We present results from numerical simulations of expanding and colliding supershells. These large-scale spherical shocks, created by the combined feedback from several OB stars, are unstable to a number of hydrodynamical instabilities, so they quickly fragment into cold and highly structured clumps. A collision between two large shells can organize these small clumps into very filamentary structures, of tens of parsecs length and less than a parsec thick. In simulations where the flow of stellar material is followed with a tracer quantity, cold structures practically do not contain any enriched material from the OB associations at the time of their creation. In this context then, the clumps are created almost exclusively out of diffuse ISM material, containing almost no wind or supernova matter. Although the mechanism presented here is possibly not the only route for filament creation, this predicted property may help identify regions of sequential star formation.