Modification of Clasts in Lodgement Tills by Glacial Erosion

• Published in: Journal of Glaciology
• Main Author: Sharp, Martin
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 1982
• Subjects: Iceland; Journal of Glaciology
• Online Access: http://dx.doi.org/10.1017/s0022143000005062; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022143000005062

Abstract 7.8% of 1 105 clasts with a B-axis diameter of >32 mm in lodgement tills from Skálafellsjökull, south-east Iceland, had a stoss-and-lee form. Clasts deeply embedded in the tills were more likely to have such a form than those resting lightly on the surface. Deeply embedded basalt and andesite clasts were also more rounded than lightly embedded clasts of similar lithology, and more striated and more likely to have a stoss-and-lee form than deeply embedded gabbro and granophyre clasts. Lightly embedded basalt and andesite clasts were more striated than lightly embedded gabbro and granophyre clasts. Within each lithological class, larger clasts were more likely to have a stoss-and-lee form. It is suggested that deeply embedded boulders acquire their rounded stoss-and-lee form by abrasion and fracture by over-riding debris-laden ice. This occurs after they have become firmly lodged by ploughing into the subglacial till bed, and before they become buried by lodgement of debris around them. The degree of modification depends in part upon clast size and lithology. The long-axis orientation of deeply embedded boulders is a good indicator of former ice-flow directions.