Determining glacier flow direction from till fabrics

Field and experimental data indicate that fabrics of sheared basal tills, based on particle orientations and on anisotropy of magnetic susceptibility, are inclined relative to the macroscopic plane of shear. As a result, if this plane is tilted with a transverse component, one of the longest-standing axioms of glacial geology-that particles in basal till are preferentially oriented parallel to the direction of shear-is not strictly true. The resultant correction to the shearing direction increases with the transverse and longitudinal components of shear-plane inclination and with the magnitude of the fabric inclination relative to the shear plane. Azimuthal corrections calculated herein range from being negligible to a few tens of degrees for reasonable ranges of shear-plane tilt. The past success of fabric azimuths as indicators of flow direction, despite the absence of this correction, likely reflects the clustering of shear planes about the horizontal in basal tills. However, on the flanks of accreting subglacial bedforms, shear-plane attitudes may well conform to the local bedform slope. As illustrated with hypothetical fabrics of drumlins and flutes, azimuthal corrections can in that case be sufficiently large and systematic to seriously affect interpretations of bedform genesis.