Petrified forest, PEHT ruh fyd, is made up of tree trunks that were buried in mud, sand, or volcanic ash ages ago and have turned to stone. This action is caused by water that carries dissolved mineral matter. The water seeps through the mud and sand into the buried logs. There it fills the empty cells of the decaying wood with this matter until the structure has become solid stone. This stone shows many details of the original wood structure, especially under the microscope.