| 1. |
- Hall, Adrian M., et al.
(author)
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Glacial ripping: geomorphological evidence from Sweden for a new process of glacial erosion
- 2020
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In: Geografiska Annaler Series a-Physical Geography. - : Informa UK Limited. - 0435-3676 .- 1468-0459. ; 2:4, s. 333-53
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Journal article (peer-reviewed)abstract
- In low relief Precambrian gneiss terrain in eastern Sweden, abraded bedrock surfaces were ripped apart by the Fennoscandian Ice Sheet. The resultantboulder spreadsare covers of large, angular boulders, many with glacial transport distances of 1-100 m. Boulder spreads occur alongside partly disintegrated roches moutonnees and associated fracture caves, and are associated withdisrupted bedrock, which shows extensive fracture dilation in the near surface. These features are distributed in ice-flow parallel belts up to 10 km wide and extend over distances of >500 km. Our hypothesis is that the assemblage results from (1) hydraulic jacking and bedrock disruption, (2) subglacial ripping and (3) displacement, transport and final deposition of boulders. Soft sediment fills indicate jacking and dilation of pre-existing bedrock fractures by groundwater overpressure below the ice sheet. Overpressure reduces frictional resistance along fractures. Where ice traction overcomes this resistance, the rock mass strength is exceeded, resulting in disintegration of rock surfaces and ripping apart into separate blocks. Further movement and deposition create boulder spreads and moraines. Short boulder transport distances and high angularity indicate that glacial ripping operated late in the last deglaciation. The depths of rock mobilized in boulder spreads are estimated as 1-4 m. This compares with 0.6-1.6 m depths of erosion during the last glaciation derived from cosmogenic nuclide inventories of samples from bedrock surfaces without evidence of disruption. Glacially disrupted and ripped bedrock is also made ready for removal by future ice sheets. Henceglacial rippingis a highly effective process of glacial erosion.
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| 2. |
- Krabbendam, M., et al.
(author)
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Glaciotectonic disintegration of roches moutonnées during glacial ripping in east Sweden
- 2022
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In: Geografiska Annaler. Series A, Physical Geography. - : Informa UK Limited. - 0435-3676 .- 1468-0459. ; 104:1, s. 35-56
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Journal article (peer-reviewed)abstract
- Roches moutonnées are typical landforms of glacial erosion developed in hard rocks, with an asymmetric profile caused by abrasion and lee-side plucking. In eastern Sweden, some roches moutonnées show extensive damage, including open fractures, disintegration into blocks, fracture caves and short boulder trains. Disintegration increases along ice-flow directions during deglaciation of the last Weichselian Fennoscandian Ice Sheet, indicating a subglacial origin: limited edge rounding can be explained by a combination of hard rock, slow abrasion rates and disintegration just prior to deglaciation. The roches moutonnées initially developed in kernels of gneissic rocks with a wide fracture spacing (large block size) and interlocking fracture pattern, and hence high overall rock mass strength. Dilated fractures and ‘fracture caves’ occur up to 15 m below the ice-bed interface. It is proposed that hydraulic jacking by overpressured water opened up the rock mass along pre-existing fractures. Jacking reduced rock mass strength, allowing glaciotectonic deformation of the roches moutonnées. Uneven hydraulic jacking led to uplift of individual fracture-bound blocks above the pre-existing smooth, abraded surface of the roches moutonnées, creating blunt, step-like edges. These edges allowed high ice pushing forces to act on large blocks: where blocks extend into the deeper rock mass, they further aided the disintegration of the rock mass. The disintegrated roches moutonnées can be regarded as transient features between intact bedrock and complete disintegration into boulders. The jacking-disintegration-transport sequence is characteristic of glacial ripping and very different from classic lee-side plucking.
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| 3. |
- Sugden, David E., et al.
(author)
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Plucking enhanced beneath ice sheet margins : evidence from the Grampian Mountains, Scotland
- 2019
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In: Geografiska Annaler. Series A, Physical Geography. - : Informa UK Limited. - 0435-3676 .- 1468-0459. ; 101:1, s. 34-44
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Journal article (peer-reviewed)abstract
- Concentrations of boulders are a common feature of landscapes modified by former mid-latitude ice sheets. In many cases, the origin of the boulders can be traced in the up-ice direction to a cliff only tens to hundreds of metres distant. The implication is that a pulse of plucking and short boulder transport occurred beneath thin ice at the end of the last glacial cycle. Here we use a case study in granite bedrock in the Dee Valley, Scotland, to constrain theory and explore the factors involved in such a late phase of plucking. Plucking is influenced by ice velocity, hydrology, effective ice pressure, the extent of subglacial cavities and bedrock characteristics. The balance between these factors favours block removal beneath thin ice near a glacier margin. At Ripe Hill in the Dee Valley, a mean exposure age of 14.2 ka on blocks supports the view that the boulder train formed at the end of ice sheet glaciation. The late pulse of plucking was further enhanced by ice flowing obliquely across vertical joints and by fluctuations in sub-marginal meltwater conditions. An implication of the study is that there is the potential for a wave of ice-marginal plucking to sweep across a landscape as an ice sheet retreats.
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