Superimposed on some glacially deformed permafrost are unde-formed dykes and sills of intrusive ice and some undeformed massive segregated-intrusive ice (Fig. 64.2D). Such ice clearly post-dates the time of glacial deformation. Because the underlying sedimentary sequence has remained frozen ever since it was deformed, water supplied by pore-water expulsion during permafrost aggradation is unlikely. Instead, an external source of overpressurized water is inferred, probably from glacial meltwa-ter flowing beneath proglacial permafrost during the retreat ofthe LIS (Murton, 2005).
These interactions illustrate the dynamic coupling of the cold-based margin of an ice sheet, first with subglacial permafrost and later, during deglaciation, with proglacial permafrost.
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