Surface morphology evolution of flat GaAs epitaxial layers grown on mesa structured substrates is studied under annealing in the presence of a saturated Ga-As melt. At high temperatures T > 700 °C, when the annealing conditions are shifted from the equilibrium towards sublimation, the initially flat surface roughens by unwinding spiral monatomic steps, which are induced by screw dislocations. This process leads to the formation of “inverted pyramid” spiral pits. Along with the spiral pits, the roughening consists in multilayer island formation on terraces, at the surface spots where the sublimation is inhibited. The mechanism of island formation is reproduced in the Monte Carlo simulation. The obtained data allowed us to estimate the relative undersaturation of −0.015 and Ga adatom diffusion length of 15 nm at T = 750 °C. Annealing at higher temperature T = 775 °C resulted in a peculiar “vortex”-like surface morphology, which consists in deep spiral pits covering the major part of the surface area.