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1.
Overdeepened valleys and basins are commonly found below the present landscape surface in areas that were affected by Quaternary glaciations. Overdeepened troughs and their sedimentary fillings are important in applied geology, for example, for geotechnics of deep foundations and tunnelling, groundwater resource management, and radioactive waste disposal. This publication is an overview of the areal distribution and the geometry of overdeepened troughs in the Alps and their foreland, and summarises the present knowledge of the age and potential processes that may have caused deep erosion. It is shown that overdeepened features within the Alps concur mainly with tectonic structures and/or weak lithologies as well as with Pleistocene ice confluence and partly also diffluence situations. In the foreland, overdeepening is found as elongated buried valleys, mainly oriented in the direction of former ice flow, and glacially scoured basins in the ablation area of glaciers. Some buried deeply incised valleys were generated by fluvial down-cutting during the Messinian crisis but this mechanism of formation applies only for the southern side of the Alps. Lithostratigraphic records and dating evidence reveal that overdeepened valleys were repeatedly occupied and excavated by glaciers during past glaciations. However, the age of the original formation of (non-Messinian) overdeepened structures remains unknown. The mechanisms causing overdeepening also remain unidentified and it can only be speculated that pressurised meltwater played an important role in this context.  相似文献   

2.
Lian, O. B. & Hicock, S. R. 2009: Insight into the character of palaeo‐ice‐flow in upland regions of mountain valleys during the last major advance (Vashon Stade) of the Cordilleran Ice Sheet, southwest British Columbia, Canada. Boreas, 10.1111/j.1502‐3885.2009.00123.x. ISSN 0300‐9483. A detailed glacial geological study was done on Vashon till, formed during the last (Fraser) glaciation, in upland areas of two relatively short and narrow mountain valleys which open onto the Fraser Lowland in southwest British Columbia. The orientation and association of glaciotectonic structures in till and bedrock, a‐axis fabrics of stones in till and abrasion features, indicate that Vashon till formed initially by lodgement and that brittle deformation processes dominated at least during the latter stages of glaciation. The presence of local glacigenic bedrock quarrying suggests that ice flow experienced localized enhanced compressive flow along valley sides. These observations indicate that ice flow was relatively slow and they contrast with a previous study of bedrock geomorphology undertaken in some larger south Coast Mountains valleys and a model of ice‐flow velocity in the Puget Lowland that suggest rapid ice flow. This indicates that either ice‐flow conditions in the larger valleys were different from those in the valleys studied here, or that the observations from our study reflect subglacial conditions following the Last Glacial Maximum (LGM), but immediately prior to deglaciation when ice had thinned and slowed. If the latter scenario is correct, and if processes inferred from this study were also common along the upland parts of other southwest Coast Mountains valleys after the LGM, then the rate at which ice was supplied to lowland piedmont glaciers would have been reduced, and this may have accelerated decay of the southwest margin of the last Cordilleran Ice Sheet.  相似文献   

3.
Glacial erosion has caused overdeepening of many alpine valleys. After retreat of the ice, they were filled with heterogeneous deposits of glacial, lacustrine and fluvial sediments. A typical example of such a valley segment and its infill is the Reuss delta on the southern shore of Lake Lucerne in Switzerland. To obtain a detailed three‐dimensional image of this valley segment, the ETH Institute of Geophysics has acquired several two‐dimensional, high‐resolution seismic and georadar profiles, and conducted a three‐dimensional georadar survey. Interpretations of these geophysical data were constrained by a geological core extracted from a borehole 300 m deep near the investigation site. The seismic profiles imaged ca. 600 m of sediment infill above bedrock. Based on their reflection characteristics, five different deposition units were distinguished. These units were interpreted as a succession of clay/silt at the base, followed by different sand units with variable but generally increasing amounts of gravel. This succession represented a prograding delta that filled the southern part of Lake Lucerne. The latest fluvial development of the region is best represented by the georadar data. In particular, the three‐dimensional georadar data set provides a detailed view of an ancient braided river channel Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

