**5. Debris input to glacial systems**

To the basic glacial parameters of Figure 1 weathered rock debris needs to be added to the system for there to be traces in the geological record. This complication is rarely considered; not just a morainic marker but to consider where, when and from where the debris addition was made. It may well have a considerable effect on the system as a whole. The total amount may be important. Dead ice preserved at the snout of a glacier long after the debrisfree glacier has melted may give hummocky moraine or even a rock glacier form. Further, the debris flux may have an effect upon the ice extent. For example, a glacier in equilibrium that receives a debris input near the snout (as from a large rockfall) could produce a glacier advance as the ablation area is reduced. The timing of debris input (at the start or end of a glacier advance phase for example) may be significant. Some work has been done on this in the British Isles, eg. (Ballantyne & Kirkbride, 1987). Although dating such events via cosmogenic ratio methods is now becoming easier (Ballantyne & Stone, 2004; Ballantyne et al., 1998) care must be taken in the temporal interpretation.

Figure 2 indicates the potential complexity here, again related to altitude, continentality and temporal input variations. Rock debris can be added to a glacial, permafrost or periglacial system. Unknowns include the relative and absolute amounts of ice/water and debris but also the flux changes in time (Nakawo et al., 2000; Whalley et al., 1996). Even 'simple' glacial systems may show this. For example a large rockslide on or near the snout of a glacier may allow the snout to advance but if away from the snout the glacier may be hardly affected. In the relatively small glacier in the British Isles however, substantial, but largely unknown, effects may be produced.

Figure 3 illustrates possible scenarios produced by the relative additions of weathered debris quantities to snow/ice bodies. This should not be taken to show that certain features *will* form but that they are possible given the relative components at any time. For example, the time element is not considered as part of the formative process. Some features might be form 'rapidly', others take some time. For the most part process studies do not give a good indication of the time needed to produce a feature. If the debris is lacking then there may be no feature formed at all. However, the diagram does suggest that there is a continuum of features and it does help guide interpretation of what is seen or mapped.

Using Discrete Debris Accumulations to Help Interpret

Upland Glaciation of the Younger Dryas in the British Isles 7

Fig. 3. A schema illustrating the relative proportions (and perhaps fluxes) of snow/ice and

Permafrost warming post Younger Dryas may also have had a significant part to play as has been suggested for present-day rockfall production and (Davies et al., 2003; Whalley et al., 1996) have shown that large debris accumulations are often associated with Little Ice Age events. It is not yet clear how substantial and variable was the production of debris in the Younger Dryas, although some attempts have been made (Ballantyne & Kirkbride, 1987). More recently, Jarman (2009) and Wilson (2009) have examined rockfalls and slope failures associated with Younger Dryas slope activity and the production of discrete debris accumulations. What does seem to be the case is that fossil rock glaciers and protalus lobes are relatively rare in the British Isles and Ireland compared with many mountainous regions. This may well be a consequence of the lack of weathering or rockfalls from the Caledonide rocks that comprise much of upland Britain (Harrison et al., 2008). It is perhaps not surprising

rock weathering debris in a 'glacial' geomorphic system. From Whalley (2009).

Fig. 2. An illustration of the weathered rock debris constituent needs to be taken into account when considering 'glacial, proglacial, perglacial or permafrost conditions. Not only may the debris addition be sudden (rock avalanche) or slow and continuous (scree formation); after Whalley (2009).

A further formational aspect not shown in Figure 3 are the possible altitudetemperature/precipitation-continentality controls (Figures 1 and 2). Thus, it is by no means clear where the 'best' analogues for the YD in the British Isles should be taken. For example, it was once thought that rock glaciers were only found in 'continental' mountains until examples from Iceland were found. The answer lay in the relative amounts of debris supplied to small glacier systems. Furthermore, Icelandic rock glaciers are found where there is no (or only sporadic) permafrost. Hence, the inverse interpretation; relict rock glacier = former permafrost, needs to be used carefully. This applies in fact to most of the features here classed as DDAs
