An unsteady one-dimensional MHD model is developed to study (1) the essential physical processes involved in the development of the forward reverse shock pair at large heliocentric distances, (2) the interaction of the shock pair with the rarefaction regions of the stream structure, and (3) the merging of two forward or reverse shocks. We use a method of solution which is quite different from the finite differenced methods: MHD shocks (or contact surfaces) are treated as boundary surfaces. The presence of moving shocks divides the domain of interest in the r-t plane into several flow regions. The jump conditions of MHD shocks describe the flow conditions across the moving boundaries between flow regions. The method of characteristics describes the variation of flow conditions in each region. The solutions explain the formation of the shock pair in the leading edge region as resulting from the merging of fast waves. The strong MHD disturbances generated in the corotating interaction region (CIR) propagate at a fast speed relative to the moving material. The wave propagation speed is greater in CIR than in its surroundings. This causes the disturbances in CIR to pile up to form a shock pair. During the formation process the shocks continuously grow into a fully developed state. The newly formed shock pair will then propagate outward from the leading edge to interact with the ambient rarefraction regions. The double-sawtooth configuration of the velocity profile is a result of this interaction. We also obtain solutions to demonstrate that the merging of two shocks produces a stronger shock and a contact surface on its backside.