Petrification

By gravity, the water seeps. Sometimes it passes areas which are heated by volcanic activity. The water heats, dissolves minerals it carries and charges up in carbon dioxide. Lightened by the carbon dioxide, it goes back up and retrieves minerals. Once in the open area, the chemical reaction (high level of CO2, air, water movement) allows the limestone to deposit itself on everything it touches. Strategically place any object for several months in the water, it will be petrified.