In a glacier, the contact pressure between a clast in basal transport and the underlying bed: the greater the pressure, the more abrasion will occur. Boulton (in D. R. Coates, ed. 1974) assumes that the pressure between a particle in contact with the glacier bed is related to the effective normal pressure, while Hallet (1981 Annals. Glaciol 2, 3) argues that clasts are completely surrounded by the ice and can be considered to be essentially floating within it. In this case the contact pressure between a clast and the glacier bed is a function of the rate at which ice flows towards the bed; see M. Bennett and N. F. Glasser (2009) p. 97.