The towers rest on underwater caissons made of southern yellow pine. Traditionally the boards were made from the flexible wood of a southern yellow pine tree. This zone is also known as the yellow pine forest zone. It was carved from yellow pine and is tall. The original structure was a wooden trestle made of yellow pine upon stone abutments. The deck was three inches of yellow pine beneath of wrought iron. The interior features U.S. Southern yellow pine. The wood industry classifies the species as a southern yellow pine. The harvesting of yellow pine further diversified Savannah's economy as a lumber exporter. George Washington opted for the much harder southern yellow pine at Mount Vernon instead. Oak and yellow pine trees form the majority of the forests. Jeffrey pine wood and ponderosa pine wood are sold together as yellow pine. This tree mostly inhabits redwood forests, California mixed woods, yellow pine forest, and oak woodlands. The deck was made of thick teak while the 'tween deck was yellow pine. Higher up, the yellow pine forest community gives way to a pure lodgepole forest.