The complex system of actin filaments spanning the volume of a moving cell can be subdivided into distinct zones differing in their dynamic behaviour, structure and function and ordered in space sequentially beginning from the cell leading edge towards the cell interior. The first two zones are the lamellipodium, which underlies the cell membrane at the leading edge, and the lamellum adjacent to the lamellipodium and propagating further into the cell volume. The lamellipodium and lamellar actin networks do not overlap; they are separated by a distinct interface marked by an abrupt change of the velocity of the retrograde actin flow, and by a sharp change of the actin network density and structure. Revealing the physical forces responsible for the generation and dynamics of the lamellipodium-lamellum interface is of a primary importance for understanding the factors which govern organization of actin at the cell front into the spatially segregated and essentially distinct sub-systems. The goal of the present work is to propose a physical mechanism for this phenomenon. Based on the existing knowledge on the mechanical properties of actin gels, we consider the lamellipodium actin network as a two-dimensional elastic medium, which slides towards the cell centre over a row of focal adhesions and exerts a friction-like interaction with the latter. We show that the friction-like interaction between the actin gel and the focal adhesions results in formation of a lamellipodium boundary passing through the row of the FAs and having a shape similar to that observed within cells. This boundary is suggested to represent the lamellipodium-lamellum interface. We further consider advancing of the lamellipodium-lamellum interface to a new row of focal adhesions.