PARAGLACIAL DEBRIS-CONE FORMATION ON RECENTLY DEGLACIATED TERRAIN, WESTERN NORWAY

Five debris cones have formed on terrain deglaciated since AD 1750-75 in Bergsetdalen, western Norway. Three were surveyed and studied in detail. Two accumulated over 108-215 years and the third over 57-108 years. Accumulation on all three appears to have effectively ceased around or slightly before AD 1965 through exhaustion of readily entrainable sediment in source areas upslope. The volumes of the three cones range from c. 9500 m(3) to 403 600 m(3), and imply average sediment accumulation rates of 8-44 mm yr(-1); sediment erosion at one site was calculated to be 37-93 mm yr(-1) averaged over the entire sediment source area. Paraglacial reworking of glacial drift by debris flows appears to have been the main agent of cone development at two sites, but snow avalanches are thought to represent the main process of sediment transport at the largest cone. These findings indicate that substantial debris cones may form and become inactive within 100-200 years of deglaciation, implying that many late Pleistocene paraglacial debris cones may be virtually indistinguishable in age from neighbouring moraines and drift deposits.