-16.4 cal. kyr BP). From a basin-wide perspective, the Irish Sea Basin sector of the British-Irish Ice Sheet had many of the characteristics of an ice stream, such as a zone of flow convergence up-ice, a grounding line in the southern Celtic Sea and recessional limits characterized by proglacially tectonized and thrust dead-ice landscapes indicative of a rapidly oscillating ice margin.  相似文献   

Talking about Antarctica     

DAVID I. M. MACDONALD 《Geology Today》1989,5(6):199-200

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Malacological records from the Upper Pleistocene at Portelet (Jersey, Channel Islands): comparisons with western and central Europe     

DENIS-DIDIER ROUSSEAU  DAVID HENRY KEEN 《Boreas: An International Journal of Quaternary Research》1989,18(1):61-66

New sampling of Mollusca from Portelet, Jersey, Channel Islands, has allowed characterization of local conditions during deposition of the pre-Kesselt Weichselian loess. The Portelet fauna also allows a reassessment of the distribution of the Columella fauna in western Europe. Comparison of the Portelet fauna with those of eastern France and central Europe shows a decrease of the number of species present in this far-western locality which appears to be either a result of the operation of a typical 'peninsula' effect comparable to that seen in other zoological groups or a consequence of their occurrence in a marginal part of the European loess belt.  相似文献   

ISLANDS,LOVERS, AND OTHERS     

DAVID LOWENTHAL 《Geographical review》2007,97(2):202-229

ABSTRACT. Geography, experience, and imagination are all crucial to how we take measure of islands. Prime foci of legend and invention, islands have haunted humanity since the dawn of history. Why are they so intensely loved and loathed, desired and rejected, minutely scrutinized yet often perilously misjudged? On islands we feel alternately landed and adrift, magnified and reduced, fulfilled and voided, at home and in exile. These and other polarities are reviewed here with examples from Caribbean, Atlantic, Mediterranean, antipodean, and imaginary islands fancied by the creator.  相似文献   

Micromorphology and geochemistry of groundwater silcretes in the eastern South Downs, UK     

J. STEWART ULLYOTT  DAVID J. NASH 《Sedimentology》2006,53(2):387-412

Groundwater silcretes have been recognized recently as major terrestrial silicon sinks and yet their origins are poorly understood. This paper aims to further the understanding of the micro‐fabric, geochemistry and formation of groundwater silcretes, through detailed analyses of silcrete boulders from the South Downs, Sussex, UK. In‐depth petrological investigations of silcrete blocks identified three varieties of silcrete across the study area (saccharoid, hard and pebbly conglomeratic or silcrete breccia), similar to those elsewhere in England. Silcrete fabrics were universally simple and preserved host material structures. Optically continuous quartz overgrowths were the dominant cement and developed on framework grains in the absence of silt‐ and clay‐sized particles. Finer‐grained silica cements occurred in isolated patches and as vein‐ or cap‐like geopetal features. These caps and veins formed through the silicification of illuviated clay‐rich material that entered the host sediment prior to, or in the early stages of, overgrowth formation. Titanium contents were related to the amount of fine‐grained silica and appear to reflect variations in host material chemistry. Subsequent to silicification, the silcretes were altered by at least two phases of ferruginization, characterized by the ingress of iron minerals and partial fabric dissolution or replacement. The study reveals strong similarities in the micromorphology and geochemistry of groundwater silcretes in the study area and those developed in sandy host sediments in neighbouring parts of northwest Europe. Variations that do occur could be explained by differences in the host sediment, geological setting or tectonic history of the respective basins (or sub‐basins), suggesting that there may be a common mechanism for silcrete formation across north‐west Europe. Silcrete development appears to be related to the release of silica accompanying acid leaching of the host material or adjacent strata. In the South Downs, this probably resulted from oxidation of pyrite in the argillaceous and lignitic sediments overlying the host sands. It is envisaged that palaeoenvironmental conditions were of limited importance for silcrete development and that there was no specific ‘era’ of formation, with phased development accompanying landscape evolution through the Neogene into the Pleistocene.  相似文献