Florissant's fossils: a lake that preserved insects' wings

A petrified redwood stump in the Florissant valley.

Thirty-four million years ago, in what is now central Colorado, a volcanic mudflow dammed a valley and made a lake. Ash settling to its bottom buried insects, leaves and flowers so gently that their wings and veins survive, and mudflows entombed a forest of giant redwoods where they stood. Florissant Fossil Beds National Monument protects what is left: one of the richest and most varied fossil deposits in the world.

The monument in brief

Delicate fossils like these are missing or extremely rare at most fossil sites, which is why Florissant's record of an Eocene ecosystem — and of how its climate and life changed while the land was rising — is so valuable.

How the fossils were made

1. The floor. The valley was cut into the Pikes Peak Granite, about 1.08 billion years old.
2. The dam. Eruptions in the nearby Thirtynine Mile volcanic field sent lahars — volcanic mudflows — down the valley, damming the streams and forming Lake Florissant.
3. The lake. Fine ash and clay settled in thin layers on the lake bed, making the "paper shales" that hold the insects and plants. The lake probably formed twice: a lower shale records the first lake, and middle and upper shales, divided by a conglomerate caprock, the second.
4. The burial. Lahars and pumice gravel eventually buried the lake beds — and the forest standing around them.

The rocks of the Florissant Formation — lava flows, pumice-rich tuffs, river gravels, volcanic breccias and the shales — sit in the Southern Rocky Mountains physiographic province.

Maps and reports

The Park Service's Geologic Resources Inventory has published a geologic map and report for the monument (KellerLynn, 2006), free on the NPS DataStore. Its fossils are protected by the Paleontological Resources Preservation Act of 2009; collecting them is prohibited.

Sources

Written from "NPS Geodiversity Atlas—Florissant Fossil Beds National Monument, Colorado", National Park Service Geologic Resources Division, a work of the United States government in the public domain, which cites KellerLynn (2006), Wobus (2001) and Evanoff and others (2001).