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Systematic fissuring of a matrix-dominated lodgement till at Church Wilne, Derbyshire, England
Published online by Cambridge University Press: 01 May 2009
Summary
A set of sub-vertical curving fissures in a Pennine-derived lodgement till of Wolstonian age is described from Church Wilne near Derby. The fissures are at least 5 m long and curved (radius of curvature c. 3 m) in both vertical and horizontal planes, the concavity facing SE in the direction of regional ice movement. The origin of the structures is considered in the light of a review of modes of formation of structural discontinuities in till. Origins due to stress relief following excavation, desiccation, frost action, direct ice push, ice-core meltout, compaction of the substrate and shear failure are rejected. It is proposed that the fissures were produced by point loading of a frozen till surface by englacial (basal) boulders which provided periodically high compressive stresses and induced tensional failure. Analogous fissures occur in a Devensian lodgement till in the Wirral peninsula in Cheshire. It is concluded that curved fissure sets are localized features of lodgement tills but that they may be quite widely distributed.
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