[balrog666]

You'd figure they could date these things fairly precisly.

It's just rock and limestone. How are you going to date when some random limestone was dug up, carved up, or put in place?

[TalBlack]

The specific make-up of the rock, cross checked with where it was quarried and when. Tool marks. Styles of stone work or craftsmanship. Methods of transport, weather erosion. Cross-checking with known seismic activity etc.