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EDWARD C. LITTLE LIONEL E. JACKSON Jr. THOMAS S. JAMES STEPHEN R. HICOCK ELIZABETH R. LEBOE 《Boreas: An International Journal of Quaternary Research》2001,30(1):43-52
In southwestern Alberta, Canada, a westward-rising last-glacial-maximum continental ice limit has been identified. This limit is defined by the upper elevation of Canadian Shield erratics deposited by last-glacial-maximum continental ice along the flanks of prominent ridges and buttes within the region. The interpolation between ice-limit data points has produced two distinct slope profiles: 2.9 m/km to the east, and 4.2 m/km to the west of Mokowan Butte. Three hypotheses are proposed to explain this westward rise of the last-glacial-maximum continental ice limit: (1) regional tectonic uplift, (2) glacio-isostatic uplift, and (3) continental ice-flow convergence due to topographic obstacles and interaction with montane ice. Inferred long-term rates of tectonic uplift and glacio-isostatic modelling show that these two mechanisms account for less than 25% of the observed absolute elevation increase of the limit between the Del Bonita uplands and Cloudy Ridge in southwestern Alberta. The remaining rise in elevation of the continental ice-sheet margin in this region is thought to result from continental ice-flow convergence due to the combined effects of the regional topography and interaction with montane glaciers to the west. The steeper rise in the former continental ice surface west of Mokowan Butte can be explained by the topographic obstruction and interaction with montane glaciers in the area of the Rocky Mountain front. 相似文献
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GEOFFREY S. P. THOMAS A. JOHN SUMMERS 《Boreas: An International Journal of Quaternary Research》1984,13(1):5-12
A series of large-scale glacio-dynamic structures exposed in 10 km of coastal cliff cut through the Screen Hills kame-morainc, Co. Wexford. Ireland, is descrihed. The structures range from simple ovcrfolds to complex multiple thrusts, have a consistent structural trend, and result from marginal deformation by an ice-sheet moving from the NE. The relationship between the structures and other, gcomorphic and stratigraphic components is resolved into a simple model of repeated ice-marginal oscillation during a phase of overall glacial retreat. 相似文献
176.
LARS CHRESTEN LUND-HANSEN EINER LARSEN KURT THOMAS JENSEN KIM N. MOURITSEN CHRISTIAN CHRISTIANSEN THORBJØRN JOEST ANDERSEN 《Marine Georesources & Geotechnology》2013,31(1-2):115-122
A newly developed video and camera system for tidal flat microtopographic studies is presented. It consists of a SONY handy cam placed in an underwater housing mounted on a frame about 70?cm above the sediment surface. A rectangular surface area of 30?×40?cm is imaged by the camera. The camera records video sequences and/or digital images at predetermined time lapses. The total number of images is about 540, and a similar number of 10-second long video sequences can be recorded. The camera is programmed with a PC before deployment, and the total deployment time depends on the time lapse between recordings. The camera is connected to an external power supply (12?volt), and a halogen projector pointing towards the sediment surface ensures that the system is operable on a 24-hours scale. The system has been tested in the Danish Wadden Sea. It has proved to be a very useful tool in studies of topographic effects of erosion and deposition sequences, and for studies of benthic organisms-sediment interactions. The test site was further equipped with sensors for water and seabed measurements, which proved to be indispensable regarding the interpretation of recorded image time-series. 相似文献
177.
THOMAS SKAUGEN 《水文科学杂志》2013,58(6):943-956
Abstract A disaggregation procedure is presented to render forecast values of precipitation from an atmospheric model with spatial resolution of 11 × 11 km suitable as input for a distributed hydrological model with spatial resolution of 1.1 × 1.1 km. Statistical and morphological properties of the input field, such as spatial mean, variance, correlation structure and intermittency, are respected in the disaggregated field. The adopted approach is a combination of interpolation and simulation. The four nodal points of the atmospheric model grid cell are used both for determining the parameters of the exponential distribution for simulating precipitation values, and in a simple interpolation procedure to determine the spatial location of the precipitation values. A shifted distribution with two parameters is used in the case of full coverage of the grid cell, and a one-parameter distribution with a theoretically derived intermittency parameter is used if intermittency is present. The results are promising with respect to the statistical and morphological properties of the disaggregated field. 相似文献
178.
The vertical component of the turbulent flow acceleration term, , is used to determine the net positive vertical force that may support a suspended sediment load. A dimensionless criterion, Λ, is proposed for the maintenance of suspension, defined as the ratio of the maximum vertical turbulent stress to immersed weight of the suspended load above a unit bed area. In order that a suspension be maintained: where v ′ is instantaneous vertical turbulent velocity, σ and ρ are solid and fluid densities, respectively and m is the suspended load dry mass. The Λ criterion is dynamic, being a ratio of stresses and is analogous in this respect to Shields dimensionless stress criterion, θ, for the initiation of bedload motion. The new criterion is successful in predicting the maintenance of steady-state suspended sediment transport in open channel shear flow and deposition from non-uniform particulate density flows of wall jet type. 相似文献
179.
MICHAEL P. EKBERG THOMAS V. LOWELL ROBERT STUCKENRATH 《Boreas: An International Journal of Quaternary Research》1993,22(3):189-204
Despite the application of radiocarbon dating for more than three decades along the southern margin of the Laurentide Ice Sheet, fundamental questions about the timing of glacial advances remain. For one of its sublobes, the Miami, we undertook areal mapping, detailed lithostratigraphic analysis, and radiocarbon dating to interpret four pulses of ice advance. On top of the undated sediments deposited during the first advance is a major unconformity. The second advance occurred about 20,000 BP and marks the beginning of the late Wisconsin glaciation. A minor recession (more than 30 km) ensued, but plants did not reoccupy the landscape. A third advance of the ice margin produced a stone-rich lodgement till to within 20 km of the late Wisconsin maximum. The final ice motion only occurred in the northern part of the study area and may be of local extent. Large accumulations of supraglacial gravity flowtills and outwash mark the final ice-margin retreat. Of these. only the second advance is well dated. This study implies that the number of advances of the ice margin is fewer than previously suggested. Consequently we argue that several of the sublobes across the southern margin of the Laurentide Ice Sheet acted in unison for the interval of 22.000 to 18.000 BP implying ice-sheet external forcing. 相似文献
180.