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1.
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2.
Debris transported by glacier is derived either supraglacially from nunataks and valley sides or from erosion of the subglacial bed. Debris produced above the glacier by fracturing of rock walls has a dominant coarse fraction with angular boulders. Subsequent englacial or supraglacial transport is relatively passive and little comminution occurs. Debris eroded from subglacial bedrock is initially transported in a basal zone of traction, where particles frequently come into contact with the glacier bed and are retarded by it so that large forces may be generated between particles and the bed and at interparticle contacts. The material introduced into this tractional zone may be subglacial bedrock which has undergone a crushing-plucking event and which has a dominant coarse fraction, or supraglacially derived material which finds its way to the glacier bed. These parent debris assemblages are further comminuted by failure in response to locally concentrated compressive stresses, and attrition at shearing interfaces. Boulders transported through the tractional zone will tend to be rounded and bear several directions of striation. Large boulders embedded in lodgement till will tend to be streamlined with striae parallel to glacier flow and with an abruptly truncated distal extremity, rather like a roche moutonnée. Textural and boulder shape characteristics can be used to help distinguish different types of till.  相似文献   

3.
Quaternary sedimentary successions are described from the Linda Valley, a small valley in western Tasmania that was dammed by ice during Early and Middle Pleistocene glaciations. Mapping and logging of exposures suggest that an orderly sequence of deposits formed during ice incursion, occupation and withdrawal from tributary valleys. Four principal sediment assemblages record different stages of ice occupation in the valley. As the glacier advanced, a proglacial, lacustrine sediment assemblage dominated by laminated silts and muds deposited from suspension accumulated in front of the glacier. A subglacial sediment assemblage consisting of deformed lacustrine deposits and lodgement till records the overriding of lake-bottom sediments as the glacier advanced up the valley into the proglacial lake. As the glacier withdrew from the valley, a supraglacial sediment assemblage of diamict, gravel, sand and silt facies formed on melting ice in the upper part of the valley. A lacustrine regression in the supraglacial assemblage is inferred on the basis of a change from deposits mainly resulting from suspension in a subaqueous setting to relatively thin and laterally discontinuous laminated sediments, occurrence of clastic dykes, and increasing complexity of the geometry of deposits that indicate deposition in a subaerial setting. A deltaic sediment assemblage deposited during the final stage of ice withdrawal from the valley consists of steeply dipping diamict and normally graded gravel facies formed on delta foresets by subaqueous sediment gravity flows. The sediment source for the delta, which prograded toward the retreating ice margin, was the supraglacial sediment assemblage previously deposited in the upper part of the valley. A depositional model developed from the study of the Linda Valley may be applicable to other alpine glaciated areas where glaciers flowed through or terminated in medium- to high-relief topography.  相似文献   

4.
Composition and genesis of glacial hummocks, western Wisconsin, USA   总被引：1，自引：0，他引：1  
Glacial hummocks associated with the Superior Lobe in western Wisconsin are stagnant-ice features composed of melt-out till, meltwater-stream sediment, and flow till. The greater proportion of melt-out till in these hummocks than in hummocks described elsewhere suggests that a model of extensive, supraglacial reworking of supraglacially released debris does not apply to the western Wisconsin hummocks. Interpretation of melt-out till in hummock exposures is based on its strong fabric oriented parallel to regional ice-flow direction. Other features of this melt-out till include poorly developed stratification (color banding and discontinuous thin sandy lenses), and minor faulting, both of which support a melt-out origin. We suggest that as stagnant, debris-rich ice began to melt, supraglacially released debris was deposited as flow till and meltwater-stream sediment (with some debris-flow sediment and lake sediment), but as the thickness of supraglacial debris increased, debris melting out at depth was stabilized, allowing features characteristic of melt-out till to be retained. Because the supraglacial debris was sandy and the stagnant ice was likely at the pressure-melting point, the supraglacial debris was well drained and did not readily fail and flow. Debris volume in the glacier generally was greater at the glacier margin, but lateral and longitudinal variations within this zone were caused by thrusting, freezing-on, or ice-margin fluctuations, which in turn resulted in variations in hummock relief. Ice-walled-lake plains are commonly associated with the hummocks and developed where debris volume was small.  相似文献   