4.
This paper investigates the processes governing bedrock bedform evolution in ice sheet and ice stream areas in central West Greenland, and explores the evidence for a cross‐shelf ice stream at the Last Glacial Maximum (LGM). To the east of Sisimiut the formation of streamlined bedforms with high elongation ratios and high bedform density has been controlled by geological structure and topography in slow‐flowing ice sheet areas. At the coast, the effects of regional flow convergence, caused by coastal fjord orientation, routed ice into the Sisimiut/Itilleq area where it formed an ice stream onset zone. This funnelled ice into an offshore trough (Holsteinsborg Dyb), resulting in a southwesterly regional ice flow direction and the formation of a topographically routed ice stream (Holsteinsborg Isbrae). To the south of this, striae and bedform evidence show that local valley glaciers initially flowed east to west across the coast, but were later redirected by the Itilleq Fjord ice which turned southwestward due to diffluent flow and deflection by Holsteinsborg Isbrae. Roches moutonnées in this area have low elongation ratios and high bedform density, but do not provide unequivocal support for ice streaming, as they are a product of both bedrock structure and changes in ice flow direction, rather than enhanced flow velocities. Cosmogenic surface exposure ages limit maximum ice sheet surface elevation to ca. 755–810 m above sea level in this region. Such ice thickness enabled Holsteinsborg Isbrae to reach the mid/outer continental shelf during the LGM, and to contribute to the formation of a trough mouth fan and the Outer Hellefisk moraines. Initial deglaciation across this region was driven by rising sea level and increasing air temperatures prior to the Bølling Interstadial at ca. 14.5 cal. ka BP. Between 12 and 10 cal. ka BP both increased air and ocean temperatures post the Younger Dryas, and peak sea‐level rise up to the marine limit, caused accelerated thinning and marginal retreat through calving, although dating evidence suggests ice streams remained along the inner shelf/coast boundary until at least ca. 10 cal. ka BP, their longevity maintained by increased ice thickness and ice discharge. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

5.
Buried palaeo‐valley systems have been identified widely beneath lowland parts of the UK including eastern England, central England, south Wales and the North Sea. In the Midland Valley of Scotland palaeo‐valleys have been identified yet the age and genesis of these enigmatic features remain poorly understood. This study utilizes a digital data set of over 100 000 boreholes that penetrate the full thickness of deposits in the Midland Valley of Scotland. It identified 18 buried palaeo‐valleys, which range from 4 to 36 km in length and 24 to 162 m in depth. Geometric analysis has revealed four distinct valley morphologies, which were formed by different subglacial and subaerial processes. Some palaeo‐valleys cross‐cut each other with the deepest features aligning east–west. These east–west features align with the reconstructed ice‐flow direction under maximum conditions of the Main Late Devensian glaciation. The shallower features appear more aligned to ice‐flow direction during ice‐sheet retreat, and were therefore probably incised under more restricted ice‐sheet configurations. The bedrock lithology influences and enhances the position and depth of palaeo‐valleys in this lowland glacial terrain. Faults have juxtaposed Palaeozoic sedimentary and igneous rocks and the deepest palaeo‐valleys occur immediately down‐ice of knick‐points in the more resistant igneous bedrock. The features are regularly reused and the fills are dominated by glacial fluvial and glacial marine deposits. This suggests that the majority of infilling of the features happened during deglaciation and may be unrelated to the processes that cut them.  相似文献   

