Vegetation flourished on the shores of this lake, and the valley came to teem with life, until about two million years later. At that time, about thirty-four million years ago, a new series of periodic volcanic eruptions began about fifteen miles away, in what is now South Park, Colorado. The fine ash rapidly buried countless plants, insects, snails, and other animals, preserving them beautifully in the process. Today, these ancient ashfalls are named the Florissant Formation, after the Florissant Valley and the nearby town of Florissant. Most of the Florissant beds are protected by the Federal Government, making up Florissant Fossil Beds National Monument.
Some private quarries do exist in the area where the general public can collect fossils.
The fossilization process known as petrifaction usually begins when a tree or log is buried in silica-rich, alkaline volcanic ash or mud. Water seeping through the ash or mud dissolves silica. If silica-rich water soaks into a buried piece of wood, where conditions are more acidic, silica is precipitated inside the hollow cells of the wood. This is what happened to these huge tree stumps, which have been identified as Sequoia trees. Before the Florissant beds received Federal protection, several redwood stumps were destroyed or damaged by souvenir seekers and collectors.