The radiocarbon clock's limit
The famous multi-colored logs of Petrified Forest National Park are a geological puzzle. They are composed of ancient organic material, yet they cannot be dated using the familiar radiocarbon method. The reason lies in the physics of radioactive decay. Radiocarbon dating relies on the unstable isotope Carbon-14 (C-14), which all living things absorb from the atmosphere. When an organism dies, it stops taking in C-14, and the existing C-14 begins to decay into stable Nitrogen-14. This decay happens at a known rate, defined by a half-life of about 5,730 years. After about 10 half-lives, or roughly 50,000 to 60,000 years, so little C-14 remains that it becomes impossible to measure accurately against background radiation.
The trees of the Petrified Forest are vastly older than this limit. They lived during the Late Triassic Period, a time when the first dinosaurs were just beginning to appear. The organic wood has long since been replaced by minerals, primarily silica, in a process called permineralization. Even if the original carbon were present, it would be far too ancient for C-14 analysis. The radioactive clock of carbon has simply wound down completely, requiring scientists to use different clocks that tick much, much slower.