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1.
Geomorphological and glacial geological surveys and multiple cosmogenic nuclide analyses (10Be, 26Al, and 21Ne) allowed us to reconstruct the chronology of variations prior to the last glacial maximum of the East Antarctic Ice Sheet (EAIS) and valley glaciers in the Terra Nova Bay region. Glacially scoured coastal piedmonts with round-topped mountains occur below the highest local erosional trimline. They represent relict landscape features eroded by extensive ice overriding the whole coastal area before at least 6 Ma (pre-dating the build-up of the Mt. Melbourne volcanic field). Since then, summit surfaces were continuously exposed and well preserved under polar condition with negligible erosion rates on the order of 17 cm/Ma. Complex older drifts rest on deglaciated areas above the younger late-Pleistocene glacial drift and below the previously overridden summits. The combination of stable and radionuclide isotopes documents complex exposure histories with substantial periods of burial combined with minimal erosion. The areas below rounded summits were repeatedly exposed and buried by ice from local and outlet glaciers. The exposure ages of the older drift(s) indicate multiple Pleistocene glacial cycles, which did not significantly modify the pre-existing landscape.  相似文献   

2.
High-level weathering limits separating ice-scoured topography from frost-weathered detritus were identified on 28 mountains in Wester Ross at altitudes of 700–960 m, and a further 22 peaks support evidence of ice scouring to summit level. Weathering limits are defined most clearly on sandstone and gneiss, which have resisted frost shattering during the Late Devensian Lateglacial, but can also be distinguished on schists and quartzite. Schmidt hammer measurements and analyses of clay mineral assemblages indicate significantly more advanced rock and soil weathering above the weathering limits. The persistence of gibbsite above weathering limits indicates that they represent the upper limit of Late Devensian glacial erosion. The regular decline of weathering-limit altitudes along former flowlines eliminates the possibility that the weathering limits represent former thermal boundaries between protective cold-based and erosive warm-based ice. The weathering limits are therefore interpreted as periglacial trimlines that define the maximum surface altitude of the last ice sheet. Calculated basal shear stresses of 50–95 kPa are consistent with this interpretation. Reconstruction of ice-sheet configuration indicates that the former ice-shed lay above 900 m along the present watershed, and that the ice surface descended northwestwards, with broad depressions along major troughs and localised domes around independent centres of ice dispersal. Extrapolation of the ice surface gradient and altitude suggests that the ice sheet did not overrun the Outer Hebrides, but was confluent with the independent Outer Hebrides ice-cap in the North Minch basin. Erratics located up to 140 m above the reconstructed ice surface are inferred to have been emplaced by a pre-Late Devensian ice sheet (or ice sheets) of unknown age. © 1997 John Wiley & Sons, Ltd.  相似文献   

3.
Analysis of climatic and topographic evidence from the Cascade Range of Washington State indicates that glacial erosion limits the height and controls the morphology of this range. Glacial erosion linked to long-term spatial gradients in the ELA created a tilted, planar zone of 373 cirques across the central part of the range; peaks and ridges now rise ≤600 m above this zone. Hypsometric analysis of the region shows that the proportion of land area above the cirques drops sharply, and mean slopes >30° indicate that the areas above the cirques may be at or near threshold steepness. The mean plus 1σ relief of individual cirque basins (570 m) corresponds to the ∼600-m envelope above which peaks rarely rise. The summit altitudes are set by a combination of higher rates of glacial and paraglacial erosion above the ELA and enhanced hillslope processes due to the creation of steep topography. On the high-precipitation western flank of the Cascades, the dominance of glacial and hillslope erosion at altitudes at and above the ELA may explain the lack of a correspondence between stream-power erosion models and measured exhumation rates from apatite (U-Th/He) thermochronometry.  相似文献   

