For once, something called "Very Big" lives up to its name.
The stones are on top of a ridge between Charnizay and Obterre and were placed about 5000 years ago. There is a theory that the dolmen was never completed, hence the stone lying on the ground.
On the other hand, it could be that the stones fell from the pockets in the apron of the wife of the giant Gargantua. There is more on the legends of the Touraine here.