Hidden Treasures

Postcard showing physical location of wayside in front of a hill of exposed shale. — 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.

Photograph of a hill with an exposure of gray layers of rock. Towards the bottom are light gray layered rocks of shale. On top is a blocky, concrete looking rock called conglomerate. — Exposure of the caprock conglomerate and the middle shale.
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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".

A close up of a rock outcrop with conglomerate on top and shale on the bottom. The two rock units are divided by a red line. The letters A, B, and C are in black circles located on the rocks. — 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 pair of hands holding two half's of a fossil leaf, the fossils are mirror images of each other. — A piece of shale split to show the mirrored images of a fossil.
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Two halves of a beetle fossil side by side showing the beetles back on the left and it's underside and legs on the right. — 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.

A fossil flower calyx on a tan piece of shale. — 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 dark spider fossil on a piece of tan shale. — 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.

A translucent brown pollen grain under magnification on a pale green background. — 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.