An AFM and SEM study of the topography induced by 20 keV Si+, Ar+ and Xe+ ion bombardment of Si at 45 ° incidence angles and for ion fluences between 1017 and 1020 cm-2 has been undertaken at room temperature. All species generate an atomic scale random roughness, the magnitude of which does not increase extensively with ion fluence, suggesting the operation of a local relaxation process. This nanometre scale roughness forms, for Ar and Xe, a background for coarser micrometre scale structures such as pits, chevrons and waves. Apart from isolated etch pits Si+ irradiation generates no repetitive micrometre scale structures. Xe+ irradiation produces well developed transverse waves while Ar+ irradiation results in isolated chevron-like etch pit trains and ripple patches. This latter pattern evolves, with increasing ion fluence, to a corrugated facet structure. The reasons for the different behaviours are still not fully clarified.