Clasts with Stoss-Lee Form in Lodgement Tills: A Discussion

• Published in: Journal of Glaciology
• Main Author: Krüger, Johannes
• Format: Article in Journal/Newspaper
• Language: English
• Published: Cambridge University Press (CUP) 1984
• Subjects: glacier; Iceland; Journal of Glaciology; Mýrdalsjökull
• Online Access: http://dx.doi.org/10.1017/s0022143000006006; https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022143000006006

Abstract Clasts modified by glacial erosion are described from lodgement tills in front of the glacier Mýrdalsjökull, south Iceland. Many clasts show modification of their lower surfaces in the same way as their upper ones. However, the lower surfaces have a smoothed down-glacier face and a truncated up-glacier face, which is the opposite orientation to that of the upper surfaces. This so-called double stoss-lee form is interpreted as a response to basal transport over abrading materials, following deposition of the clast and succeeded by glacial erosion. It is suggested that clasts with a double stoss-lee form are a diagnostic criterion for subglacial deposition by lodgement. Furthermore, the distribution and orientation of clasts with a stoss-lee form was investigated on a ground-moraine surface. 17.3% of 2199 clasts with an a -axis diameter > 30 cm had a stoss-lee form. The proportion of clasts with their smoothed ends facing up-glacier within ± 22.5° of the ice-flow direction was 72.7%. Thus, the preferred stoss-side orientation is closely related to the ice movement and indicates the direction from which the ice came. Only 11.7% of boulders with a divergent stoss-side orientation are located in connection with annual moraines. It is suggested that such boulders have mainly been re-deposited beneath the ice and not at the ice front by minor advances of the glacier.