BUILDING UP TILLS BY SUB-MARGINAL, INCREMENTAL THICKENING

Chiefly on theoretical grounds, it has been established that the most important till-producing mechanisms in a sub-glacial setting must be melt-out, lodgement and deformation. These three mechanisms are well established and virtually undisputed: they have been presented as principal end-members of the sub-glacial till deposition process.

Although as theoretical concepts the definition of till forming end-members may be useful, we believe that it is unlikely that these processes may individually be responsible for the formation of most sub-glacial tills. This is in line with current discussions on the actual extent of the process of deformation.

We are currently testing a hypothesis that proposes the incremental thickening of till sequences in sub-marginal or sub-glacial settings. Our investigations involve assessments of the relative contributions of the depositional mechanisms lodgement and melt-out and the role of sub-glacial deformation in the delivery and emplacement of sub-glacial tills. In a multi-scale approach, we employ both field methods (sedimentary, structural and clast fabric analyses) and laboratory techniques (micromorphology). We here present examples from contemporary glacial settings in SE-Iceland and a Loch Lomond Readvance setting in central Scotland.

In an attempt to explain the stacked till architecture in all of these examples, we propose a complex interplay of processes that accounts for sub-glacial debris undergoing cycles of release and (re)freezing onto the base of the glacier before it eventually becomes deposited as slabs of till. We envisage that the cyclic processes eventuate beneath an oscillating ice front, either seasonally or in phases of retreat and (re)advance.