Petrify' means - change (organic matter) into a stony concretion by encrusting or replacing its original substance with a calcareous, siliceous, or other mineral deposit.
A living tree dies and loses its leaves, branches and bark.
The river undercuts the dead tree.
The tree topples into the water and transported a short distance downstream.
The log is buried under sand and mud in the stream channel.
Deep burial seals the tree away from bacteria and oxygen preventing decay.
Eventually Silica in the ground water infiltrates the tree replacing the organic material with quartz crystals and the log is "Petrified".
Erosion of the surrounding rock layers re-exposes the petrified logs.