5.
Breaks in the slope of log-probability plots of cumulative grain-size distributions of bed material are compared with frequency distributions of bedload and suspended sediment over a range of discharges at two stations on the Platte River in south-central Nebraska. The break between suspension and intermittent suspension as determined from the bed-material curve coincides with the upper limit of the grain-size overlap between bedload particles and suspended-sediment particles, whereas the break between intermittent suspension and traction corresponds to the grain size at the lower limit of overlap of bedload particles and suspended-sediment particles. Although grain-size distributions of bedload change little with discharge, the size of the coarsest grains in suspension increases with increasing discharge. Thus, the length of overlap of bedload and suspended-sediment distributions increases with increasing discharge. The limits of grain-size overlap of bedload and suspended-sediment distribution curves associated with near-flood discharges most closely approximate the breaks in the bed material grain-size distribution.  相似文献   

6.
This paper focuses on the structural glaciology, dynamics, debris transport paths and sedimentology of the forefield of Soler Glacier, a temperate outlet glacier of the North Patagonian Icefield in southern Chile. The glacier is fed by an icefall from the icefield and by snow and ice avalanches from surrounding mountain slopes. The dominant structures in the glacier are ogives, crevasses and crevasse traces. Thrusts and recumbent folds are developed where the glacier encounters a reverse slope, elevating basal and englacial material to the ice surface. Other debris sources for the glacier include avalanche and rockfall material, some of which is ingested in marginal crevasses. Debris incorporated in the ice and on its surface controls both the distribution of sedimentary facies on the forefield and moraine ridge morphology. Lithofacies in moraine ridges on the glacier forefield include large isolated boulders, diamictons, gravel, sand and fine-grained facies. In relative abundance terms, the dominant lithofacies and their interpretation are sandy boulder gravel (ice-marginal), sandy gravel (glaciofluvial), angular gravel (supraglacial) and diamicton (basal glacial). Proglacial water bodies are currently developing between the receding glacier and its frontal and lateral moraines. The presence of folded sand and laminites in moraine ridges in front of the glacier suggests that, during a previous advance, Soler Glacier over-rode a former proglacial lake, reworking lacustrine deposits. Post-depositional modification of the landform/sediment assemblage includes melting of the ice-core beneath the sediment cover, redistribution of finer material across the proglacial area by aeolian processes and fluvial reworking. Overall, the preservation potential of this landform/sediment assemblage is high on the centennial to millennial timescale.  相似文献   

7.
Hummocky terrain composed of boulder gravel and a wavy contact between stratified till and sand are described and explained as products of subglacial meltwater activity beneath the Saginaw Lobe of the Laurentide Ice Sheet in south-central Michigan. Exposures and geophysical investigations of hummocky terrain in a tunnel channel reveal that hummocks (˜100m diameter) are glaciofluvial bedforms with a supraglacial melt-out till or till flow veneer. The hummocky terrain is interpreted as a subglacial glaciofluvial landscape rather than one of stagnant ice processes commonly assumed for hummocky landscapes. Sandy bedforms at another site are in-phase with a wavy contact at the base of a stratified till exposed for 50m along the margin of a tunnel channel. The 0.4m thick stratified till is overlain by up to 5m of compact, pebble-rich, sandy subglacial melt-out till. The contact between the till and sand has a wave form with a 0.5m amplitude and 3-5m wavelength. Bedding within the stratified till, sandy bedforms and melt-out till are mostly in-phase with each other. Clasts from the overlying stratified till penetrate and deform the underlying sand recording recoupling of the ice to its bed. Ice ripples cut into the base of river ice have a similar morphology and are considered analogs for cavities cut into the base of the glacier and subsequently filled with sand. Subglacial meltwater activity was not coeval at each study site, indicating that subglacial meltwater played important roles in the evolution of the subglacial environment beneath the Saginaw Lobe at different times.  相似文献   

8.
The macro‐ and micro‐sedimentology of a supraglacial melt‐out till forming at the Matanuska Glacier was examined in relationship to the properties of the stratified basal zone ice and debris from which it is originating. In situ melting of the basal ice has produced a laminated to bedded diamicton consisting mainly of silt. Macroscopic properties include: discontinuous laminae and beds; lenses of sand, silt aggregates and open‐work gravel; deformed and elongate clasts of clay; widely dispersed pebbles and cobbles, those that are prolate usually with their long axes subparallel to parallel to the bedding. Evidence for deformation is absent except for localized bending of beds over or under rock clasts. Microscopic properties are a unique element of this work and include: discontinuous lineations; silt to granule size laminae; prolate coarse sand and rock fragments commonly with their long axis subparallel to bedding; subangular to subrounded irregular shaped clay clasts often appearing as bands; sorted and unsorted silt to granule size horizons, sometimes disrupted by pore‐water pathways. Limited deformation occurs around rock clasts and thicker parts of lamina. This study shows that in situ melting of debris‐rich basal ice can produce a laminated and bedded diamicton that inherits and thereby preserves stratified basal ice properties. Production and preservation of supraglacial melt‐out till require in situ melting of a stagnant, debris‐rich basal ice source with a low relief surface that becomes buried by a thick, stable, insulating cover of ice‐marginal sediment. Also required are a slow melt rate and adequate drainage to minimize pore‐water pressures in the till and overlying sediment cover to maintain stability and uninterrupted deposition. Many modern and ancient hummocky moraines down glacier of subglacial overdeepenings probably meet these process criteria and their common occurrence suggests that both modern and pre‐modern supraglacial melt‐out tills may be more common than previously thought.  相似文献   

