Help Needed: The Essential Blender

I’m on the first Mesh Modeling tutorial, where you build the bridge and such and at the end i find that when i try to extrude the top to make the little pillar out of the top it does it out of the bottom too. That’s because they’re mirrored. Is there some way that i should ‘un-attach’ them or the like? I can’t really understand it, because the book takes it for granted that it won’t do the same out of the bottom.

Also, another less worrying problem, whenever i render a scene some objects are fuzzy gray and some are solid gray. Why is that? I haven’t done any color yet.