Wissahickon Schist, commonly called Chestnut Hill Stone is a very old, historical stone which dates to the Ordovician period, some 550 million years ago. It is so old, that it contains no fossils. It is a metamorphic rock, which means “undergoing a change”. It was originally gravel on the sea floor which then underwent vast pressure when volcanic action moved it upward to form a mountain range.