6.
Subglacial landsystems in and around Okanagan Valley, British Columbia, Canada are investigated in order to evaluate landscape development, subglacial hydrology and Cordilleran Ice Sheet dynamics along its southern margin. Major landscape elements include drumlin swarms and tunnel valleys. Drumlins are composed of bedrock, diamicton and glaciofluvial sediments; their form truncates the substrate. Tunnel valleys of various scales (km to 100s km length), incised into bedrock and sediment, exhibit convex longitudinal profiles, and truncate drumlin swarms. Okanagan Valley is the largest tunnel valley in the area and is eroded >300 m below sea level. Over 600 m of Late Wisconsin-age sediments, consisting of a fining-up sequence of cobble gravel, sand and silt fill Okanagan Valley. Landform–substrate relationships, landform associations, and sedimentary sequences are incompatible with prevailing explanations of landsystem development centred mainly on deforming beds. They are best explained by meltwater erosion and deposition during ice sheet underbursts.During the Late-Wisconsin glaciation, Okanagan Valley functioned as part of a subglacial lake spanning multiple connected valleys (few 100s km) of southern British Columbia. Subglacial lake development started either as glaciers advanced over a pre-existing sub-aerial lake (catch lake) or by incremental production and storage of basal meltwater. High geothermal heat flux, geothermal springs and/or subglacial volcanic eruptions contributed to ice melt, and may have triggered, along with priming from supraglacial lakes, subglacial lake drainage. During the underburst(s), sheetflows eroded drumlins in corridors and channelized flows eroded tunnel valleys. Progressive flow channelization focused flows toward major bedrock valleys. In Okanagan Valley, most of the pre-glacial and early-glacial sediment fill was removed. A fining-up sequence of boulder gravel and sand was deposited during waning stages of the underburst(s) and bedrock drumlins in Okanagan Valley were enhanced or wholly formed by this underburst(s).Subglacial lake development and drainage had an impact on ice sheet geometry and ice volumes. The prevailing conceptual model for growth and decay of the CIS suggests significantly thicker ice in valleys compared to plateaus. Subglacial lake development created a reversal of this ice sheet geometry where grounded ice on plateaus thickened while floating valley ice remained thinner (due to melting and enhanced sliding, with significant transfer of ice toward the ice sheet margin). Subglacial lake drainage may have hastened deglaciation by melting ice, lowering ice-surface elevations, and causing lid fracture. This paper highlights the importance of ice sheet hydrology: its control on ice flow dynamics, distribution and volume in continental ice masses.  相似文献   

7.
A 30 m-deep drill core from a glacially overdeepened trough in Northern Switzerland recovered a ~180 ka old sedimentary succession that provides new insights into the timing and nature of erosion–sedimentation processes in the Swiss lowlands. The luminescence-dated stratigraphic succession starts at the bottom of the core with laminated carbonate-rich lake sediments reflecting deposition in a proglacial lake between ~180 and 130 ka ago (Marine Isotope Stage MIS 6). Anomalies in geotechnical properties and the occurrence of deformation structures suggest temporary ice contact around 140 ka. Up-core, organic content increases in the lake deposits indicating a warming of climate. These sediments are overlain by a peat deposit characterised by pollen assemblages typical of the late Eemian (MIS 5e). An abrupt transition following this interglacial encompasses a likely hiatus and probably marks a sudden lowering of the water level. The peat unit is overlain by deposits of a cold unproductive lake dated to late MIS 5 and MIS 4, which do not show any direct influence from glaciers. An upper peat unit, the so-called «Mammoth peat», previously encountered in construction pits, interrupts this cold lacustrine phase and marks more temperate climatic conditions between 60 and 45 ka (MIS 3). In the upper part of the core, a succession of fluvial and alluvial deposits documents the Late Glacial and Holocene sedimentation in the basin. The sedimentary succession at Wehntal confirms that the glaciation during MIS 6 did not apparently cause the overdeepening of the valley, as the lacustrine basin fill covering most of MIS 6 is still preserved. Consequently, erosion of the basin is most likely linked to an older glaciation. This study shows that new dating techniques combined with palaeoenvironmental interpretations of sediments from such overdeepened troughs provide valuable insights into the past glacial history.  相似文献   

