摘要:

ABSTRACTThe paper describes several types of small scale forms on the stoss side of a roche moutonnée surface, recently exposed in a gravel pit in the Fjärås bräcka complex, SW Sweden. The forms were filled with weathered rock debris. When excavated, they showed a striking resemblance to forms generally accepted to be shaped by glacial/glaciofluvial erosion. In addition to these forms there were irregular cavities for instance smoothly rounded throughs and irregular rough surfaces resembling those claimed to be preglacial weathering forms. In spite of the few observations made, many small scale glacial erosion forms are likely to be the result of a three-step process i.e. glacial pressure deformation of the bedrock, subsequent weathering of the deformed rock, and finally excavation of the deformed rock by exogenic processes. Further on it seems probable that many rough surfaces of the type described in the paper are the result of glacial pressure deformation and subsequent weathering.

ISSN:0435-3676    

DOI:10.1080/04353676.1991.11880328

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

ABSTRACTCirques at low altitudes in Sweden are, from a climatological point of view, not likely to be a result of cirque glaciation but rather of subglacial erosion under a wet based inland ice-sheet. In this paper a comparison is made between cirque forms and forms excavated under the presently existing Storglaciären in northern Sweden.Cirques have been observed in Sweden at low altitudes on locations where todays climate is continental (Agrell 1977, Rapp 1984, Rudberg 1984, Vilborg 1984). It is difficult to visualize the colder and more maritime climate needed to create wet-based cirque glaciers. The glaciation level is approximately 1500–2000 metres higher than the highest summits in these areas. A lowering of the glaciation level of this amount corresponds to a more than 10°C lowering of the annual temperature. As the interior of northern Sweden today is close to the limit of discontinuous permafrost, such drop in temperature would most certainly lead to continuous permafrost in the entire area. Under such climatic circumstances small cirque glaciers do not erode their bottoms. Thus, either many of these cirques are incorrectly interpreted or there is something wrong with the definition of cirques.

ISSN:0435-3676    

DOI:10.1080/04353676.1991.11880329

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

ABSTRACTThis paper contains results from studies of ice flow directions on the islands of Bornholm and Christiansø, Denmark, as indicated by striae and other marks of glacial erosion on the bedrock surface. The Jesuits indicate old northwesterly and northerly directions of ice-flow, followed by a powerful ice-flow from the northeast and later ice-flows from the east and the southeast. These directions may be correlated with observed directions of ice-flow in southern Sweden. The weathered bedrock surface in northeastern Bornholm may be due to Tertiary or earlier weathering followed by small or no glacial erosion.

ISSN:0435-3676    

DOI:10.1080/04353676.1991.11880330

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

ABSTRACTSmall-scale hydraulic laboratory models based on Froude modelling criteria were used to investigate: i) the influence of gradient and discharge on the average cross-sectional geometry and channel pattern of braided streams; ii) the temporal variability of braided stream morphology at constant gradient and discharge; iii) the relationship between this temporal variability in morphology, the occurrence of bed-load pulses and the nature of bed-load transfer processes in gravel-bed braided streams.Repeated transect measurements of braided channels, formed at several different combinations of slope and discharge, show that, despite large temporal and spatial fluctuations, mean active channel width and depth show an orderly relationship equivalent to the classical downstream hydraulic geometry. Similarly, for a given particle-size distribution, braiding intensity increases with increasing discharge and total stream power, as predicted from theory.Temporal fluctuations in channel form parameters correlate with periodic pulses in bed load over a time scale of hours in the models (probably equivalent to days or weeks in prototype streams). The pulses are generated internally by local degradation and aggradation. Increasing bed-load input to a stream reach causes local aggradation, increased braiding intensity and an increase in the number of migrating, avalanche-face, unit bars. These groups of migrating bars are the primary means of downstream transfer of bed-load pulses. Transfer tends to occur as a downstream progression from one aggradational area (complex bar) to another, through an intervening confluence zone. This provides the framework of longer-term braided stream mechanics within which detailed, reach-scale investigations of short-term channel changes and bar development can be conducted.

ISSN:0435-3676    

DOI:10.1080/04353676.1991.11880331

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor

摘要:

ABSTRACTWe studied recent shoreline forest degradation caused by erosion occurring during high-water events on an upper St. Lawrence estuary site in Canada. Uprooted trees show a significant suppression pattern in annual growth rings, allowing accurate dating of erosion events. During recent decades, landward transgression of shoreline was observed at many sites in the estuary. Dates of tree uprooting indicate two major erosion periods, one centered in the late 1940s - early 1950s and the other in 1974–76. Tree establishment at shore interface occurred during a low disturbance period from 1920 to 1940, but increasing geomorphic activity due to a peak frequency of flood was depicted subsequently. While the studied site is lying in a fresh-water meso-tidal estuary, seasonal variability of flood occurrence is linked to climatic fluctuations affecting hydrological regime and, through geomorphic processes, causing ecological disturbance allowing dendrochronological dating of features. However, it seems possible that the sea level rising trend observed along the North American seacoast and even in the Gulf St. Lawrence could be a trigger factor for flood-related shoreline displacement toward forest margins in the upper estuary, where tides are still important.

ISSN:0435-3676    

DOI:10.1080/04353676.1991.11880332

出版商:Taylor&Francis    

年代:1991

数据来源: Taylor