STONEWALLING: Enemies of the proposed Elsmere Canyon dump in the Santa Clarita Valley hope they have put a stonewall in the dump's path . . . a real one. Dump opponents are attempting to get a rock formation on the site--called La Puerta del Camino Viejo (Spanish for the Gate to the Old Road)--designated a state historic site. They figure it wouldn't be easy to dump on history (B1).