The rich variety of micron-scale features observed in the orientation-dependent surface morphology of crystalline silicon during anisotropic wet chemical etching is shown to have its origin at the atomistic scale. Realistic Monte Carlo simulations show that the pyramidal hillocks on Si(100) are the result of local stabilization of distributed apex atoms by (metal) impurities from solution. In the absence of this stabilization, shallow round pits are formed on Si(100) as a result of the anisotropy between (one layer deep) pit nucleation and (isotropic) step propagation. It is also concluded that nosed zigzag structures at vicinal (110) are the combined result of misaligment and the etching anisotropy, showing that the nucleating mechanisms of morphologically related structures such as pyramidal hillocks and nosed zigzags are not necessarily the same. The simulations confirm that the formation of (one layer deep) triangular and hexagonal pits on exact Si(111) and of polygonal (saw-shaped) and straight terraces in vicinal Si(111) depends on the relative rate of [11] and [2] step propagation and on the misorientation of the surface with respect to Si(111).