Hmmm... How come all the photos are "cut off" on the side where the object is? (At least on my screen, they are.) I see a bunch of brick paving but only a tiny sliver of one side of the rock and the rest of it goes off the page.
When I looked at that first photo with the rounded, whiter looking stone, though, I immediately thought of "Staddler Stones" that are these mushroom-looking, low, roughly hewn, stone "seats" they've used for hundreds of years as outdoor seats in English gardens.
They still make them (using "chainsaws", I've heard -- I think they're actually made from "slate" which is relatively easy to carve). I remember seeing antique ones advertised for sale in an upscale gardening catalog a long time ago. All I could find was a couple of links for info for you:
Here's a pretty good photo of one:
http://www.hollowaysgardenantiques.co.uk/media/details/cotswold-stone-mushroom-473.jpg and that website does have some explanation, as well.
Here's another photo showing a couple that look more like the ones I remember seeing in that catalog all those years ago:
http://www.andrewhartleyfinearts.co.uk/AuctionImages/691JUN2010.jpg
But it's really hard to tell WHAT the entirety of the piece you're showing us really looks like (if everybody else is having the same problem viewing them that I am) since we (I) can only see what looks like about a 6th of one edge of the entire object in the photos.