4.
Nunataks of the last ice sheet in northwest Scotland   总被引：1，自引：0，他引：1  
High-level weathering limits separating ice-scoured topography from an upper zone of frost-weathered detritus were identified on 17 mountains in NW Scotland at altitudes of <600 m to< 900 m, and a further 6 peaks were found to support evidence of ice scouring to summit level. Weathering limits are most clearly defined on Torridon Sandstone, which is resistant to frost shattering, but can also be mapped on Cambrian Quartzite, Lewisian Gneiss and Moine Schist. Contrasts in degree of rock surface weathering above and below the weathering limits were evaluated using measurements of joint depth and rock surface hardness, and through X-ray diffraction analyses of clay mineral assemblages. The results indicate significantly more advanced rock and soil weathering above the weathering limits. Widespread persistence of gibbsite above the weathering limits suggests that they represent the upper limit of Late Devensian glacial erosion, and the regularity of the decline in weathering limit altitude along former flowlines eliminates the possibility that it represents a former thermal boundary between protective cold-based and erosive warm-based ice. The weathering limits are therefore interpreted as periglacial trimlines defining the maximum surface altitude of the last ice sheet around former nunataks. Calculated basal shear stresses of 50–78 kPa are consistent with this interpretation. The altitude of the trimlines implies that the former ice shed lay at 900–930 m in the Fannich Mountains and descended gently northwards, and that the ice surface descended NW from the ice shed to >500 m over the extreme NW tip of Scotland and to 700–730 m at the head of Little Loch Broom.  相似文献   

5.
Erosion-driven uplift of the modern Central Alps   总被引：2，自引：0，他引：2  
We present a compilation of data of modern tectono-geomorphic processes in the Central European Alps which suggest that observed rock uplift is a response to climate-driven denudation. This interpretation is predominantly based on the recent quantification of basin-averaged Late Holocene denudation rates that are so similar to the pattern and rates of rock uplift rates as determined by geodetic leveling. Furthermore, a GPS data-based synthesis of Adriatic microplate kinematics suggests that the Central Alps are currently not in a state of active convergence. Finally, we illustrate that the Central Alps have acted as a closed system for Holocene redistribution of sediment in which the peri-Alpine lakes have operated as a sink for the erosional products of the inner Central Alps.While various hypotheses have been put forward to explain Central Alpine rock uplift (e.g. lithospheric forcing by convergence, mantle processes, or ice melting) we show with an elastic model of lithospheric deformation, that the correlation between erosion and rock uplift rates reflects a positive feedback between denudation and the associated isostatic response to unloading. Thus, erosion does not passively respond to advection of crustal material as might be the case in actively converging orogens. Rather, we suggest that the geomorphic response of the Alpine topography to glacial and fluvial erosion and the resulting disequilibrium for modern channelized and associated hillslope processes explains much of the pattern of modern denudation and hence rock uplift. Therefore, in a non-convergent orogen such as the Central European Alps, the observed vertical rock uplift is primarily a consequence of passive unloading due to erosion.  相似文献   

6.
Reconstructions of the last (late Devensian) British ice sheet have hitherto been based on assumptions regarding its extent and form. Here we employ observational evidence for the maximum altitude of glacial erosion (trimlines) on mountains that protruded through the ice (palaeonunataks) to reconstruct the form of the ice sheet over ≈ 10 000 km2 of NW Scotland. Contrasts in the clay mineralogy of soils and exposure ages of rock surfaces above and below these trimlines confirm that they represent the upper limit of late Devensian glacial erosion. The reconstruction yields realistic values of basal shear stress and is consistent with independent evidence of ice movement directions. The ice sheet reached ≈ 950 m altitude over the present N–S watershed, descended northwards and north-westwards, was deflected around an ice dome on Skye and an independent Outer Hebrides ice cap, and probably extended across the adjacent shelf on a bed of deforming sediments.  相似文献   