8.
The sedimentary infills of subglacially eroded bedrock troughs in the Alps are underexplored archives for the timing, extent and character of Pleistocene glaciations but may contain excellent records of the Quaternary landscape evolution over several glacial–interglacial cycles. The onset of sedimentation in these bedrock troughs is often reflected by diamicts and gravels directly overlying bedrock in the deepest basin segments. Subglacial or proglacial depositional environments have been proposed for these coarse‐grained basal units but their characteristics and origin remain controversial. This article presents results from drill cores that recovered a coarse‐grained basal unit in a major buried bedrock‐trough system in the Lower Glatt Valley, northern Switzerland. The excellent core recovery allowed a detailed study combining macroscopic, microscopic and geochemical methods and gives unprecedented insights into the transition from erosion to deposition in overdeepened bedrock troughs. These results show that the basal infill comprises diamicts, interpreted as subglacial tills, separated by thin sorted interbeds, originating from subglacial cavity deposition. The stacking of these units is interpreted to represent repeated switching between a coupled and decoupled ice–bed‐interface indicating an ever‐transforming mosaic of subglacial bed conditions. Decoupling in response to high basal water pressures is probably promoted by the confined subglacial hydraulic conditions resulting from the bedrock acting as aquitards, the narrow reverse sloping outlet and a large catchment area. While stratigraphic and lithological evidence suggests that erosion and the onset of basal sedimentation occurred during the same glaciation, different scenarios for the relative timing of infilling in relation to formation and glaciation of the bedrock trough are discussed. Overlying deltaic and glaciolacustrine sediments suggest deposition during subsequent deglaciation of the bedrock trough. The basal sediment characteristics are in agreement with previous reports in hydrogeological and seismic exploration and suggest the occurrence of similar basal successions in other subglacially overdeepened basins in the Alps and elsewhere.  相似文献   

9.
We estimate using gravity data the thickness of post-glacial unconsolidated sediment filling two major glacial valleys in northern Pyrenees: the Gave de Pau valley between Pierrefitte-Nestalas and Lourdes, and the Garonne valley between Saint-Béat and Barbazan. One hundred and eighty-four new gravity data complete 74 measurements obtained from the International Gravimetric Bureau database. Negative residual anomalies resulting from the presence of small-density unconsolidated sediment approach 4 mgal in both the Gave de Pau and the Garonne valleys. Estimating the sediment thickness requires knowing the density contrast between Quaternary sediments and the underlying bedrock. Supposing this density contrast is 600 kg/m3, the maximum estimated thickness of post-glacial sediment is ∼ 230 and 300 meters, and the volume of sediment is 2.1 and 3.2 km3 in the Gave de Pau and Garonne valleys, respectively. In both valleys, the depth of Quaternary sediment suddenly increases at the confluence between two major glacial valleys (Gave de Pau – Gave de Cauterets, and Garonne – Pique confluences). Overdeepened basins are less deep downstream when approaching terminal moraines (Lourdes and Barbazan area), illustrating that the efficiency of glacial erosion depends on the ice flux flowing through valleys.  相似文献   

10.
Coastal valleys in the west part of Mid-Wales, such as the Mawddach, Dysynni, Tal-y-llyn and Dyfi, acted as corridors for ice which drained the Welsh Ice Cap during the Devensian. Analyses of detailed digital elevation models, and interpretation of satellite images and aerial photographs, show the existence of large variations in the amount of glacial modification between these valleys. Although all the valleys are glacially over-deepened along late Caledonian fault lines, only the Dyfi basin exhibits a dendritic pattern, with V-shaped cross-profiles and valley spurs typical of valleys formed by fluvial processes. Connectivity analysis of the Dyfi basin shows that it exhibits an almost completely dendritic pattern with connectivity α and β values of 0.74 and 1.01, respectively, with little glacial modification of the preglacial fluvial valley pattern in the form of glacial valley breaching. Several examples of glacial meltwater incision into a well-developed pre-existing river valley system, causing river capture across watersheds, have been identified in the Dyfi basin. The degree of preservation of the preglacial fluvial valley system within the Dyfi basin indicates limited modification by glacial processes, despite the area being subjected to glacier activity during the Late Devensian at least. It is possible that major parts of the basin were covered by cold-based or slow-moving ice, close to, or under, a migrating ice-divide, with the major ice drainage occurring along the weaker zone of the Pennal Fault along which teh Dyfi valley is located, causing minor adjustments to the surrounding interfluves and uplands. It is proposed here that the general river valley morphology of the Dyfi basin is of a pre-Late Devensian age.  相似文献   

