Written by an internationally recognised expert in the field, this book represents an important step forward in the modern understanding of glacial process sedimentology. As such, Till: A Glacial Process Sedimentology is an indispensable resource for advanced undergraduates and researchers in sedimentology, glacier science and related areas. David J A Evans is a Professor in the Department of Geography at Durham University, Durham, UK. A leading expert in the field of glacial process sedimentology, his research on the landforms and sediments of modern and ancient glaciated basins has been the catalyst for the compilation of glacial landsystems models and the reconstruction of palaeo-glacier dynamics.
Glacigenic Diamictons - A Rationale for Study
The Glacial Drifts ... are known to us all but too well. We cannot escape them; their clays, their sands and gravels confront us at every turn, so masking the underlying rock that they are a positive curse to the 'solid' geologist.
Carruthers (1947-48, p. 43)
The process sedimentology of tills is crucial to the understanding of the glacier ice-bed interface as a complex depositional, erosional and shear boundary layer. Consequently, it also plays a central role in deciphering the genesis of enigmatic subglacial bedforms such as drumlins, flutings and ribbed terrain. Yet, unlike the study of other boundary layers such as those that operate at the bed of fluvial, aeolian and deep water systems, our knowledge of subglacial process-form relationships is relatively impoverished, largely due to the inaccessibility of glacier and ice sheet beds. Notwithstanding the important contributions now being made to this research problem by remotely sensed and localised borehole observations as well as reductionist laboratory experiments, it is critical that glacial scientists continue to refine their interpretations of ancient archives of subglacial processes, specifically those that are represented by tills and associated deposits, as these archives form the most widespread and accessible record of processes at the ice-bed interface ( Figure 1.1 ).
Figure 1.1 Flow diagram to illustrate the inter-relationships between the main glaciological and sedimentological processes associated with the subglacial environment (modified from Menzies and Shilts (1996) to acknowledge glacitectonic processes as deformational rather than depositional).
Such an inductive approach to the reconstruction of former subglacial processes has some considerable shortcomings, largely because it relies on actualist principles that are in turn based on process-form relationships that we cannot as yet unequivocally validate. This often has been compounded by the glacial geomorphology literature, wherein the traditional, uncritical acceptance of thick sequences of diamicton as 'lodgement tills' has assumed a definitive knowledge of process-form relationships even though that knowledge base is far from definitive. This has been exposed more recently in the apparent incompatibility between modern process measurements (indicating thin subglacial deformation/till construction) and ancient glacigenic sequences interpreted to contain often very thick subglacial tills. Moreover, the existence of ambiguous diagnostic criteria for identifying processes of subglacial sedimentation in ancient diamictons does not inspire confidence in the glacial research community when turning to till sedimentology for some guidance!
What we can now be confident in espousing are the concepts of debris entrainment and transport pathways together with concomitant clast modification within the glacial debris cascade ( Figure 1.2 ). Till sedimentology should reflect the nature of the debris cascade, or more specifically: (1) the entrainment and transport history; (2) the continuum of clast modification during various phases or repeat cycles of transport and deposition; (3) the debris release processes and (4) any secondary displacement processes such as deformation. Glacial systems are complex in that these three aspects of the debris cascade are juxtaposed in a contemporaneous process-form regime. The result is a temporal and spatial mosaic of process operation that can be shut down at any stage of development once a landscape undergoes deglaciation. This temporal and spatial mosaic is a recurring theme throughout this book, because it allows us to visualise the till forming environment more appropriately as a process-form hierarchy more akin to the principles of sequence stratigraphy as they are applied to other geomorphological syst