7.
A wide variety of Late Devensian periglacial landforms developed on Scottish mountains both before ca. 13,000 BP and during the Loch Lomond Stadial of ca. 11,000-10,000 BP. Nearly all such features are now inactive. Late Devensian periglacial weathering produced three types of regolith mantle (openwork block deposits, sandy diamicts and silt-rich frost-susceptible diamicts), each of which supports a characteristic assemblage of relict landforms. On upper slopes these include large-scale sorted circles and stripes, earth hummocks and nonsorted relief stripes, sorted and nonsorted solifluction features, massive boulder sheets and lobes, and nivation benches. Talus, protalus ramparts, rock glaciers and alluvial fans also developed at the base of mountain slopes.The distribution of Late Devensian periglacial features on Scottish mountains is locally controlled by topography, the response of underlying rocks to periglacial weathering and the limits of former glaciers. Regional variations in the altitude of certain forms of Loch Lomond Stadial age (particularly protalus ramparts and rock glaciers) indicate a decrease in former snowfall eastwards across the Scottish Highlands and northwards from the Highland Boundary Fault. Several upland periglacial features are also diagnostic of former permafrost, and complement palaeotemperature reconstructions based on ice-wedge casts and the equilibrium firn line altitudes of stadial glaciers. These suggest that under stadial conditions mean January temperatures at 600 m and 1000 m on mountains in the Western Grampians must have been no higher than −20°C and −23°C respectively, and possibly several degrees lower.  相似文献   

8.
Cosmic ray exposure ages of frost-weathered bedrock from mountain summits in the Outer Hebrides exceed the age of Late Devensian glaciation. Exposure ages of most glacially-abraded bedrock surfaces at low and intermediate elevations are younger than the age of maximum Late Devensian glaciation. These results confirm that previously mapped periglacial trimlines in the Outer Hebrides define the upper limit of bedrock erosion by Late Devensian ice. They are consistent with the interpretation, based on geomorphological evidence, that the trimlines mark the approximate upper limit of a Late Devensian Outer Hebrides Ice Cap. A postglacial exposure age from the summit of Oreval (662 m) suggests that this mountain was overrun during the last glaciation, indicating thicker ice cover and a lower surface gradient west of the ice-cap divide than previously inferred. Although bedrock surfaces below the trimlines are strongly ice-moulded, some show evidence of prior cosmic ray exposure, which we attribute to limited erosion during Late Devensian glaciation. If this interpretation is correct, the youngest apparent ages from these surfaces give the most reliable dates for deglaciation, at ca. 14.5–14 ka. This implies that ice persisted at favourable sites through the warm opening phase of the Windermere Interstade. Comparison with radiocarbon-dated evidence from offshore cores suggests net ice margin retreat of ∼74 km eastwards across the adjacent shelf in > 2.3 ± 1.0 ka. The dating evidence is consistent with relatively rapid retreat of calving margins to the coast, then slower withdrawal of ice margins to high ground. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

9.
青藏高原及周边山地拥有地球上最高大且最广阔的高山高原,是除两极外最大的现代冰川作用中心,这也使得中国成为中低纬度地区现代冰川最发育的国家之一. 现代冰川平衡线分布具有纬度地带性特征,在青藏高原上还呈不对称的环状. 根据相关研究资料估算,中国末次冰期最盛期时的冰川面积约为50×104 km2,是现代的8.4倍. 基于平衡线处年降水量和夏季平均气温（6-8月）之间的相关关系重建的中国西部（105° E以西）末次冰期最盛期时的平衡线分布图与现代的相似. 在青藏高原内部与西北部,平衡线下降值在500 m以内,小的仅为200~300 m;在青藏高原东南边缘下降值约800 m,最大可达1 000~1 200 m. 天山与阿尔泰山平衡线下降值均在500 m左右. 中国东部（105° E以东）没有发育现代冰川,仅有数处中高山地,如贺兰山、太白山、长白山与台湾山地保存有确切的末次冰期冰川地形,末次冰期最盛期时的平衡线下降800~900 m,大于青藏高原、天山与阿尔泰山地区的下降值. 根据中国东部末次冰期的平衡线分布图以及相关的古气候与古环境研究资料,海拔2 000 m以下的中低山地在第四纪期间任何一次冰川作用中都不具备冰川发育所需的地势条件.  相似文献   

