Abstract
In glacial science, the origin(s) of that deceptively simple sounding sediment referred to as till is ‘one of those’ research topics. You know the type: the sort that elicits ‘vigorous’ debate, keeping the protagonists warm on a chilly, wet day in the field and providing entertainment for the more casual observer. Possibly, the main reason for this is the fact that these deposits provide us with valuable insights into the processes occurring beneath glaciers and ice sheets, a complex, hostile environment which is extremely difficult to observe firsthand in contemporary glacial settings. However, these enigmatic, typically massive-looking (in the field) sediments do not reveal their secrets easily. So it is a brave man who steps forward to write the book Till: A Glacial Process Sedimentology. That man is David JA Evans, an internationally recognised expert in the field of glacial process sedimentology. At this point, I must confess to being a ‘till convert’ and to having been one of Dave’s collaborators on the subject of tills for a number of years now – although it is fair to say we don’t always agree, or understand what it all means.
Till: A Glacial Process Sedimentology is aimed at academics, postgraduate students and advanced undergraduates who are just starting their research careers in this field or already have an established interest in subglacial processes. So this book is not for the amateur Earth scientist, but for the more dedicated researcher; it provides a comprehensive review of the published literature on ‘till science’, as well as drawing on the author’s own research and photograph archive. Furthermore, those new to this field of study will no doubt find the extensive reference list (over 40 pages) to be a valuable source of information and further reading.
The book is divided into a total of 17 chapters. The opening sections explain why the study of tills is important with respect to our understanding of the processes occurring at that critical interface between the ice and the underlying glacier bed (Chapter 1). This is followed by a brief history of research highlighting the key players in this field (Chapter 2) and finishes with a discussion of when (and when not) to use the ‘till’ word (Chapter 3), effectively asking the question – ‘What is a till?’ These early chapters provide the reader with an ‘appetiser’, which is followed by a well-illustrated, in-depth ‘main course’ that critically reviews the field, experimental and laboratory-based evidence, as well as the conceptual models that have evolved to explain subglacial till formation and deformation, and their implications for glacier motion.
Despite a long history of investigation (over a century), scientists have yet to reach a consensus on the criteria for identifying/recognising the processes involved in till genesis in the geological record. Consequently, Chapter 4 builds upon the earlier sections to establish a nomenclature and methodology for the description and analysis of tills in both the field and laboratory. This is followed by an examination of the sedimentary processes leading to the formation and final grain size of the till matrix (Chapter 5). Chapter 6 continues this ‘processes’ theme with a detailed review of the range of subglacial processes observed in modern glacial environments which are believed to have a significant impact on till formation, including clast lodgement, lee-side cavity fill, ploughing, sediment deformation, soft-bed sliding, meltwater drainage and ice–bed coupling, melt-out and glacitectonite production.
Chapters 6 and 7 deal with laboratory and numerical modelling experiments of till evolution and the techniques employed in measuring the strain signatures recorded by glacial deposits, in particular the use of clast macrofabrics. Before the sedimentary process theme is then revisited in Chapters 9–11 with an in-depth discussion of the evidence for clast lodgement, clast pavements, deforming bed deposits and sliding bed deposits found within the geological record. Chapter 10 also includes a section illustrating how microstructural analysis is being increasingly used to shed light on till deformation processes. Chapter 12 assesses the impacts of pressurised water and the formation of clastic dykes in glacial environments. The scale of these naturally occurring hydrofracture systems varies from just a few millimetres up to several metres across, with these sediment-filled water-escape features being traced laterally for several tens of metres. At the microscale, it can be seen that the sediment filling these features can record a complex history of sedimentation, recording repeated phases of pressurised fluid flow. The issue of the preservation and recognition of subglacial melt-out tills formed by the release of sediment from melting, debris-rich basal ice is dealt with in Chapter 13. The following chapter (Chapter 14) examines the impacts of subglacial deformation (shearing) on pre-existing sediments and bedrock leading to the formation of glacitectonite, with field examples from as far afield as southern and western Ireland, Scotland, eastern England, northern Germany, Denmark and southern Alberta, Canada.
Chapter 15 reviews the literature on glacial diamictons which are unrelated to subglacial processes and covers a range of deposits formed by the release of sediment from melting glacier ice. As the author mentions in the opening paragraphs to this chapter, any visit to the margin of a modern receding glacier will leave you with a clear impression that mass flows are the most common diamicton-forming process, as well as the feeling that forefield of many glaciers resembles a badly bulldozed building site. So it is a puzzle why there appears to be more confidence in identifying subglacial tills in the geological record, rather than these gravity-driven slope deposits. The nature of glacigenic deposits, in particular the vertical and lateral variation in tills, in the geological record is the focus of Chapter 16. This includes a review of the models for till emplacement and the evidence for development of a mosaic of ‘slippery’ and ‘sticky’ spots within the beds of both modern and ancient glaciers and ice sheets. The final chapter (Chapter 17) is a short one and is reserved for a few concluding remarks as well as presenting the need for an updated, simplified till nomenclature based upon the knowledge gained from the field- and laboratory-based studies of glacial diamictons presented earlier in the book.
So in summary, Till: A Glacial Process Sedimentology is aimed at advanced undergraduates, postgraduate students and established researchers who have an interest in the complex processes occurring beneath both modern and ancient glaciers and ice sheets. It is a thought-provoking, well-illustrated book which is written by an internationally recognised expert in this field. The more ‘seasoned’ till researchers may not agree with all of the interpretations presented in this book. But in my opinion everyone who reads it will find it to be an indispensable resource which can be repeatedly ‘dipped into’, provoking new ideas, speculations and research approaches which can be used to tackle the outstanding issues regarding till genesis.