A piece of rock with mysterious markings that lay largely unstudied for 4,000 years is now being hailed as a "treasure map" for archaeologists, who are using it to hunt for ancient sites around north-western France.

A piece of rock with mysterious markings that lay largely unstudied for 4,000 years is now being hailed as a “treasure map” for archaeologists, who are using it to hunt for ancient sites around north-western France.