9.
西天山托木尔峰南麓大型山谷冰川冰舌区消融特征分析   总被引：4，自引：3，他引：1  
基于对托木尔峰南麓托木尔型山谷冰川的野外考察和典型冰川的定位观测,对冰面被表碛广泛覆盖的所谓“托木尔型”冰川冰舌区表碛与冰面消融的关系进行了研究. 结果表明：表碛对冰面消融、冰川水文过程、冰川变化等均具有重要影响,当表碛厚度超过3 cm时,表碛对冰面消融就产生明显抑制作用,且随着厚度增加,冰面消融显明减弱. 科其喀尔冰川表面的观测表明,由末端向上,表碛厚度逐渐减薄. 受表碛影响,科其喀尔冰川区最大的消融量出现在海拔3 800~3 900 m之间、表碛物厚度小于10 cm的区域内;冰川消融强度由此向上随着海拔的升高而下降,向下随表碛厚度的增大而减弱. 冰面湖的发育是表碛覆盖冰川的又一主要特征,湖水对冰面的融蚀和快速排泄成为冰面产汇流的主要过程. 科其喀尔冰川研究表明,两三个冰面湖排泄形成的融蚀冰量就相当于冰川末端退缩造成的冰量损失. 因此,冰面湖等热喀斯特地形的形成、扩张融蚀、融穿排泄、形成湖区低地,这一周而复始的过程不仅是其主要消融方式之一,而且也强烈的影响着冰川水文及冰川变化. 托木尔峰南麓地区大型冰川变化主要以厚度减薄为主,而不是像大多数冰川显著的变化主要表现在末端和面积减少方面.  相似文献   

10.
冰岩界面的冰川动力学是冰川系统的重要组成部分, 海螺沟冰川地处温暖湿润的海洋环境, 冰川运动速度较快, 冰川底部接近压融点, 是研究冰下过程的较理想地点. 在海螺沟冰川大型磨光面上浅显侵蚀坑内发现了碎屑物质. 对碎屑物质理化特征研究表明: 粒度特征、地球化学与石英砂SEM 分析表明沉积在冰岩界面上的物质来自于冰川底部的底碛层, 而不是冰上环境的产物. 偏光显微镜下观察到的冰下沉积物呈现出一系列塑性变形(微旋转、褶皱)和脆性变形(线性结构、支撑结构、断层)微观结构和构造. 两种变形结构的存在是碎屑物质在形成过程中其含水量波动情况的反映. 冰下碎屑物质是冰下融出、滞碛作用的共同产物. 在整个冰下碎屑物质形成与变形过程中, 由于冰下水系季节性变化带来的冰岩界面上冰川融水含量的波动起了决定性作用.  相似文献   

11.
Subglacial and subaqueous sediments deposited near the margin of a Late-glacial ice-dammed lake near Achnasheen, northern Scotland, are described and interpreted. The subglacial sediments consist of deformation tills and glacitectonites derived from pre-existing glaciolacustrine deposits, and the subaqueous sediments consist of ice-proximal outwash and sediment flow deposits, and distal turbidites. Sediment was delivered from the glacier to the lake by two main processes: (1) subglacial till deformation, which fed debris flows at the grounding line; and (2) meltwater transport, which fed sediment-gravity flows on prograding outwash fans. Beyond the ice-marginal environment, deposition was from turbidity currents, ice-rafting and settling of suspended sediments. The exposures support the conclusion that the presence of a subglacial deforming layer can exert an important influence on sedimentation at the grounding lines of calving glaciers.  相似文献   

