Sandstone with hydrothermal plumbing and fractures, in the Mojave Desert, California. This sandstone shows fractures at cross angles, apparently hydrothermally altered and eroded. The tubular features may be fossilized 'pipes', channels for volcanically heated water to rise, forming calcium carbonate tufa and plugging the pipes, forcing the hydrothermal solution to seek new paths. This is called forest cross bedding.