The "forest" lies on a small sandstone rise and covers an area of 800m by 300m in the Aba-Huab River valley. The trees occur in sandstone of the Ecca Group, a subdivision of the Karoo Sequence, and are about 260 million years old. The name "Petrified Forest' is a misnomer the trees did not grow where they are found. It has been suggested that they were carried here by floodwaters, following the onset of warmer climatic conditions after the Dwyka glaciations. Evidence for the assumption that the trees grew somewhere else is the fact thatno roots or branches are present and only trunks are found. The trees were uprooted and transported by rivers to their present site where they were stranded on sandbank or shoals. This is inferred from the position of many stems which are orientated parallel to each other. Subsequently they were embedded in sand also deposited by the rivers. Opal-filled cracks in the logs suggest that many of the trees dried out before being embedded in the sand. 

The trees were deposited in an oxygen-depleted environment, preventing decay of the organic material and creating ideal conditions for petrifaction. Silica-rich water penetrated the logs, filling the cells, bark and other parts, where silica precipitated. This long process went together with the hardening of the sediments into sandstones. Nearly 200 million years later, after uplift of the whole area, erosion in a warm, often-arid climate removed the overlying rocks and finally exposed the petrified trees. Remnants of at least 50 trees can be seen on the plateau-some only partly exposed, while others reveal their full length. A good indication of the size of these trees is a partially exposed trunk with a length of more than 30m and an estimated circumference of 6m. The growth rings and the texture of the bark are so well preserved that one can easily mistake the petrified trees for logs.