Delicate Impressions

At Florissant, weathering shale splits into paper-thin sheets that expose fossils and reveal how preservation in the valley depended on size and changing volcanic conditions. The first volcanic mudflows destroyed everything in their path except the big trees, and later mudflows dammed a stream to create a lake. Repeated eruptions covered the ground with powder-fine volcanic ash, which washed into the lake over time and weathered into thin clay layers on the bottom, while periodic die-offs of microscopic lake algae called diatoms added their silica shells. These alternating sediments compacted into shale that preserved tiny parts of insects and plants in exquisite detail. When the paper shale is split correctly, usually with a razor blade rather than a chisel, a fossil's part and counterpart appear on the two halves, sometimes showing different features of the organism. Florissant fossils include compressed carbon remains, shallow impressions in the rock, and microscopic fossils such as pollen grains that reveal plants not preserved in the fossil leaf record.

Florissant, Colorado · United States