You called?
As an archaeologist... Oh, really, I dunno if I can be bothered....
On the question of dating, it's not the stones themselves that give a date - you can date the working of stone in some situations, but it requires them to be unearthed and processed in very controlled conditions, IIRC correctly, and having stood on Salisbury Plain for hundreds of years is not controlled conditions. What will be dated will be any artefacts or ecofacts found in sealed contexts near or under the stones. Carbon dating can only be used on organic material, non-organic stuff is more often dated according to established typologies and analogy. (eg, we know this style of brooch is found in tombs etc dated to X by other absolute methods, therefore the context it lies in may be from that date too). Dating isn't an exact science, well, not often (unless you have a coined stamped with a date or something), but it's exact enough for most people with any actual understanding to know that Stonehenge dates from before 1877. And I'm fairly sure it was marked on maps before then anyway, although bonj will just say they were made up prophetically or something.