Abstract <p>The paper provides a mathematical description of the terraced relief (TR) formation process under oblique irradiation of a silicon surface with Ga⁺ ions at fluences <i>F</i> &gt; 10¹⁸ cm^–2. The spatial nonlocal erosion model provides a rather complete description of surface relief evolution under ion bombardment conditions. The nonlocal erosion equation (NEE) allows describing terrace slopes via linear piecewise-continuous functions. These functions, similar to the horizontal plane, are equilibrium states of the NEE. The partition structure of the NEE phase space into stability domains for the horizontal plane and TR has been established. The study demonstrates that when ion beam incidence angles exceed Θ₀ &gt; 30°, the plane becomes unstable, leading to the formation of a TR with stable left and unstable right slopes. As a result of right slope instability, a wavelike relief forms with the wave vector oriented perpendicularly to the ion beam incidence plane.</p>