The dynamics of early stage of terrace formation in thin supported films of cylinder forming triblock copolymers was studied both theoretically using self-consistent-field theory (DSCFT) and experimentally by in-situ scanning force microscopy (SFM). In experiment, an initially flat film of incommensurable thickness was imaged continuously, and the evolution of a vertical orientation of cylinders into a parallel one as well as the respective development of thickness gradient (terrace formation) was captured in detail. On the grounds of these experimental observations, the parameters of the computational model A3B12A3 were determined to match the structures in experiment. Both systems show excellent agreement in details of structural phase transitions and in the dynamics of the step development, suggesting that the underlying transport mechanisms are governed by diffusion. The early stage of terrace formation is characterized by the development of the step height up to 80% of its equilibrium value and associated reorientation of cylindrical domains.