They are not concave, they are convex, like frozen bubbles. And it's 4' long, so at least half a ton. Still trying to track down the owner, to see if they want to part with it :)
And these two-what-used-to-be-one who refuse to be parted, even after millenia of erosion down middle, leaving a perfectly parellel channel 6" wide between the two. Underneath Castle Naze Hillfort.
There are several other 'pairs' scattered to the west of the fort where the cliff falls away in a similar state, a superb lesson in weathering and erosion for my boy :)