**Using Discrete Debris Accumulations to Help Interpret Upland Glaciation of the Younger Dryas in the British Isles**

W. Brian Whalley *Department of Geography, University of Sheffield, Sheffield, United Kingdom* 

#### **1. Introduction**

With no glaciers in the British Isles in the last 10, 000 years or so, the acceptance of the 'Glacial theory' propounded by Agassiz and Charpentier in the Alps in the 1830-40s was late to be accepted in the British Isles (Chorley, et al., 1973). Scientists from the British Isles had travelled to areas with glaciers, yet even popular tourist areas such as the English Lake District area were not considered to have been affected by glaciers until the 1870s (Oldroyd, 2002). In Scotland however, the ideas of James Croll, giving a theoretical reason for changes in climate, were persuasive in an earlier acceptance of glacial interpretations (Oldroyd, 2002). Field mapping by the Geological Survey in Edinburgh helped to displace the 'floating iceberg' and 'diluvial' theories, especially in the explanation of erratics (Rudwic*k*, 2008). Once accepted, the basic tools of mapping the former extent of glaciers, for example by the recognition of moraines, became commonplace. More recently, aerial and satellite imagery have made drumlins and cross-cutting depositional modes important in elucidating the limits of the terrestrial Pleistocene glacial record (Clark et al*.*, 2004). However, and perhaps inevitably, interpretations of the significance of some moraines and their corresponding glaciers has been in debate. Together with chronological methods, detailed mapping of glacial limits and altitudes allow comparisons with climatological models and general climatic interpretations such as the recent interpretations of the Younger Dryas in Scotland summarised by Golledge (2010).

In this paper I take an overview of the problems associated with a variety of features, other than moraines, associated with mapping the glacial limits (and associated climatic conditions) in the Younger Dryas Stadial in the British Isles. It does not aim to be comprehensive in treatment of these features in the British Isles but is concerned with the problems of mapping and interpreting a variety of features. Recognising the genesis is important as it may help to provide evidence for the magnitude-frequency of selected events as well as help to distinguish between a variety of events that may produce similar-looking landforms. Furthermore, as some features seen and mapped may be post-glacial slope failures rather than glacial deposits, their identification and correct interpretation may be useful for mapping slope failures in an area rather than glacial features. First however, it is necessary to identify some terms and meanings that will be used or have been used in mapping the Late Holocene in the uplands of the British Isles.

Using Discrete Debris Accumulations to Help Interpret

**3. Discrete Debris Accumulations and terminology** 

between periglacial, proglacial, paraglacial and permafrost in these studies.

2007; 2009)? This problem will be considered in more detail below.

Slaymaker (2009).

follow below.

(Bakke et al., 2005; 2008).

Upland Glaciation of the Younger Dryas in the British Isles 3

glaciated landscapes adjust to nonglacial conditions during and after deglaciation'. However, in many cases this does not help with the interpretation as it may not be at all clear what was glacier or snow or permafrost-related. Further discussion can be found in

If there are problems in interpreting the significance of moraines this is also true of landscape features where the ice-debris mix is of less certain origin and formative process unclear. For example, the ice-cored moraines investigated by Østrem in Scandinavia (Østrem, 1964) were interpreted by Barsch (1971) as being 'rock glaciers'. This dispute (Østrem, 1971) is still not resolved. There are several reasons for this uncertainty; problems of observation as well as nomenclature and understanding of the geological processes and mechanisms involved and their rates of operation. This is despite advances of glacial theory, sedimentology and dating techniques. Additionally, researchers coming from diverse backgrounds have tended to have different, often divergent, views about the processes operating and therefore the interpretations. Further discussion on this will

The term Discrete Debris Accumulations (DDA) has been used to encompass a range of features that can be mapped in the field or from aerial photographs (Whalley, 2009). It is used here as a non-genetic and descriptive term, such that focus can be given to whatever is under study without any preconceived notion of origin or significance. The actual interpretation of these features is, in the British Isles, very much related to the use of analogues. This is especially significant when the presence of ice (of some origin) is considered and so the recognition of ice-debris features and their mechanisms is considered. Although DDAs *may* be paraglacial it could be that some are fossil glacial features and not at all modified by post YD activity. Hence, there needs to be some care in distinguishing

This paper is specifically concerned with recognising the process and mechanisms of debris accumulations rather than dating *per se*. In particular, the association of specific features can be associated with climatic conditions. For example, moraines are associated with glaciers and the size (mass balance) of the glaciers. In the British Isles there is an assumed west-east gradient in glacier net balance, such has been found in Scandinavia (Chorlton & Lister, 1971). There are also possibilities of changes in prevailing winter storm tracks that may influence the size of glaciers (Whalley, 2004) that have not yet been investigated for the palaeo-conditions for the British Isles compared with suggestions for northern Scandinavia

The traditional view of the relationship between glacier extent, mass balance and glacial record is the linear set of boxes in Figure 1. The 'geological record' is usually taken as being manifest in the simplest (or least complicated) debris accumulation associated with glaciers; a moraine. Although the basic idea may hold, interpretation is more complicated for periglacial features such as snowbanks and their rock debris remnants such as 'protalus lobes'. Is such a feature classed as periglacial, proglacial or indeed paraglacial (Slaymaker,
