The Comninou–Dundurs effect (C-D effect) has yet to be considered in slip processes at the base of ice sheets and glaciers. This effect exists whenever slip occurs between two solids of differing mechanical properties and at least one of the solids is compressible. It gives rise to an additional normal traction stress across a non-uniformly slipping interface, a stress which otherwise would not exist if the two solids had identical properties. the C-D effect should exist to some degree, therefore, wherever ice slips over glacial till. It will be enhanced where large slip gradients exist, such as at boundaries of subglacial lakes. In this paper it is shown that, because of the C-D effect, subglacial lakes that lie at a basal till/ice interface may migrate in the direction of increasing ice thickness.