An extensive study of the ripple formation on Si is presented. The ripples are characterized with atomic force microscopy (AFM) as a function of sputter depth and also the effect of the introduction of oxygen gas near the sample is investigated. Based on these results, a two-step model is proposed for the formation of the ripples on Si. The first step relates to the formation of small topography (seeds) caused by the heterogeneity of the internal layer. The second step relates to the rapid development of these seeds into the regular ripple structure. The driving force for the latter is the surface oxidation of the different faces of the ripples. The validity of the separation in two steps is additionally checked by studying the effect of deliberately roughened samples. The chemically induced topography with a height of no more than 10 nm is shown to move forward the transient region quite severely suggesting that the origin of the roughness does not have any influence on the subsequent ripple formation. Interestingly enough, the AFM pictures of the ripple development reveal that the random distribution of the original topography gradually evolves into a structure aligned in a direction normal to the incident beam. The shape of the final structure is not linked with the original topography indicating that it is dictated by ion beam induced effects. The importance of the bulk heterogeneity compared with the surface oxidation is explored using argon bombardment in combination with 02 flooding. As expected on the basis of the proposed model, no ripples were formed when bombarding an unetched sample but large ripples were generated when starting from the rough samples.