THE INTERPRETATION AND CLASSIFICATION OF SUBGLACIALLY-DEFORMED MATERIALS

A general classification of subglacially-deformed materials is propose d, based on sed iment properties and their relationship to styles of s ubglacial strain. Deformation till is defined as homogenized, usually diamictic material formed by glacially-induced shear of subsole materi als. Three types are recognized: Type A, formed by pervasive, ductile deformation; type B, formed by brittle shear; and comminution till, pr oduced by the reduction of void space by in situ crushing and abrasion . The term glacitectonite is adopted for materials that have undergone subglacial shear but retain some of the structural characteristics of the parent material. Original structures may be truncated by glacitec tonic fabric elements (Type A) or distorted but not truncated (Type B) . The principles of strain and material response, particle orientation mechanisms, fabric development and drainage conditions are reviewed. Examples of deformation tills and glacitectonites from Breidamerkurjok ull, Iceland; Slettmarkbreen, Norway; Loch Lomond, Scotland; and East Yorkshire, England, are used to illustrate the nature of subglacially- deformed materials and their relationship to former subglacial strain and drainage conditions. The geologic evidence suggests a continuum of bed strengths can be recognized in deforming substrata, ranging from high strength, low-strain Type B deformation till and comminution till to low strength, high strain Type A deformation (C) 1996 Elsevier Sci ence Ltd