12.
A typical stratigraphy below a streamlined till plain in Northumberland, England, consists of cross-cutting lodgement till units, within and between which occur repeated shoestring interbeds of ‘cut and fill’ channels. Till units have erosional lower contacts; in certain cases marked changes in erratic content and local ice flow direction are evident from one till unit to another. These lodgement till complexes have hitherto been described by ‘tripartite’ schemes of lower grey till (s) and upper reddened till (s) identified with respect to ‘middle’ fluvial horizons; regional correlation proceeding on the basis of matching ‘middle’ horizons, with the whole sequence commonly interpreted as evidence for multiple glaciation. Data indicates, by way of contrast, that these lodgement till complexes were deposited during a single phase of subglacial deposition. Till deposition was not continuous but was interrupted by erosional episodes. Changes in the mix of bedrock lithologies transported by the glacier down a single flow line or by lateral displacement of basal ice flow units within the glacier result in till units of different facies to be emplaced when deposition recommences, a process referred to as ‘unconformable facies superimposition’. Subglacial meltwater flow was also a characteristic of the glacier bed; channeled glaciofluvial sediment bodies are found as ribbon-like inclusions in the till and appear to have been deposited rapidly. These so-called ‘middle’ fluvial horizons occur repeatedly in section, their lateral extent at any given exposure being dependent upon the orientation of the exposure with respect to former ice flow direction. These lenses act as internal drainage blankets and have accelerated postglacial soil formation in the drier climate of eastern Britain accounting for the reddened colour of upper till(s). It is suggested that this model of subglacial deposition can be employed in other areas of northern England characterized by subglacial (lodgement till plain) terrains.  相似文献   

13.
Tillites, conglomerates and sandstones occurring in the basal part of the Smalfjord Formation along the Varangerfjord, East Finnmark, North Norway are believed to have formed during the retreat of a glacier. At Kvalnes, on the south side of the fjord, the following sequence, up to 20 m thick, is found: (1) massive monomict tillite interpreted as a subglacial till, (2) massive polymict tillite with lenticular intercalations of stratified sandstone and tillite, interpreted as supraglacial/proglacial drift, (3) polymict conglomerate interstratified with laminated sandstones, interpreted as braided stream deposits. The last named interfingers laterally and is overlain by marine sandstones. At Bigganjargga, near the head of the fjord, a lens of tillite about 3 m thick rests on a striated pavement and is overlain by sandstones and shales. Part of the tillite, containing irregular patches of slightly winnowed tillite, is interpreted as a melt-out till, while a marginal part consisting of inclined tillite beds is interpreted as a series of flow till deposits. The lens is believed to be an oblique section through what was originally an ice-cored moraine ridge. During a subsequent transgression, the moraine was partially eroded, a lag conglomerate was formed, and overlying marine sediments were deposited. Bedded flow tills formed in a supraglacial/proglacial environment may be preserved where the extent of current reworking is very low (such as an isolated end moraine). Stratified conglomerate and sandstone, intimately intercalated with tillite, is to be expected at a glacier margin where glacial meltwater is locally and occasionally abundant, and glacier ablation permits downslope flowage of mobilized supraglacial fluid till.  相似文献   

14.
This paper presents the first detailed sedimentological study of annual moraines formed by an alpine valley glacier. The moraines have been forming since at least AD 1980 by a subsidiary lobe of Gornergletscher, Switzerland that advances up a reverse bedrock slope. They reach heights of 0.5–1.5 m, widths of up to 6 m and lengths of up to several hundreds of metres. Sediments in these moraines are composed of proglacial outwash and debris flow units; subglacial traction till is absent entirely. Based on four representative sections, three genetic process combinations have been identified: (i) inefficient bulldozing of a gently sloping ice margin transfers proglacial sediments onto the ice, causing differential ablation and dead‐ice incorporation upon retreat; (ii) terrestrial ice‐contact fans are formed by the dumping of englacial and supraglacial material from point sources such as englacial conduit fills; debris flows and associated fluvial sediments are stacked against a temporarily stationary margin at the start, and deformed during glacier advance in the remainder, of the accumulation season; (iii) a steep ice margin without supraglacial input leads to efficient bulldozing and deformation of pre‐existing foreland sediments by wholesale folding. Ice‐surface slope appears to be a key control on the type of process responsible for moraine formation in any given place and year. The second and third modes result in stable and higher moraines that have a higher preservation potential than those containing dead ice. Analysis of the spacing and climatic records at Gornergletscher reveals that winter temperature controls marginal retreat and hence moraine formation. However, any climatic signal is complicated by other factors, most notably the presence of a reverse bedrock slope, so that the extraction of a clear climatic signal is not straightforward. This study highlights the complexity of annual moraine formation in high‐mountain environments and suggests avenues for further research.  相似文献   

