  A simple (2+1) dimensional discrete model is introduced to study the
evolution of solid surface morphologies during ion-beam sputtering. The model
is based on the same assumptions about the erosion process as the existing
analytic theories. Due to its simple structure, simulations of the model can be
performed on time scales, where effects beyond the linearized theory become
important. Whereas for short times we observe the formation of ripple
structures in accordance with the linearized theory, we find a rough surface
with scaling properties for intermediate times. The long time behavior of the
model strongly depends on the surface relaxation mechanism.
