[en] A geometrical theory is presented which divides the collision cascade, created by ion bombardment, into energy groups, each with a characteristic anisotropy in the distribution in space of knock-on trajectories. An analytical solution is developed for the simplest non-trivial case: a high energy group with strong anisotropy and an isotropic low energy group. This model is particularly appropriate for high energy bombardment. The solutions, in the form of the variation of sputtering yield and interface mixing with angle of ion incidence demonstrate that experimentally observable features can be predicted by consideration of the effects of anisotropy within the collision cascade. (author)