Structures and textures in till indicating subglacial deposition

Five structural and textural features are discussed: (1) small lenses of sorted material, (2) smudges, (3) small‐scale deformations of till matrix and smudges by clasts, (4) clasts consistently striated, and (5) clasts with stoss‐and‐lee sides. Analyses suggest that these features may be produced by subglacial processes acting in the ice‐bed interface. Long axes of small sand lenses and smudges as well as the striation on the upper surface of scattered clasts in lodgement till have a strong preferred orientation in good agreement with the glacier flow direction as indicated by clast fabrics, bedrock striation, and surface fluting of ground moraine. When in traction against the till bed, clasts may plough up till banks. Clasts with stoss‐and‐lee sides development were also very distinctly oriented as their stoss sides faced significantly up‐glacier. It is concluded (1) that each of the five features discussed is useful as a criterion for subglacial deposition by lodgement, (2) that they indicate important differential movement along the ice‐bed interface and therefore suggest a temperate regime in this part of the glacier during the till deposition, (3) that very few orientation measurements of one or more of these features signify the ice movement direction; i.e. a time‐saving method to find the paleoflow direction of Pleistocene glaciers, and (4) that taken together with till preconsolidation, mechanical composition, and clast fabric, they may support each other and give good indications of the genesis of Pleistocene tills.