11.
随着水利水电和旅游经济的发展,清江流域已成为众多学科研究关注的热点,清江的形成与演化也得到了学术界重视。探讨清江的河流演化有助于研究地质灾害的发生机理,为当地减灾防灾工作提供理论支撑。在清江下游多处河段存在以双底谷中谷为代表的古宽谷,它们呈线状连续分布,与现今河道大致平行,是清江下游河道变迁的重要证据。通过古宽谷内的代表性河流阶地年龄数据,可以较为准确地梳理出河流演化过程的时间序列。从清江下游的双底谷中谷和其中的河流阶地的系统调查研究入手,通过自然历史分析法和类比法,结合阶地物质光释光年代测试方法,重建了中更新世晚期以来清江下游大致的河流演化过程,同时提出山地河流演化存在去直取弯、袭夺归并和变换的特点,指出地壳差异隆升是导致河道变迁的主要因素,提出通过河流演化过程的研究探讨岸坡稳定性的思路。  相似文献   

12.
The Wicklow Trough is one of several Irish Sea bathymetric deeps, yet unusually isolated from the main depression, the Western Trough. Its formation has been described as proglacial or subglacial, linked to the Irish Sea Ice Stream (ISIS) during the Last Glacial Maximum. The evolution of the Wicklow Trough and neighbouring deeps, therefore, help us to understand ISIS dynamics, when it was the main ice stream draining the former British–Irish Ice Sheet. The morphology and sub-seabed stratigraphy of the 18 km long and 2 km wide Wicklow Trough is described here from new multibeam echosounder data, 60 km of sparker seismic profiles and five sediment cores. At a maximum water depth of 82 m, the deep consists of four overdeepened sections. The heterogeneous glacial sediments in the Trough overlay bedrock, with indications of flank mass-wasting and subglacial bedforms on its floor. The evidence strongly suggests that the Wicklow Trough is a tunnel valley formed by time-transgressive erosional processes, with pressurised meltwater as the dominant agent during gradual or slow ice sheet retreat. Its location may be fault-controlled, and the northern end of the Wicklow Trough could mark a transition from rapid to slow grounded ice margin retreat, which could be tested with modelling.  相似文献   

13.
The extent of the last British–Irish Ice Sheet (BIIS) in northern Scotland is disputed. A restricted ice sheet model holds that at the global Last Glacial Maximum (LGM; ca. 23–19 ka) the BIIS terminated on land in northern Scotland, leaving Buchan, Caithness and the Orkney Islands ice‐free. An alternative model implies that these three areas were ice‐covered at the LGM, with the BIIS extending offshore onto the adjacent shelves. We test the two models using cosmogenic 10Be surface exposure dating of erratic boulders and glacially eroded bedrock from the three areas. Our results indicate that the last BIIS covered all of northern Scotland during the LGM, but that widespread deglaciation of Caithness and Orkney occurred prior to rapid warming at ca. 14.5 ka. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

14.
四川康定折多山末次冰盛期古冰川重建及其气候意义   总被引：1，自引：0，他引：1  
折多山位于青藏高原东缘,地势上处于由高原面向高山峡谷的过渡区,古冰川遗迹保留较好。对折多山冰川地貌进行深入研究,对揭示青藏高原东缘地形演化与古气候变化的耦合机制具有重要意义。在野外考察的基础上,结合Google Earth遥感影像、《中国第二次冰川编目》和DEM等资料,对折多山冰川地貌及其特征参数进行了识别、提取和计算。研究共识别出189条古冰川,覆盖面积为497 km2。基于研究区已有年代学资料,本区冰川地貌主要为末次冰盛期（LGM）以来冰川作用遗存。恢复研究区LGM冰川平衡线高度（ELA）,在西坡和东坡分别为4 380 m和4 110 m,相差270 m,揭示出分水岭东侧更有利于冰川发育。广泛发育的冰蚀湖、冰蚀基岩面、羊背石等,以及深切的冰川槽谷（U形谷）指示海洋性冰川作用特征;冰川作用正差、冰川朝向、冰蚀地貌的差异,揭示积累区地形条件和水汽来源对研究区冰川发育具有重要的影响。  相似文献   

