Figure 2.10 A) Lodged and striated boulder and lee-side fluting on the foreland of Skalafellsjokull, Iceland. B) Explanation of a fluting forming in the lee of a lodged boulder (from Benn, 1994).
near the outer limits of the Little Ice Age advance contain complex interbedded sequences of tills and stratified sediments. Often underlying the surface tills are stratified sediments deposited initially in ice-contact fans, braided streams and localized proglacial lakes (Howarth, 1968; Boulton, 1987; Benn and Evans, 1996; Evans, 2000; Evans and Twigg, 2002; Fig. 2.11). These sediments are truncated by the overlying tills and are characterized by internal glacitectonic disturbance in many locations. Evans and Twigg (2002) report a vertical continuum comprising a basal undisturbed zone of horizontally bedded sediments and peat beds overlain by a shear faulted and locally fluidized and hydrofractured zone in which original sedimentary structures are still discernible (non-penetrative glacitectonite of Benn and Evans, 1996). This is overlain by an intensely deformed zone or penetrative glacitectonite (Benn and Evans, 1996; Evans et al.,
1998). The whole sequence is capped by a till containing smeared inclusions of the stratified sediments and peat. Large numbers of striated and stoss-and-lee clasts in the tills on the foreland indicate that abrasion and lodgement is taking place in addition to widespread deformation. Additionally, till fabrics record clear clast alignments that are consistent with the former ice flow directions as recorded by local flutings. Therefore, the vertical sediment continuums probably record the local cannibalization of pre-advance sediments and the plastering of lodgement and deforming bed materials after their advection from up-glacier.
Conspicuous features in the subglacial tills of temperate glaciers are vertical dykes or injection structures composed of non-till sediment. On the foreland of Slettjokull, Iceland such features have been interpreted by van der Meer et al. (1999) as the products of subglacial meltwater discharges and related specifically to water escape. Previous research on clastic dykes suggests that they form by hydrofracturing due to the escape of pressurized groundwater (Mandl and Harkness, 1987; Boulton and Caban, 1995). The tensional cracks so formed are infilled by the sediment fluidized by the escaping water (Lowe, 1975; Nichols et al., 1994; Rijsdijk et al.,
1999). Important in the interpretations of van der Meer et al. (1999) is the occurrence of an apparently perennially frozen outer glacier margin. This marginal belt of permafrost acts to force increased discharges of subglacial meltwater into sub-till outwash, thereby producing downglacier dipping water escape structures. Although the occurrence of permafrost is not
Total Ral alive Strain
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