If you can't find the stone styrene sheets, you can also try carving your own rocks using foam. I did something similar in the footings for the girder bridge in the foreground. I bought two blocks of florists foam (almost any hard fine graded foam will work, except beaded styrofoam) and cut/scraped grooves to make large rough blocks, like cut from a quarry, using a knife blade and small screw driver tip. I couple of thick coats of latex paint sealed the foam air pockets, leaving a surface very much like stones. You could either cover your cardboard base (if it is salvageable) or rebuild the entire viadfuct from the foam.
Doc.