15.
A 1250 km2 3D seismic volume is used to provide a detailed spatial and geometrical analysis of fifteen Pleistocene tunnel valleys in the Danish North Sea. All the valleys are buried; they are up to 39 km long, 3–4 km wide and up to 350 m deep. The valleys are part of a vast tunnel valley province covering an area of some 0.5 million km2 of the formerly glaciated lowland areas of North West Europe. The valleys consist of non‐branching, non‐anastomosing troughs; they exhibit strongly undulating bottom profiles with numerous sub‐basins and thresholds, and are characterised by adverse end slopes. Cross‐cutting relationships and theoretical considerations suggest the occurrence of seven major episodes of valley incision attributed to ice marginal oscillations within a few glacials. Calculations considering the valley end gradients and theoretical ice‐surface profiles suggest that the valleys were formed by pressurised subglacial meltwater erosion. Given a range of theoretical ice‐surface profiles, the adverse end slopes are well beyond the supercooling threshold, which suggests that the water was not in thermal equilibrium with the basal ice and that flow was concentrated in substantial conduits with sufficient mass and flux to maintain water temperature well above the freezing point. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

16.
Field evidence of palaeoglacial records in the Verçenik valley in the Eastern Black Sea Mountains was examined and 19 samples for surface exposure dating with cosmogenic 10Be were collected with the aim of increasing knowledge on the amplitude and frequency of palaeoglacier advances in Anatolia. Glacial erosional features were mapped and the flow directions of the palaeoglaciers were determined. The Verçenik palaeoglacier advanced before 26.1 k ± 1.2 k yr. The Last Glacial Maximum (LGM) advance continued until 18.8 k ±1.0 k yr. The Verçenik palaeoglacier collapsed during Termination I. After 17.7 k ± 0.8 k yr there was no more ice in the main valley. The Verçenik palaeoglacier most probably then separated into five small glaciers that were restricted to the tributary valleys. Among these, the Hem?in palaeoglacier completed its recession around 15.7 k ± 0.8 k yr. On the basis of glacial erosion features, a Lateglacial glacier advance can be identified. Evidence of the Little Ice Age advance appears to be absent. The results from this valley system seem to be consistent with the first results from the adjacent Kavron valley and with the Anatolian LGM palaeoclimate, the sea surface temperature minima in the western Mediterranean, the deposition of red clay layers in the Black Sea and deposition of the Heinrich‐1 layer in the North Atlantic. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

17.
冰川槽谷横剖面定量化研究方法及其影响因素   总被引：2，自引：2，他引：0  
姚盼  王杰 《冰川冻土》2015,37(4):1028-1040
冰川槽谷(“U”形谷)是冰川与下伏基岩相互作用的结果, 是典型的冰蚀地形, 对其定量化研究是了解冰川作用过程以及冰川槽谷演化过程的重要途径. 二次多项式(y=A+Bx+Cx2)和幂函数(y=axb)是定量描述冰川槽谷形态的两种较普遍的方法, 二次多项式可以描述冰川槽谷的整体形态且不需要考虑高程基准面的选择, 但是该方法不能用于槽谷间的比较且其只能较准确地描述接近抛物线的横剖面; 幂函数不但可以反映不同作用过程形成的谷地, 还能在不同横剖面间进行比较, 但幂函数在应用过程过会受到坐标原点选取、 对数变化、 后期堆积以及横剖面不对称的影响, 其运用过程更加复杂. 此外, 相同的幂函数指数b可能指示不同的槽谷形态, 形态比率FR的引入并与指数b结合起来使对槽谷形态的描述更加全面. 从冰川动力和外部环境方面出发, 影响槽谷形态的因素主要有冰川作用时间、 基岩的抗侵蚀能力、 岩性的分布以及裂隙、 冰量、 气候、 构造和冰川性质, 后三者对槽谷形态的定量化影响需要进一步进行探讨. 运用不同地区槽谷形态参数所做b~FR图探讨了山地冰川槽谷的发育模式, 发现山地冰川槽谷存在对应于两种不同冰川性质的相反的发育模式, 但是由于岩性、 气候等其他因素的影响, 造成了冰川槽谷发育模式有时出现了不对应的情况.  相似文献   