15.
Terminal-moraine ridges up to 6 m high have been forming at the snout of Styggedalsbreen for two decades. Based on intermittent observations during this period, combined with a detailed study of ridge morphology, sedimentary structures and composition during the 1993 field season, a model of terminal-moraine formation that involves the interaction of glacial and glacio-fluvial processes at a seasonally oscillating ice margin is presented. In winter, subglacial debris is frozen-on to the glacier sole; in summer, ice-marginal and supraglacial streams deposit sediments on the wasting ice tongue. The ice tongue overrides an embryonic moraine ridge during a late-winter advance and a double layer of sediment (diamicton overlain by sorted sands and gravels) is added to the moraine ridge during the subsequent ablation season. Particular ridges grow incrementally over many years and exert positive feedback by enhancing snout up-arching during the winter advance and constraining the course of summer meltwater streams close to the ice margin. The double-layer annual-meltout model is related to moraine formation by the stacking of subglacial frozen-on sediment slabs (Krüger 1993). Moraine ridges of this type have a complex origin. are not push moraines, and may be characteristic of dynamic high-latitude and high-altitude temperate glaciers.  相似文献   

16.
Sediments deposited in two small ice-contact lakes with low rates of sediment input have been studied in subaerial exposures. Sediment characteristics are a function of the water source (glacial meltwater versus non-meltwater), proximity to the glacier margin and lake shore, amount of supraglacial debris, and lake duration. Calving Lake expanded (and later partially drained) as a calving ice margin retreated. Nearshore deltas contain 1 × 105 m3 stratified sand and gravel deposited at rates up to 1 m/yr during a 9-yr interval. Deltaic sediment contains types A and B ripple-drift cross-lamination, draped lamination, and scour surfaces caused by variations in water-flow velocity and the amount of sediment settling from suspension. Most water inflow came from non-subglacial meltwater sources and was sediment-poor, so overflow and interflow sedimentation processes dominated the offshore environment. Offshore sediment generally contains massive silt or silt interbedded with fine-grained sand deposited at rates of 1.3-1.5 cm/yr. Iceberg gravity craters observed on the lake plain were formed when icebergs impacted the lake floor during calving events. In Bruce Hills Lake, proximity to glacier ice and the presence of supraglacial sediment formed coarsening-upward successions when debris fell directly from an ice ledge onto silty lacustrine sediment.  相似文献   

17.
冰川冰内及冰下水系研究综述   总被引：1，自引：0，他引：1  
冰内及冰下水系的形成与演化具有时空变化性,对冰川汇水储水及径流过程产生影响,与之紧密联系的冰下水文过程(水力状况)与冰川运动、冰川侵蚀及冰川洪水形成等过程息息相关。冰内及冰下水系空间结构和形态复杂,且不同于一般喀斯特水文系统,具有明显的季节变化性,其空间分布和水力状况会因外界水体输入(降水和冰雪融水)的变化而改变。冰内及冰下水系的变化通过影响汇流对冰川融水的径流过程产生影响,冰川区一些溃决洪水事件的发生与冰内及冰下蓄水的突然释放有很大关系。冰川蓄排水还通过改变冰下水力条件来影响冰川运动,反之冰川运动不仅影响蓄排水过程的转换效率,且通过改变冰川消融强度(冰体向下游消融区输送速率的变化)影响冰川排水系统的空间分布范围。在气候变暖及冰川变化的背景下,研究冰内冰下水系演化的时空特征及其影响具有重要科学意义。综述了目前国内外针对冰川冰内及冰下水系相关研究的进展及主要成果,并对该领域的研究前景进行了展望。  相似文献   

18.
JOHN SHAW 《Sedimentology》1987,34(1):103-116
Glacigenic sediments exposed in pits around Villeneuve, near Edmonton, Alberta, are subdivided into facies based on grain size, sedimentary structure, glacially-induced deformation and faulting, and groove marks. Two diamicton facies are recognised, one of which is interpreted as a primary till, deposited directly from glacier ice, and the other as a product of mass-movement. The diamicton facies are closely associated with current bedded facies interpreted as fluvioglacial deposits. The stratigraphic sedimentological and tectonic aspects of these fluvial deposits suggest subglacial deposition in channels and cavities. At any one place the glacier appears to have alternated between being attached to the bed, causing thrusting and sole marking, and being separated from the bed by a cavity in which fluvial and mass-movement sediments accumulated. The net result is a highly complex and laterally variable stratigraphy produced by a single glacial advance. The correct interpretation of such sequences is essential if lithostratigraphy is to be used to establish glacial history. In addition, the interpretations presented here have implications regarding the formation of soft zones in ‘till’. They indicate that the soft zones are beds of sorted sediment redeposited by mass-movement.  相似文献   

