The formation of triangular-shaped adatom islands and vacancy islands on Ag(111) at room temperature is observed in a local growth experiment. The tip of a scanning tunneling microscope (STM) vibrating normal to the surface leads to a local modification. Atoms are removed from flat terraces and renucleate as adatom islands. Simultaneously with the modification process the surface is imaged with the STM. In the case of triangular islands only one out of two possible types of close-packed 110 step edges is found in the STM images. This observation is explained by differences in the step formation energy, rather than by diffusion.