10.
Evidence of small glaciers is often used to infer past atmospheric climate through calculation of steady-state ELAs. However, if topographic niches such as shading or windblown-snow augmented mass-balance then ELAs cannot reflect regional climate and determining the significance of these topoclimates is therefore important. The Brecon Beacons, South Wales, contains upland glacial landforms dating to the Younger Dryas (11,000–10,000 yr BP) when local climate was at the threshold for glaciation. This case study categorises topoclimate using three-dimensional modelling of topography and reconstructed palaeoglaciers from two sites containing mapped moraines whose orientation suggest complex patterns of deglaciation. Ablation season solar radiation is modelled over multiple ice-surfaces as shade from surrounding topography and intensity from ice-surface incidence angle. Snowblow and avalanching potential models are also used and the significance of all topoclimate variables assessed against a mass-balance deficit calculated for each glacier given palaeoclimate models for the region. Results demonstrate that both glaciers were likely to be heavily reliant on topoclimate and that previous studies underestimate the significance of solar radiation. Modelled over multiple ice-surfaces reflecting patterns of recession, results indicate that the distribution of topoclimate variables predicts the style of deglaciation at both sites, possibly explaining the complexity of glacial evidence in this environment.  相似文献   

11.
Glaciers erode bedrock but are also efficient conveyors of debris supplied during a cycle of glaciation by processes other than basal erosion. In this dual capacity as both an eroding and a transporting agent lies the ambiguity of ‘glacial erosion’ as a geomorphic process, with implications for methods of measuring the removal of rock mass by glaciers in the geological past, and for interpreting what exactly the consequences have been on topography and elevation change. A global review of ～400 Quaternary glacial denudation rates estimated from five different measurement techniques provides values ranging between 10?4 and 10 mm yr?1. We investigate the causes of such wide variability by examining the respective influences of environmental setting and methodological bias. A reference frame chosen for assessing these issues is the Massif du Carlit (Pyrenees, France), where a quantified mass balance of the well preserved glacial, periglacial and paraglacial deposits was made possible by detailed geomorphological mapping and terrestrial cosmogenic nuclide dating of extant erosional and depositional landform sequences. Resulting age brackets helped to define three main episodes of ice-cap growth and decline, each characterized by a volume of debris and a mappable source area. Erosion rates were expressed in two ways: (i) as spatially averaged denudation rates (D) during the successive stages of glacial advance to the line of maximum ice extent (MIE), post-MIE ice recession, and Lateglacial cirque readvance, respectively; and (ii) as cirque-wall recession rates (R) where moraine facies criteria indicated a supraglacial provenance of debris. Results indicate low erosion (D ≈ 0.05 mm yr?1) during the ice advance phase, probably because of thin or passive ice covering the low-gradient subglacial topography that occurs just above the late Pleistocene equilibrium line altitude (2.2–2.4 km). Erosion rates peaked (D ≈ 0.6 mm yr?1 and R ≈ 2.4–4.5 mm yr?1) during the main transition to ice-free conditions, when deglacial debuttressing promoted the rapid response of freshly exposed slope systems to new equilibrium conditions in the steep crest zone. Lateglacial D- and R-values declined to 0.2–0.3 mm yr?1, with indications of spatially variable R controlled by lithology. In this environment glaciers overall behaved more as conveyors of debris supplied by supraglacial rock exposures in the mountain crest zone than as powerful modifiers of subglacial topography. This explains the widespread preservation of deep, in situ preglacial weathering profiles on relict Cenozoic land surfaces in the deglacierized part of the Eastern Pyrenees. When plotted on the global data set analyzed and discussed in the review, the East Pyrenean erosion rates stand out as being amongst the lowest on record.  相似文献   