18.
Buried valleys are characteristic features of glaciated landscapes, and their deposits host important aquifers worldwide. Understanding the stratigraphic architecture of these deposits is essential for protecting groundwater and interpreting sedimentary processes in subglacial and ice‐marginal environments. The relationships between depositional architecture, topography and hydrostratigraphy in dissected, pre‐Illinoian till sheets is poorly understood. Boreholes alone are inadequate to characterize the complex geology of buried valleys, but airborne electromagnetic surveys have proven useful for this purpose. A key question is whether the sedimentary architecture of buried valleys can be interpreted from airborne electromagnetic profiles. This study employs airborne electromagnetic resistivity profiles to interpret the three‐dimensional sedimentary architecture of cross‐cutting buried valleys in a ca 400 km2 area along the western margin of Laurentide glaciation in North America. A progenitor bedrock valley is succeeded by at least five generations of tunnel valleys that become progressively younger northward. Tunnel‐valley infills are highly variable, reflecting under‐filled and over‐filled conditions. Under‐filled tunnel valleys are expressed on the modern landscape and contain fine sediments that act as hydraulic barriers. Over‐filled tunnel valleys are not recognized in the modern landscape, but where they are present they form hydraulic windows between deep aquifer units and the land surface. The interpretation of tunnel‐valley genesis herein provides evidence of the relationships between depositional processes and glacial landforms in a dissected, pre‐Illinoian till sheet, and contributes to the understanding of the complex physical hydrology of glacial aquifers in general.  相似文献   

19.
The morphology of the Alpine and adjacent landscapes is directly related to glacial erosion and associated sediment transport. Here we report the effects of glacio-hydrologic erosion on bedrock topography in the Swiss Plateau. Specifically, we identify the presence of subsurface valleys beneath the city of Bern and discuss their genesis. Stratigraphic investigations of more than 4,000 borehole data within a 430 km2-large area reveal the presence of a network of >200 m-deep and 1,000 m-wide valleys. They are flat floored with steep sided walls and are filled by Quaternary glacial deposits. The central valley beneath Bern is straight and oriented towards the NNW, with valley flanks more than 20° steep. The valley bottom has an irregular undulating profile along the thalweg, with differences between sills and hollows higher than 50–100 m over a reach of 4 km length. Approximately 500 m high bedrock highlands flank the valley network. The highlands are dissected by up to 80 m-deep and 500 m-broad hanging valleys that currently drain away from the axis of the main valley. We interpret the valleys beneath the city of Bern to be a tunnel valley network which originated from subglacial erosion by melt water. The highland valleys served as proglacial meltwater paths and are hanging with respect to the trunk system, indicating that these incipient highland systems as well as the main gorge beneath Bern formed by glacial melt water under pressure.  相似文献   

20.
Modelling palaeoglaciers in mountainous terrain is challenging due to the need for detailed ice flow computations in relatively narrow and steep valleys, high-resolution climate estimations, knowledge of pre-ice topography, and proxy-based palaeoclimate forcing. The Parallel Ice Sheet Model (PISM), a numerical model that approximates glacier sliding and deformation to simulate large ice sheets such as Greenland and Antarctica, was recently adapted to alpine environments. In an attempt to reconstruct the climate conditions during the Last Glacial Maximum (LGM) on Mount Dedegöl in SW Turkey, we used PISM and explored palaeoglacier dynamics at high spatial resolution (100 m) in a relatively small domain (225 km2). Palaeoice-flow fields were modelled as a function of present temperature and precipitation. Nine different palaeoclimate simulations were run to reach the steady-state glacier extents and the modelled glacial areas were compared with the field-based and chronologically well-established ice extents. Although our results provide a non-unique solution, best-fit scenarios indicate that the LGM climate on Mount Dedegöl was between 9.2 and 10.6 °C colder than today, while precipitation levels were the same as today. More humid (20% wetter) or arid (20% drier) conditions than today bring the palaeotemperature estimates to 7.7–8.8 or 11.5–13.2 °C lower than present, respectively.  相似文献   

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