19.
The Kuannersuit Glacier surged 11 km between 1995 and 1998. The surge resulted in the formation of an ice cored thrust moraine complex constructed by subglacial and proglacial glaciotectonic processes. Four main thrust zones are evident in the glacier snout area with phases of compressional folding and thrusting followed by hydrofracture in response to the build-up of compressional stresses and the aquicludal nature of submarginal permafrost and naled. Various types of stratified debris-rich ice facies occur within the marginal zone: The first (Facies I) comprises laterally continuous strata of ice with sorted sediment accumulations, and is reworked and thrust naled ice. The second is laterally discontinuous stratified debris-rich ice with distinct tectonic structures, and is derived through subglacial extensional deformation and localised regelation (Facies II), whilst the third type is characterised by reworked and brecciated ice associated with the reworking and entrainment of meteoric ice (Facies III). Hydrofracture dykes and sills (Facies IV) cross-cut the marginal ice cored thrust moraines, with their sub-vertically frozen internal contact boundaries and sedimentary structures, suggesting supercooling operated as high-pressure evacuation of water occurred during thrusting, but this is not related to the formation of basal stratified debris-rich ice. Linear distributions of sorted fines transverse to ice flow, and small stratified sediment ridges that vertically cross-cut the ice surface up-ice of the thrust zone relate to sediment migration along crevasse traces and fluvial infilling of crevasses. From a palaeoglaciological viewpoint, marginal glacier tectonics, ice sediment content and sediment delivery mechanisms combine to control the development of this polythermal surge valley landsystem. The bulldozing of proglacial sediments and the folding and thrusting of naled leads to the initial development of the outer zone of the moraine complex. This becomes buried in bulldozed outwash sediment and well-sorted fines through surface ablation of naled. Up-ice of this, the heavily thrust margin becomes buried in sediment melted out from basal debris-rich ice and subglacial diamicts routed along thrusts. These mechanisms combine to deliver sediment to supraglacial localities, and promote the initial preservation of structurally controlled moraines through insulation, and the later development of kettled dead ice terrain.  相似文献   

20.
Approximately 35 parallel, discontinuous glacial ridges occur in an area of about 100 km2 in north‐central Wisconsin. The ridges are located between about 6 and 15 km north (formerly up‐ice) of the maximum extent of the Wisconsin Valley Lobe of the Laurentide Ice Sheet. The ridges are between 1 and 4 m high, up to 1 km long, and spaced between 30 and 80 m apart. They are typically asymmetrical with a steep proximal (ice‐contact) slope and gentle distal slope. The ridges are composed primarily of subglacial till on their proximal sides and glacial debris‐flow sediment on the distal sides. In some ridges the till and debris‐flow sediment are underlain by sorted sediment that was deformed in the former direction of ice flow. We interpret the ridges to be recessional moraines that formed as the Wisconsin Valley Lobe wasted back from its maximum extent, with each ridge having formed by a sequence of (1) pushing of sorted ice‐marginal sediment, (2) partial overriding by the glacier and deposition of subglacial till on the proximal side of the ridge, and (3) deposition of debris‐flow sediment on the distal side of the ridge after the frozen till at the crest of the ridge melted. The moraines are similar to annual recessional moraines described at several modern glaciers, especially the northern margin of Myrdalsjokull, Iceland. Thus, we believe the ridges probably formed as a result of minor winter advances of the ice margin during deglaciation. Based on this assumption, we calculate the net rate of ice‐surface lowering of the Wisconsin Valley Lobe during the period when the moraines formed. Various estimates of ice‐surface slope and rates of ice‐margin retreat yield a wide range of values for ice‐surface lowering (1.7–14.5 m/yr). Given that ablation rates must exceed those of ice‐surface lowering, this range of values suggests relatively high summer temperatures along the margin of the Wisconsin Valley Lobe when it began retreating from its maximum extent. In addition, the formation of annual moraines indicates that the glacier toe was thin, the ice surface was clean, and the ice margin experienced relatively cold winters.  相似文献   

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