Complex sentence: A bluish crushed diorite was added to the concrete used to pour the walls of the abutments and the spandrel walls, which were left unfinished, and reflect the texture of the wooden forms used to create them.
Split into simpler sentences: A bluish crushed diorite was added to the concrete used to pour the walls of the abutments and the spandrel walls. The walls of the abutments and spandrel were left unfinished, and reflect the texture of the wooden forms used to create them.