A two-dimensional problem of the cliff surface evolution is considered taking into account the marine abrasion and denudation processes. The evolution is described using a set of three differential equations. When analysed the equations revealed the existence of two regimes of the marine erosion and denudation process, one of the regime being unstable. Occasional increase in debris supply to the beach may cause the transition from erosional to accumulative type of coast. Numerical solutions of the equations are obtained which describe the cliff evolution at two sites of the Black Sea coast. A technique of the erosion coefficient calculation is introduced, field observations data being used in the calculations.