Fossilization Processes

Although we generally think of volcanoes as agents of destruction, volcanic activity in central Colorado during the late Eocene was imperative for the fossilization of the more than 1800 fossil species preserved in the Florissant Formation. From the tiny, delicate fossil insects to the giant, petrified redwood stumps, the ancient Guffey volcano played a significant role in the fossilization of late Eocene life in the Florissant valley. 

Most of the fossils of the Florissant Formation have been discovered within the paper-thin shales, which is a fine-grained rock type produced by the deposition of volcanic ash, clay, and diatoms. The shales of the Florissant Formation were deposited in a low-energy, low-oxygen lake environment where plants and insects consistently fell into and were readily preserved. The eruptions of ash produced by the Guffey volcanic center would settle onto ancient Lake Florissant and enrich the water with silica. The abundance of silica in the lake led to blooms of single-celled algae called diatoms, which use silica to grow glassy “shells” (frustules). When stressed, the diatoms produced slimy mats, which coated the plants and insects as they settled to the lake bottom, inhibiting the decay process.

Furthermore, volcanic ash on hillsides weathered into clays that washed into the lake on top of the diatom layers. The alternate layers of siliceous clay and diatom mats turned into paper-thin layers of shale, and trapped organic remains (e.g., plants and insects) turned into carbon compression fossils. Fossil clams and snails have also been discovered in the paper shales of the Florissant Formation, though as mold and cast fossils.


Diagram depicting how the paper-thin shales of the Florissant Formation formed.

Vertebrate fossils in the Florissant Formation are rare because of the environmental conditions necessary for their preservation. Of the fossil vertebrates found, fish skeletons and mammal teeth are the most abundant. The more delicate vertebrates (e.g., fish and birds) have been found in the paper-thin shales, whereas the larger vertebrates (e.g., brontothere and Mesohippus) have been found in stream deposits of the Florissant Formation. Many of the larger vertebrates have been identified by fossil teeth alone. Teeth and other bones are sturdy enough to remain intact after the rest of the body decays or breaks apart. These remains turn into fossils as groundwater deposits small crystals of dissolved minerals in the tissue pores, a process called permineralization.

Similarly, the petrified redwood stumps were also fossilized through the process of permineralization. While most of life would have probably been wiped out by the influx of lahars (volcanic mudflows) into the Florissant valley during the late Eocene, the redwoods withstood such force and were destined for preservation. The lahar that flowed in was large enough to bury the valley forest under more than 16 feet (5 meters) of debris. Although the lahar killed the trees by preventing oxygen from reaching their roots, it encased and protected the lower trunks, while the roots and treetops decayed and/or broke off.  Lahar material is rich in silica, just like volcanic ash, and when water percolates through it becomes rich in silica. As the mineral-rich water penetrated the stumps, it deposited silica within the plant cells. As the wood decays and water continues to seep in, more silica minerals form inside the cells. Some plant tissue remains after wood petrifies, which helps preserve the tree anatomy in cellular detail. The process of turning wood into stone is specifically called petrifaction.


Silica-rich water (blue arrows) seeps into wood and deposits silica within the plant cells.