Lichenometric dating of slope movements, Nant Ffrancon, North Wales

Over the last 50 years, frequent debris flows on the oversteepened slopes crossed by the main A5 road in Nant Ffrancon, North Wales, have presented a significant traffic hazard. This study uses two approaches to lichenometry to date earlier debris flows. The first approach provides dating estimates based on size/age correlations of the lichen Rhizocarpon geographicum subspecies prospectans growing in two local churchyards. The correlations supplied a growth rate of 1.47 mm year−1 for the species over a 140-year period, with a delay before colonization of 18 years. Remeasurement of the same lichens after a 4.25-year interval confirmed the growth and colonization rates in the churchyards. The second lichenometric approach, based on population size frequency distributions with size increments dated using the churchyard growth rate, provided controls for the Nant Ffrancon study site, with peaks in Rhizocarpon population size frequency distributions correlating with debris flow dates recorded by Gwynedd Council Highways Department. Circumstantial evidence supporting adoption of the growth rate on the Nant Ffrancon slopes was also supplied by data from archival and meteorological sources. The lichenometric dating estimates suggest that these slopes have a history of periodic debris flows covering at least the last 110 years, with major flows occurring in the 1890s followed by further flows in the early decades of the twentieth century.