Schmidt-hammer exposure-age dating (SHD) of snow-avalanche impact ramparts in southern Norway: approaches, results and implications for landform age, dynamics and development

Schmidt-hammer exposure-age dating (SHD) was applied to the problem of dating the diachronous surfaces of five distal river-bank boulder ramparts deposited by snow avalanches plunging into the JostedOla and SprongdOla rivers in the Jostedalsbreen region of southern Norway. Approaches to local high-precision linear age calibration, which controlled in different ways for boulder roundness, were developed. The mean age (SHDmean) and the maximum age (SHDmax) of surface boulders were estimated for whole ramparts, crests and distal fringes. Interpretation was further assisted by reference to R-value distributions. SHDmean ages (with 95% confidence intervals) ranged from 520 +/- 270years to 5375 +/- 965years, whereas SHDmax ages (expected to be exceeded by <5% of surface boulders) ranged from 675 to 9065years. SHD ages from the JostedOla ramparts tended to be older than those associated with the SprongdOla, rampart crests were younger than the respective distal fringes, and use of relatively rounded boulders yielded more consistent SHD ages than angular boulders. The SHDmean ages indicate differences in recent levels of snow-avalanche activity between ramparts and provide insights into rampart dynamics as boulders are deposited on rampart crests and, in smaller numbers, on the distal fringes. SHDmax ages provide minimum age estimates of rampart age (i.e. the time elapsed since the ramparts began to form) and suggest that at least some of the ramparts have been developing since the early Holocene. Copyright (c) 2015 John Wiley & Sons, Ltd.