Riding through the dusty sagebrush of Arizona's Painted Desert in 1851, US army officer Lt Lorenzo Setgreaves stumbled by chance on the remains of a forest, the like of which he has never seen before. His discovery prompted two major questions: what were these iron-hard stumpsand branches doing in the middle of an arid desert where no known tree could survive the scorching heat?

IN fact, the trees of rock that form the largest petrified forest in the world are the result of a natural processthat start around 200 million years ago when the Arizona Desert was a broad flood plain.

The log of the forest exist in multitude od shades largely owing to the process of crystallisation. Where they exist in isolation, silica molecules crystallise into pure quartz, and many of the forest logs are formed from this substance.Slowly, wind and rain eroded the sediment, shale and sandstone covering the logs, and the petrified forest was exposed.The process that buried and then revealed the logs also created fossils of many of the plants and animals that populated this area 200 millioms years ago.In this way, the logs and dossils of the Petrified Forest are constantly being augmented by evidence of life from the days when giant reptiles roamed the earth.