The ancient trees in most petrified forests are in horizontal positions. They were carried by streams and deposited in alluvial flood plains. Petrified trees on Specimen Ridge are unusual because they are still in their upright positions. Volcanic eruptions in this region during the Eocene epoch (50 million years ago) triggered massive landslides into mountain and valley streams. The mixture of ash, water and sand buried entire forests in their original standing positions, including redwood trees very similar to those of coastal California. Before the wood decayed, silca from the volcanic mud flow replaced the cell contents (lumens), literally creating forests of stone. Unlike other petrified woods that are completely replaced with minerals, the petrified trees of this region have lignified cell walls of the original xylem tissue that are still intact. Right: A 50 million-year-old piece of petrified wood (possibly redwood) from nearby Lamar Valley compared with a piece of recent dead wood. Both are radial sections with parallel annual rings. Can you tell which of the two pieces is petrified?

These small, primitive plants are the size of mosses, only about 1-2 inches tall. Unlike mosses, they are true vascular plants with water-conducting xylem tissue. They produce reproductive spores, but do not produce flowers, fruits or seeds. Male and female spores are produced within minute structures called microsporangia (male) and megasporangia (female). Ancestral, tree-like forms existed during the Carboniferous Era, about 300 million years ago. The square, upright leafy branches contain orange microsporangia in the leaf axils (white arrow). Each microsporangium contains numerous microscopic microspores.