12.
The major climatic variations that have affected the summit slopes of the higher Apennine massifs in the last 6000 yr are shown in alternating layers of organic matter-rich soils and alluvial, glacial and periglacial sediments. The burial of the soils, triggered by environmental-climatic variations, took place in several phases. For the last 3000 yr chronological correlations can be drawn between phases of glacial advance, scree and alluvial sedimentation and development of periglacial features. During some periods, the slopes were covered by vegetation up to 2700 m and beyond, while in other phases the same slopes were subject to glacial advances and periglacial processes, and alluvial sediments were deposited on the high plateaus. Around 5740-5590, 1560-1370 and 1300-970 cal yr B.P., organic matter-rich soils formed on slopes currently subject to periglacial and glacial processes; the mean annual temperature must therefore have been higher than at present. Furthermore, on the basis of the variations in the elevation of the lower limit reached by gelifraction, it can be concluded that the oscillations in the minimum winter temperatures could have ranged between 3.0°C lower (ca. 790-150 cal yr B.P.) and 1.2°C higher (ca. 5740-5590 cal yr B.P.) than present minimum winter temperatures. During the last 3000 yr the cold phases recorded by the Calderone Glacier advance in the Apennines essentially match basically the phases of glacial advance in the Alps.  相似文献   

13.
地带性与非地带性夷平面*   总被引：6，自引：0，他引：6  
夷平面是指由剥蚀和夷平作用所产生的,以截面形式横切所有在年龄上先于它的地层和构造的一种平缓地形。前人普遍认为,夷平面是在构造相对稳定条件下经过长期外营力作用形成的一个接近基准面的平坦地形,是地貌长期演化的终极产物。 流水侵蚀是夷平面形成的动力学基础。"夷平面"一直是解释新生代以来大的构造格局和地貌演化的重要理论依据,如我国青藏高原隆升的研究。 基于气候地貌学原理,文章提出了寒冻夷平面和干旱夷平面的概念;从坡地演化过程理论模型入手,探讨了寒冻、干旱夷平面和流水夷平面的形成机理;提出有别于传统"构造侵蚀旋迴"理论的"构造隆升和地面剥夷动态平衡"的气候夷平地貌假说;此外,还简要分析了这三类夷平面的全球分布规律。  相似文献   

14.
Geomorphological mapping of southern Skye indicates evidence for a single readvance of locally-nourished glaciers. These comprised a major icefield that occupied c. 155 km2 of the main mountain area, a small icefield c. 10 km2 in extent in the Kyleakin hills and ten corrie glaciers with a total area of c. 16 km2. The absence of Lateglacial pollen sites, shorelines and periglacial features within the limits of local glaciation implies a Loch Lomond Readvance age for this glacial event. The area-weighted mean equilibrium line altitude (ELA) of the reconstructed Loch Lomond Readvance glaciers (319 m) conforms with a regional eastwards rise in ELAs that indicates dominant westerly airstreams during the Loch Lomond Stadial. An easterly decline in ELAs across the former icefields is interpreted in terms of easterly transfer of snow across ice-sheds by westerly winds, though the altitudes of corrie glacier ELAs suggest that the dominant snow-bearing winds were southerlies. Calculations based on the area-weighted mean ELA for the major icefield (308 m) indicate a stadial mean July sea-level temperature of c. 6 °C. The anomalously low gradients of certain former icefield outlet glaciers are attributed to deformation of subglacial sediment, an effect that may vitiate the assumption of linear ablation/accumulation gradients in the calculation of former ELAs for reconstructed glaciers.  相似文献   

