Having found silicified sandstones are common in France I tracked some down to Fontainbleau.
The boulders are so large and common there is a whole sport of climbing them.
And then I found a video that describes their geology.
The sarsen stones of Wiltshire which Stonehenge and Avebury are built of are the same sort of rock. The difference is that all the sand has been washed away leaving just the stones on the chalk downlands.