The vertical extent of ice sheets in Nordfjord, western Norway: measuring degree of rock surface weathering

Abstract

Degree of rock surface weathering was measured on sites in Oldedalen and Brigsdalen, where dates of deglaciation have been estimated. and on an altitudinal transect on the slopes of Skåla. representing one of the highest supra‐marine reliefs in western Norway. The Schmidt hammer is useful only for distinguishing sites deglaciated during the Little Ice Age from those deglaciated during the Lateglacial and early Holocene. Degree of roughness of granitic augen gneiss bedrock surfaces was quantified from profiles measured in situ using a micro‐roughness‐meter and profile gauge. There is a significant increase in surface roughness above a clear trimline at c. 1350 m a.s.I. but no significant increase above a higher trimline previously proposed as the vertical limit of the last ice sheet in this area (c. 1560 m a.s.I.). The roughness of boulder surfaces on the summit blockfield does not direr significantly from the roughness of bedrock surfaces downslope as far as the lower trimline. These unexpected results suggest that bedrock surfaces between the two trimlines were not glacially abraded during the Late Weichselian, so that the upper trimline is unlikely to represent the vertical limit of ice during either the Late Weichselian or a subsequent readvance. Preliminary results of 10Be dating of surface quartz samples from above the lower trimline support the proposal that the site was not abraded during the last glaciation. The results can be interpreted in two ways: (1) The upper trimline represents the vertical limit of a pre‐Late Weichselian advance. During the Late Weichselian the mountains were completely covered but surfaces down to the lower trimline were protected by cold‐based ice. (2) The lower trimline marks the vertical limit of the Late Weichselian ice and the upper limit an older and more extensive glaciation.

Thickness evolution of the Scandinavian Ice Sheet during the Late Weichselian in Nordfjord, western Norway: evidence from ice-flow modeling

“…; another trimline is evident at 1350 m a.s.l., and the lowest trimline, which has been interpreted as the Younger Dryas ice limit, is positioned at c. 1100 m a.s.l. (Nesje et al 1988;McCarroll & Nesje 1993). While a thin, cold-based ice sheet covering mountain peaks in the area and protecting the highest elevations from erosion cannot be ruled out by cosmogenic results alone (Brook et al 1996), comparison with our model results indicates that the highest trimline most likely represents the maximum ice thickness during the last glaciation.…”

“…However, new preconsolidation tests on samples from the same site (Skorgedalen, western Norway, at c. 63 N), using the constant strain rate method rather than incremental loading, indicate substantially lower minimum ice thickness close to the elevation of the trimline (Hildreth 2001). Based on roughness measurements at Skåla, in the Nordfjord area, which indicate the degree of rock surface weathering, McCarroll & Nesje (1993) concluded that the ice during the LGM either extended over all summits, but was cold-based and therefore did not erode the bedrock, or its maximum vertical limit was even lower than previously suggested (at c. 1350 m a.s.l. ).…”

Improved Schmidt-hammer exposure ages for active and relict pronival ramparts in southern Norway, and their palaeoenvironmental implications

“…on Finnan (cf. Nesje and Dahl, 1990;McCarroll and Nesje, 1993;Nesje et al, 1994;Brook et al, 1996;Goehring et al, 2008). Despite its westerly location, permafrost is likely to have extended into the bedrock at altitudes above and beneath relatively thin coldbased ice at this site at this time (cf.…”

Geomorphology and weathering characteristics of erratic boulder trains on Tierra del Fuego, southernmost South America: Implications for dating of glacial deposits

“…Rock surface roughness can be used to compare the effects of weathering where it is assumed that initial surface texture was roughly the same (McCarroll and Nesje, 1993Nesje, , 1996. This can then be used as an indicator of the relative time that a rock surface has been exposed to weathering processes.…”

“…Rebound values (R-values) produced by the impact of the hammer decline with increased rock-surface weathering, and so can be used as a relative measure of exposure history (McCarroll, 1991;McCarroll and Nesje, 1993;Nesje et al, 1994;Goudie, 2006;Shakesby et al, 2011). We used an N-type Schmidt hammer to analyse 50 boulders from two of the boulder trains thought to be from different glacial cycles (BI 1 and RC 1).…”