These are lodgepole pines that died by shifting hot spring run-off. By capillary action the trees soaked up mineral laden water, which colors their bases white and retards decay. Dissolved minerals in the water deposited a crust of white silica on the bottom of the dead trees and stumps. This silica, a non-crystalline compound, slowly impregnates the wood and over time, with the absence of oxygen, could eventually petrify the wood. information credit: flickr