摘要:

Large subglacial cavities in basal ice. the presumed precursors of depositional drumlins, can be created by corrasion from suspended sediment during a large water sheet outburst flood. The cavities are primarily the result of vortex action. For a given flood discharge the corrasion rate of the ice roof increases with water sheet velocity, V, to a power greater than V16/3, Because of ice displacment during a flood, V can vary spatially along a flow line by up to an order of magnitude. The highest velocities normally occur downstream from major bed depressions or near the ice margin. Fields of large drumlins frequently occur at such sites. The process of formation of large subglacial cavities is predicted to be more velocity‐sensitive than the erosion of bedrock. No accurate estimate of water sheet flood volumes can be made at this time but they could be at least an order of magnitude less than the 80 000 km3, which was previously estimate

ISSN:0300-9483    

DOI:10.1111/j.1502-3885.1995.tb00622.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The basal ice of many glaciers contains debris structures that reflect subglacial processes. Presented here is an unusually clear photograph of ice and debris in the lowest 2 m of the basal layer at the margin of the Greenland ice sheet. The photograph shows ice‐debris relationships and deformation structures that reflect entrainment processes and flow histor

ISSN:0300-9483    

DOI:10.1111/j.1502-3885.1995.tb00623.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Tills from four Pleistocene glaciations were recovered from drill cores in Estonia and subjected to particle size and microtexture analyses by Scanning Electron Microscope (SEM). All tills were deposited by thick continental ice‐sheets following the transport of, at most, several hundred kilometers during four Fennoscandian glaciations. The main problem is to determine if the type and range of microtextures present on the grain surfaces are diagnostic of transport in continental ice. The frequency of occurrence of microtextures including fractures, abrasion, and relief features are used to test the ability of continental ice to damage quartz particles emplaced as till. The range of quartz dissolution and presence of coatings on grains are also used to reconstruct the paleoenvironment that existed prior to transport as well as to estimate diagenetic effects that occurred following emplacement. The available data indicate a high degree of reworking of quartz grains from one glaciation to another. While the shapes and microtextures of grains from source rocks are not known, the great range of fracture and abrasion microfeatures, and high frequency of occurrence on grains in all tills, indicate that glaciers are effective crushing agents. An increase in the prevalence of chemically etched grains from older to younger tills suggests that some grains (c. 50%) escape crushing, either because of preservation in the ice and lack of grain‐to‐grain contact, or as a result of massive reworking of weathered grains following interglacia

ISSN:0300-9483    

DOI:10.1111/j.1502-3885.1995.tb00624.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

This paper identifies an anomaly between the currently acccpted isobasc pattern for raised shorelines in Scotland. which is a simple ellipsoid centred over the Western Highlands, and current models for the morphology of the Late Devensian ice‐sheet. which show a primary dome in the Western Highlands but a prominent secondary dome over the Southern Uplands. One explanation of this anomaly is that it is an artefact of inadequate shoreline data for south‐west Scotland. To test this hypothesis, altitude data on the Main Rock Platform in the Firth of Clyde area have been collected and analysed. and demonstrate patterns that probably result from the influence of Southern Upland ice. The ice‐sheet models in this area are therefore supp

ISSN:0300-9483    

DOI:10.1111/j.1502-3885.1995.tb00625.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Bottomsets from glaciomarine deltas situated stratigraphically below and above the Weichselian maximum glaciation till at Skorgenes, western Norway, were tested for consolidation in an attempt to quantify the ice thickness at the time of deposition of the till. The value of the preconsolidation pressure in the lower unit (15 ± 1 MPa), indicates an ice thickness over the site of some 1350 ± 90 m. This is, however, only considered a minimum because values of preconsolidation pressures normally are lower than actual ice thickness would suggest due to incomplete drainage of the bed during consolidation. The estimated ice thickness indicates an ice surface some 400 m above the lower limit of the block field in the area, suggesting that this limit can not be used as a criterion for reconstructing the upper glacier surface for the Weichselian maximum glaciation in western Norway. Also, the nearest mountain peaks seemingly were completely ice covered, suggesting that no nunataks were present in that are

ISSN:0300-9483    

DOI:10.1111/j.1502-3885.1995.tb00626.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

The late Weichselian sequence in the northern part of the Norwegian Trench is composed of eight units. The two lowermost units are massive, firm to stiff diamictons, interpreted to have been deposited beneath ice‐streams that in all likelihood reached the shelf edge. They are overlain by glaciomarine and normal‐marine sediments deposited after 15000BP. The first phase of glacial retreat from the Norwegian Trench (15000–14800 BP) was very rapid and left a thin layer of proximal sediments on top of the tills. This was followed by a period with lower accumulation rates (14800–13600 BP), probably as a result of rapid source retreat and cold meltwater inhibiting dropstone fall‐out. The end of this interval marks the change from ice‐stream calving in cold water to melting on land. According to lithologic and isotopic data, the maximum rate of Fennoscan‐dian ice‐sheet disintegration took place around 12500 BP. The water temperatures declined significantly and rates of sedimentation and ice‐rafting fell in association with the Younger Dryas period. The final retreat of the ice began as early as 10 500 BP, and the transition to normal‐marine sedimentation is reflected by precipitation of iron oxide followed by pyrite, reduced sedimentation rates, and a change from terrigenous to bio

ISSN:0300-9483    

DOI:10.1111/j.1502-3885.1995.tb00627.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

This paper examines the data obtained during a complex study of a 17 m deep peat sequence in a small glaciokarstic kettle hole mire in SE Estonia. The pollen and macrofossil analysis and the14C data were used for the spatio‐temporal reconstruction of the development of the mire, including its vegetation and microclimatic conditions during the Holocene. The comparative analysis of the data gained allows us to estimate the relative influence of various local, extra‐local, and regional natural processes on the dynamics of the ecosyst

ISSN:0300-9483    

DOI:10.1111/j.1502-3885.1995.tb00628.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Landform assemblages may be useful for elucidating glaciological changes in West Antarctica. Numerous nunataks in the Antaretic Peninsula are highly dissected by alpine valley heads (corries, cirque‐headed valleys, etc.). It is believed that these have been cut by wet‐based mountain glaciation before and during the build up of the ice‐sheet. though the smallest ones may have formed later at times when the ice‐sheet was insufficiently thick to submerge them. Dimensions of the features suggest that the main alpine glaciation was in the Miocene. Morphometric analysis of 1663 alpine valley heads has been based on satellite imagery, concentrating on regional variations in frequency, size and orientation. Length of alpine glaciation is believed to be an important control of size. Orientation is believed to have been influenced by palaeoclimate as well as topography. During early, more marginal glaciations poleward and lee‐side sites were occupied by corrie glaciers, with more windward sites being added as glaciation intensified, followed by northerly orientations under more recent polar conditions. Comparisons have also been made with other morphometric studies of corries in Antarctica and

ISSN:0300-9483    

DOI:10.1111/j.1502-3885.1995.tb00629.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY