Hidden Treasures

Geoheritage Highlight!

The majority of Florissant's world-famous fossils are plants and insects preserved in these delicate shales. Some pieces of shale are as thin as a sheet of paper, which makes them very fragile records of the past. A significant aspect of Florissant's Geoheritage is that there are 1,800 described fossil species, making Florissant one of the richest fossil sites in the world. The fossil record reveals that the climate and forest were much different in the warmer world here during the Eocene 34 million years ago.

 

This humble hillside hides a bonanza of fossils hidden between paper-thin rocks!

NPS:SIP Mariah Slovacek

The hillside pictured here contains thousands of tiny, fragile fossils of insects and leaves. The fossils are hidden between layers of rock called shale. The shale was formed at the bottom of an ancient lake that once existed in the Florissant valley.

 

Exposure of the caprock conglomerate and the middle shale.

NPS

Plants and animals that died in or near the ancient lake settled to the bottom where they were buried in layers of clay, ash and diatoms. Eventually, the layers hardened into rock, and the plants and animals became fossilized. As outcrops of shale weather, they separate into paper-thin sheets exposing fossils on their surfaces. A glimpse of the diverse Eocene ecology is revealed within these delicate "pages".

 

Close up of the contact between the caprock conglomerate and middle shale units as denoted by the red line.

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Contact!

The outcrop shown here includes a very important geologic contact within the Florissant Formation, shown by the red line. This contact formed when a debris flow settled in the lake to from the caprock conglomerate unit. The debris flow radically disrupted the lake environment and resulted in a distinct contact on the lake bottom with the middle shale unit. The thick, durable conglomerate protects the easily eroded shale.

A) The caprock conglomerate unit

B) Tuff formed from volcanic ash in the middle shale unit

C) Laminated shale beds in the middle shale unit

 

A piece of shale split to show the mirrored images of a fossil.

NPS

 

A fossil beetle and counterpart demonstrating how some fossils can preserve different halves of the same individual. Left: Dorsal (top) of beetle with elytra (wing covers), Right: Ventral (underside) including legs. (FLFO-8787)

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A split piece of shale usually displays two halves of the fossil organism—the part and the counterpart. These can show different aspects of the organism, such as an insect's legs on one half and the back on the other. Many of Florissant's shale fossils are compressed carbon remains—a thin film of dark residue. Others may be a shallow impression of the organism in the rock.

 

Calyx of a fossil flower named for the monument. (UCMP-3619)

Courtesy of the University of California Museum of Paleontology

Florissantia speirii

This fossil preserves a calyx that matured into a fruit. It belongs to an extinct genus of the cocoa family.

 

A large spider fossil. (AMNH-FI-19032A)

Courtesy of the Department of Invertebrate Paleontology, American Museum of Natural History

Lycosa florissanti

This wolf spider, like its modern relatives, hunted its prey on the ground. It most likely lived on the forest floor under rocks and plants.

 

Pollen grain found at the monument. Equatorial diameter 40 µm.

Hugh Wingate, provided courtesy of Doug Nichols

Some of the most spectacular fossils found in the Florissant valley are revealed only under the lens of a microscope. The distinctly shaped Xylonagra pollen grains reveal the presence of plants that were not preserved in the fossil leaf record.

 

Stop 2: Arc of Heritage

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Stop 4: The Land Provides

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