What happened to the wood that made it that way in the beautiful petrified trees in the forests of Arizona? They believe that petrified wood is so old that in the prehistoric period it has emerged. But do you know how petrified wood was made? This guide will show you how. What is petrified wood and how is it formed? Fossil wood is considered to have grown when the material of the plant is buried by sediment. When the wood is buried deep in the muck, it is protected from decay caused by exposure to oxygen and organisms.