The nature of the inversion problem

The glacial inversion problem is quite different from the genetic problem, i.e. deciphering of the processes by which, and the conditions under which, particular landforms are created (Plate 38.3). Most of our direct process knowledge pertains to marginal landforms such as end moraines, meltwater channels, proximal glaciofluvial deposits, etc. Observations of subglacial processes are restricted to marginal or thin-ice (<100 m) situations or rely on borehole or geophysical data where either the three-dimensional spatial context or the information content and resolution are extremely restricted. Hence, the landforms that are the most important in the inversion context, i.e. glacial lineations and ribbed moraine, are the least understood in terms of genetic processes. Yet, answers to genetic questions are necessary as basic assumptions in inversion models.

The formative processes for individual landforms have attracted considerable attention. For example, linkages between particular marginal landform assemblages and glacier dynamics have been established (e.g. Sharp, 1988; Krüger, 1993; Hart, 1995a; Evans et al., 1999; Glasser & Hambrey, 2002). The understanding of linkages between interior ice-sheet dynamics and particular subglacial landform assemblages, which is critical for ice-sheet-scale palaeoglaciological reconstructions, has also experienced significant progress in the past decade (e.g. Kleman, 1992; Dyke, 1993; Clark, 1993, 1994, 1999; Kleman & Borgström, 1994; Hättestrand & Kleman, 1999; Kleman & Hättestrand, 1999).

The problem of genesis of glacial landforms can, at least in theory, be approached in a manner similar to other laboratory sciences, with the main problems being related to practical and logistical considerations and the extreme hostility and inaccessibility of the subglacial environment. The glacial inversion problem, on the other hand, is a reconstruction problem based on proxy data of phenomenal richness and complexity. The sheer size of glaciated areas requires that literally hundreds of independent data sources be utilized, if credibility in terms of space, time and glaciological inference is to be achieved. These circumstances require that any full-scale ice-sheet reconstruction has to be performed in a stringent manner in order to be trustworthy.

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