The important fact that shear margins support a significant fraction of ice stream gravitational driving stress is caused ultimately by excess basal lubrication of ice streams, which leads to the situation in which td < tb or even td << tb. (Raymond, 2000; Tulaczyk et al., 2000a,b; Kamb, 2001). Early models of ice-stream mechanics focused on application of the hard-bed sliding theory assuming that the basal lubrication is due to ice-bed separation by a subglacial water film/layer (e.g. Weertman & Birchfield, 1982). However, in the recent decades students of ice streams focused on the soft bedded end-member of basal ice-stream conditions, as exemplified by the trunks of the Siple Coast ice streams (Alley et al., 1986a, 1987a,b, 1989; Blankenship et al., 1986, 1987, 2001; Bentley, 1987; MacAyeal, 1989a,b, 1992b; Kamb, 1991; Whillans & van der Veen, 1993, 1997, 2001; Echelmeyer et al., 1994; Engelhardt & Kamb, 1997; Smith, 1997a,b; Harrison et al., 1998; Hulbe & MacAyeal, 1999; Anandakrishnan et al., 2003; Vaughan et al., 2003b; and many others). Kamb (2001) provides a comprehensive and recent overview of borehole observations of basal conditions beneath three of these ice streams. Although much of the recent work on ice streams focused on the soft-bedded conditions, it is important to remember that ice streams develop in hard-bedded settings as well. This caution is perhaps best underscored by findings of Stokes & Clark (2003), who identified a footprint of a palaeo-ice stream in northern Canada, where the ice stream was flowing over a predominantly hard bed.
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