**6. Rock debris production in upland British Isles**

As the ideas shown in Figures 2 and 3 depend upon debris input some consideration will now be given to the production of rock debris. At the present day, however, there is very little active rock fall production. There are some active scree slopes (talus), as shown by the lack of vegetation, but these are relatively uncommon.

Rockfalls (of indeterminate size) may be associated with periods of cliff instability related to a number of possible factors. These include glacial unloading and seismic, neotectonic, tremors associated with isostatic readjustment (e.g. Jarman, 2006) large-scale weakening of rock buttresses caused by intense periglacial weathering; permafrost melting or a combination of these factors. Some characteristics of these rock glaciers includes: location within mapped Younger Dryas glacier limits, high and steeply-angled cliffs upslope of the landform and largely unvegetated surfaces. A review of large 'felssturz' (rockfall events) in extra-glacial areas of Austria by (Meissl, 1998) shows how significant such events may be even today. There is compelling evidence (Whalley, 1984; Whalley et al., 1983) that nearglacial conditions were, and are, important in the development of rockfalls and slides.

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

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

As the ideas shown in Figures 2 and 3 depend upon debris input some consideration will now be given to the production of rock debris. At the present day, however, there is very little active rock fall production. There are some active scree slopes (talus), as shown by the

Rockfalls (of indeterminate size) may be associated with periods of cliff instability related to a number of possible factors. These include glacial unloading and seismic, neotectonic, tremors associated with isostatic readjustment (e.g. Jarman, 2006) large-scale weakening of rock buttresses caused by intense periglacial weathering; permafrost melting or a combination of these factors. Some characteristics of these rock glaciers includes: location within mapped Younger Dryas glacier limits, high and steeply-angled cliffs upslope of the landform and largely unvegetated surfaces. A review of large 'felssturz' (rockfall events) in extra-glacial areas of Austria by (Meissl, 1998) shows how significant such events may be even today. There is compelling evidence (Whalley, 1984; Whalley et al., 1983) that nearglacial conditions were, and are, important in the development of rockfalls and slides.

formation); after Whalley (2009).

features here classed as DDAs

**6. Rock debris production in upland British Isles** 

lack of vegetation, but these are relatively uncommon.

Fig. 3. A schema illustrating the relative proportions (and perhaps fluxes) of snow/ice and rock weathering debris in a 'glacial' geomorphic system. From Whalley (2009).

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

Using Discrete Debris Accumulations to Help Interpret

**8. Interpreting Discrete Debris Accumulations** 

glaciers (Hamilton & Whalley, 1995; Martin & Whalley, 1987).

Blockfield \* If autochthonous (*in situ*):

Hummocky moraine\*

Østrem-type moraine

ice sheets? ii How old is it?

push moraines

Upland Glaciation of the Younger Dryas in the British Isles 9

Table 1 (after Whalley, 2009) lists the main features likely to be seen as Younger Dryas Discrete Debris Accumulations in the uplands of the British Isles. This must, at present, be taken as a rather rough typology. It has not proved possible to provide a key system to help identify features. There are three reasons for this. First, the features themselves are somewhat variable in form and location on a hillside. Secondly, the debris input location and type needs to be taken into account (following from Figure 3). Thirdly, the interpretation itself may change. Thus, some of the following photographs show variations in form. The diverse papers about the origin of the Beinn Alligin 'rockslide' exemplifies both the second and third reasons starting with the original description (Sissons, 1975; Whalley, 1976) and with further detailed interpretations (Ballantyne, 1987; Ballantyne & Stone, 2004; Gordon, 1993). A clear example of a change in opinion is that by Wilson, already mentioned, in revising his formation model of some rock glaciers in northern Ireland to be massive rockslides. This view then casts doubt on the interpretation of the rock glacier (in the same geology) on Islay (Dawson, 1977). This also illustrates a further difficulty, that of terminology, a problem that has long bedevilled this area of research, especially that of rock

**Feature name Comments on formation etc Environmental interpretation use** 

Landslide Any YD or post-glacial event Ice probably not involved but the

i Was it deformed by over-riding

Passive formation (ablation) In some cases might be related to

Originally, frontal debris deposition over 'old' snowbank;

Possible confusion with:

iv Hummocky moraine

ii Rock glacier (glacier ice or

glacier/snowbank ice + debris

i Push moraine

permafrost) iii Protalus lobe

Protalus lobe i Involvement with

**or caution** 

exposure data.

here.

might be helpful.

If undeformed or not removed how is this interpreted? Possible cosmogenic ratio

Use of tors related to blockfield

Various interpretations, related to moraines, debris transport

location, ice deformation; possible link to Østrem-type moraine

resultant landform may look like one or other of the features listed

Relict feature difficult to interpret due to lack of ice and (as far as known) a significant relict feature. May look like a rock glacier which then provides possible interpretation problems. To date, these have not been attributed to any feature in the British Isles.

Glacial, nival or permafrost maintenance, length of time of

that Norway, similar in a geology of hard old rocks, also seems deficient in rock glaciers and protalus lobes. Unsurprisingly however, present-day scree formation in Norway does seem more active than in Britain because of more severe weathering conditions.
