Abstract

Clast-fabric strength is commonly used as a quantitative tool to determine the depositional history of subglacial tills. This paper emphasizes the variance in fabric strength relative to clast size. Data on 46 clast-fabric analyses were sampled from two basal till localities in southeast Denmark and another eleven analyses from a basal till locality beyond a glacier in south Iceland. It is demonstrated that clasts longer than 2-3 cm show a stronger preference for parallel orientation than do shorter clasts. This implies that two operators sampling fabric data a few meters apart within the same till unit, but selecting clasts longer or shorter than 20 mm, may classify the till as two different types of basal till because of differences in clast size alone. Additionally, this study demonstrates considerable variations in fabric strength over short lateral distances within the same basal till bed. It is concluded that caution should be exercised in using eigenvalues to infer depositional processes and strain histories in subglacial environments; sedimentological criteria other than clast fabric must also be considered before drawing final conclusions on subglacial deposition.

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