J. Rose

Till fabric patterns and significance: particle response to subglacial stress

Abstract The analysis of particle arrangements within glacial sediments (fabric) has long been considered to reflect the behaviour of the ice that transported and deposited, or deformed the sediment. Particles can develop specific fabric arrangements as a response to bulk sediment strain driven by effective stress at the bed of a glacier. As such, fabric analysis has been used to try and understand the complex relationship between glacier and ice sheet dynamics and the geological evidence of former glaciation. Developments have focused on the use of stereonet patterns and statistical measures (vector and eigenvector analysis) of clast fabric to infer the process of sediment deposition/deformation, and consequent ice flow directions and dynamics. However, as this study identifies, fundamental processes of particle orientation (March, Jeffery and Taylor rotation mechanisms) would suggest more complex explanations than have been previously reported. This study reports the results of a detailed field and laboratory investigation of macro-scale (clast) and microfabrics from a till sequence in Central Scotland. The results suggest a systematic behaviour of different particle size fractions, which may be interpreted in terms of the strain response of sediment. Three fabric patterns are identified that illustrate consistent, progressively shifting and more complex particle behaviour. It is suggested these orientation patterns indicate the degree to which subglacial sediment has undergone strain, and the nature of the deforming bed (erosional or accretionary). It is concluded that this approach may yield fundamental information about the strain conditions underlying different ice masses, providing critical data linking glaciological modelling with the sedimentary evidence of glaciation.

Palaeoenvironmental and stratigraphic significance of the Valley Farm and Barham Soils in Eastern England

Abstract The Valley Farm and Barham Soils are important stratigraphic units representing an Early and/or Middle Pleistocene depositional hiatus within parts of Eastern England. The Barham Soil, buried beneath Anglian sediments, is recognised by a variety of large- and small-scale cryogenic features which developed in Cromerian sediments or were superimposed upon the Valley Farm Soil during the early Anglian. The Valley Farm Soil is rubified, mottled and contains substantial quantities of translocated clay. Originally attributed solely to temperate (Cromerian stage) pedogenesis, its stratigraphic status has changed over recent years as the age range of its Kesgrave Sands and Gravels parent material has been extended. Currently, the Valley Farm Soil is regarded as a complex stratigraphic unit with component soils developed on a series of different age surfaces, perhaps spanning cold and temperate intervals over more than a million years. Detailed micromorphological studies of the Valley Farm Soil on some of these surfaces have confirmed its variable pedological complexity. A number of theoretical and practical problems, however, need to be confronted and solved before its pedological record can be used routinely to provide an independent means of dating and reconstructing detailed palaeoenvironments.

15 – Glacial stratigraphy

Publisher Summary This chapter provides an overview of glacial stratigraphy. Stratigraphy is the discipline to understand the history of how, when, and why glacial deposits occur. Glacial stratigraphy is closely related to Quaternary stratigraphy, which is concerned with the history of the whole environment over the past 2.6 Ma, even though glaciations have occurred on many other occasions in Earth history. Much of glacial stratigraphy has a solid scientific foundation that is based on an extended and more powerful version of the International Stratigraphic Code. However, the complex nature of the glacial system means that interpretation is often far more complex than that of other depositional systems that dominate the premiddle Quaternary parts of the geological column. In these circumstances, it is no wonder that in some instances, stratigraphic frameworks that were constructed some decades ago now appear naive. The chapter reviews the principles behind glacial stratigraphy and outlines the appropriate stratigraphic methods available. Emphasis is given on the methods, associated complexities, and stratigraphic problems.