15.
The plateaus between 400 and 800 m a.s.l. around the water-divides on central and eastern Janieson Land are covered by the 'Jameson Land Drift' up to 50 m thick glacial. placiotluvial and glaciolacustrine deposits. A high content of far-travelled wcsterii rocks indicates the overriding by extensive glaciers channelled from the west through the Scoresby Sund basin. The Jameson Land Drift deposits have bccn lithostratigraphically divided into two groups. each representing the sedimentary successions from one glaciation in the wider sense of the word. sediments from the lower Lollandselv glaciation are upwards delimited by a distinct periglacial surface. TL-dates suggest a prc-Saalian (approximately isotope stages 11–9) age. The following Scoreshby Sund glaciation . when most of the studied Jameson Land Drift sediments were laid down. is of Saalian age (e. isotope stages 8 6). The deposits from the Scoresby Sund glaciation are interpreted as representing a complete glaciation deglaciation succession, including proglacial sandur and glaciolacustrine sediments. followed by till deposition, with an overlying succession of glaciolacustrinc and glaciofluvial sediments. From 200–250 m to c . 400 m a.s.l. there is a driftless area, exposing Jurassic sandstones, probably a result of intensive and long-lasting periglacial erosion. Extensive occurrences of tors and of glaciofluvially (subglacially as well as subaerially) eroded canyons and channels characterize the landscape. A similar. although less well defined. upper driftless zone is found above c 500 m a.s.l. on northern Jameson Land, north of the drift-covered plateaus. During the Wcichsclian (isotope stages 5d 2). thick glacial. fluvial and marine deposits were laid down in a coastal zone below c . 200 m a.s.l., and only cold-based local ice caps seem to have existed on the interior plateaus of Jameson Land. The now driftless areas were characterized by periglacial erosion during this period.  相似文献   

16.
新疆天山地区是我国高海拔、高纬度、大温差典型代表冰缘山区,本文基于气象水文站及现场监测获取的大气温度和降水变化规律,建立了天山地区冻融侵蚀强度评价模型,结合野外三维激光扫描观测结果初步探讨了冻融侵蚀时空分异规律对岩体产屑率的影响。研究结果表明:(1)大气温度、岩体结构、降水量、坡度、地震烈度和坡向等因素对冻融侵蚀强度的贡献值依次减小。(2)天山地区冻融侵蚀下界海拔约为2600~2900 m,冻融侵蚀的空间分布具有明显的垂直分带性和纬度坡降性,时间上具有明显的季节变化特征,融化和冻结交替时大气温度在冻融侵蚀基准线0 ℃上下波动频繁,冻融侵蚀最为强烈。(3)溜砂坡形成过程被划分为岩体冻融侵蚀产屑、岩屑运移和堆积3个阶段;花岗岩(硬岩)、砂岩(中硬岩)和千枚岩(软岩)斜坡产屑率随冻融侵蚀评价指数增加而增加,随岩石冻融系数增加而减小,建立不同岩性斜坡产屑率与冻融侵蚀强度和岩石冻融系数的定量关系式。该成果可以为冰缘山区岩体冻融侵蚀理论研究、工程建设及防灾减灾提供参考。  相似文献   

17.
Geomorphological mapping of northern Arran provides evidence for two advances of locally nourished glaciers, the younger being attributable to the Loch Lomond Stade (LLS) of ca. 12.9–11.5 k yr BP, primarily through the mutually exclusive relationship between glacial limits and Lateglacial periglacial features. The age of the earlier advance is unknown. Inferred LLS glacier cover comprised two small icefields and eight small corrie or valley glaciers and totalled 11.1 km2. ELAs reconstructed using area–altitude balance ratio methods range from 268 m to 631 m for individual glaciers, with an area‐weighted mean ELA of 371 m. ELAs of individual glaciers are strongly related to snow‐contributing areas. The area‐weighted mean ELA is consistent with a north–south decline in LLS ELAs along the west coast of Great Britain. This decline has an average latitudinal gradient of 70 m 100 km?1, equivalent to a mean southwards ablation‐season temperature increase of ca. 0.42°C 100 km?1. Mean June–August temperatures at the regional climatic ELA, estimated from chironomid assemblages in SE Scotland, lay between 5.7 ± 0.1°C and 4.1 ± 0.2°C. Empirical relationships between temperature and precipitation at modern glacier ELAs indicate equivalent mean annual precipitation at the ELA lay between 2002 ± 490 mm and 2615 ± 449 mm. These figures suggest that stadial precipitation on Arran fell within a range between +8% and ?33% of present mean annual precipitation. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

