This is one of the “Alderney Stones”, a landscape installation art project by Andy Goldsworthy on the island. The stones are HUGE, and they will erode away exposing the things that are inside. Each stone weighs a respectable 3 Tons!
More info and a record of the erosion here at the ALDERNEY STONES website
(This is the “Fretis” Stone for anyone interested – The are 11 Stones on the island
3 exp HDR processed in photomatix & PS4