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Norsk Geografisk Tidsskrift 31, A lichenometric test of the hypothesis: Storbreen gletschervorfeld, southern Norway. Glacier and climatic fluctuations inferred from tree-growth variations over the last years, central southern Norway. A comment on Neoglaciation in south Norway using lichenometric methods. Norsk Geografisk Tidsskrift 41, Snow-avalanche impact landforms in southern Norway: Arctic and Alpine Research 26, Timing of events The clustering of debris flow dates between and suggests that this was a period of general instability in which major slope movements were common, while dating estimates from to from the slopes above the road either indicate discrete rockfall events or re-mobilization and re-exposure of debris Fig.
Lichen evidence for debris flows between and mids is lacking, but if there was any, this could have been covered by V. Apart from a date of on the interfluve near the junction of the two most southerly streams, there is no lichen evidence for slope activity before and there could be a number of reasons for this. A limited lichenometric dating range using Rhi- zocarpon species, which in Nant Ffrancon could be less than years. However this limited range is unlikely since a date of , supported by archival evidence, was found for a Rhizocarpon specimen on a church roof in Cumbria Winchester, Land use changes, with the introduction of sheep over the last years and high deer stocks could have led to overgrazing and hence increased erosion.
However, there is little evidence for this, and apart from lichen disturbance introduced by erosion, a weakened vegetation cover could be beneficial to lichen propagation. Lichen growth has been excluded by revegetation of deposits. Fire incidents could also have discouraged growth and colonization, but if bracken was reduced by grazing then widespread damage to lichens, espe- cially those on larger boulders whose upper surfaces rise well above any bracken cover is unlikely.
How- ever, Rhizocarpon species can thrive in regions where there are less than 3. Large-scale slope activity beginning in the s could have obscured earlier lichenometric evidence for slope movements. This last option would seem the most likely here in view of the clustering of debris flow dates focussing on a period when the meteoro- logical records for North Wales show that there were numerous extreme rainfall events, with some partic- ularly intense episodes in October , , , and November Symonds, — ; Rumsby and Macklin, Although intense rainfall events in upland areas may be strictly local, some regional generalizations could be justified Merrett and Macklin, In the Tawe River catchment, South Wales, there were six severe and eight major flooding events linked to intense rainfall between and with two of the events occurring in September, three in October, five in November and four in December Walsh et al.
Recent rainfall events over the last two decades in North Wales, linked to debris flows in Nant Ffrancon, occurred in September , January , October and November , with the only notable event not conforming to this pattern being that of the debris flow in July Based on this evidence, it would seem reasonable to assume that events in the late nineteenth century that triggered debris flows on the Braich Ty Du farm slopes are likely to have taken place mostly in the autumn and winter months.
The chief problem for lichenometry is verification of growth rates and colonization periods.