18.
姚盼  王杰  林文旺  曾兰华  陈仁容 《冰川冻土》2022,44(4):1260-1269
确定冰川侵蚀的主控因素及其与各影响因素间的相互作用方式,不仅对深入认识冰川侵蚀的物理机制和理解冰川作用区地貌演化具有重要的意义,也是探讨构造、气候、地形间相互关系的根本。然而,以往的学者仅在构造活动略单一的区域研究冰川侵蚀的主控因素,致使对构造的影响认识不足,那么,构造是否是冰川侵蚀的主控因素呢？又是如何作用于冰川侵蚀？北天山第四纪冰川作用规模巨大,留下了丰富的冰川遗迹,其气候与构造条件也多样,因而成为探讨上述问题的理想区域。本文在北天山北坡自西向东选取了7个冰川流域,基于每个流域的Hkr值和冰川侵蚀影响因子的定性定量数据,分析了该区域冰川侵蚀的分布规律及其影响因素。结果表明,北天山各流域冰川侵蚀自西向东有减小趋势,该变化趋势是构造、气候、地形共同作用所致。其中,山顶高度和降水对冰川侵蚀的影响最显著,两者均通过对冰川规模施加作用来控制冰川侵蚀,而构造也可能通过影响顶点高程、积累区面积、冰川规模,进而作用于冰川侵蚀,但是其是否发挥主要作用有待进一步认识。因此,冰川规模可能才是导致北天山各冰川流域侵蚀差异的根本原因。  相似文献   

19.
Across Ireland there is a striking topographical contrast between predominantly limestone-floored lowlands and uplands developed on silicate-dominated rock types. This arises from the fundamentally different way in which limestone and other rocks are removed. Limestone is removed through dissolution, a low-energy process enhanced by vegetation. Other rock types are removed by erosion, a high energy process that is inhibited by vegetation. In Ireland countless ‘soft days’ over the last 60 Ma have been more effective at removing limestone than other rock types. Limestone uplands have survived only where they were protected by a cover of insoluble rock, such as sandstone or mudstone, which has been stripped away relatively recently by glacial erosion. The large-scale removal of considerable thicknesses of limestone across Ireland has increased the relief of non-limestone uplands through the effects of isostatic uplift. Denudation across the Irish landscape has led to changing outcrop patterns of limestone and other rocks, resulting in profound long-term changes in topography and drainage patterns.  相似文献   

20.
The creation of the huge fans observed in the western Barents Sea margin can only be explained by assuming extremely high glacial erosion rates in the Barents Sea area. Glacial processes capable of producing such high erosion rates have been proposed, but require the largest part of the preglacial Barents Sea to be subaerial. To investigate the validity of these proposals we have attempted to reconstruct the western preglacial Barents Sea. Our approach was to combine erosion maps based on prepublished data into a single mean valued erosion map covering the whole western Barents Sea and consequently use it together with a simple Airy isostatic model to obtain a first rough estimate of the preglacial topography and bathymetry of the western Barents Sea margin. The mean valued erosion map presented herein is in good volumetric agreement with the sediments deposited in the western Barents Sea margin areas, and as a direct consequence of the averaging procedures employed in its construction we can safely assume that it is the most reliable erosion map based on the available information. By comparing the preglacial sequences with the glacial sequences in the fans we have concluded that 1/2 to 2/3 of the total Cenozoic erosion was glacial in origin and therefore a rough reconstruction of the preglacial relief of the western Barents Sea could be obtained. The results show a subaerial preglacial Barents Sea. Thus, during interglacials and interstadials the area may have been partly glaciated and intensively eroded up to 1 mm/y, while during relatively brief periods of peak glaciation with grounded ice extending to the shelf edge, sediments have been evacuated and deposited at the margins at high rates. The interplay between erosion and uplift represents a typical chicken and egg problem; initial uplift is followed by intensive glacial erosion, compensated by isostatic uplift, which in turn leads to the maintenance of an elevated, and glaciated, terrain. The information we have on the initial tectonic uplift suggests that the most likely mechanism to cause an uplift of the dimensions and magnitude of the one observed in the Barents Sea is a thermal mechanism.  相似文献   

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