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The Florissant Fossil Beds

Guffey volcano lahar - petrified redwood tree stump

Petrified redwood tree stump (Sequoia affinis, an extinct species), the remains of a forest buried by a lahar from the Guffey volcano 34 million years ago. Florissant Fossil Beds National Monument.

The Florissant Fossil Beds

By Jack Barkstrom

The Guffey volcano lahar - Buried in mud and water...

After the volcanic eruption of 36.7 million years ago near Mount Princeton, there was relative quiet for several million years. When it began again the center of activity had shifted east forty miles. The Guffey volcano was one of several volcanoes which combined to make up the Guffey volcanic center, about ten to fifteen miles west-southwest of present-day Florissant.  The Guffey volcanic center was, in turn, part of what is known as the Thirtynine Mile volcanic field. While eruptions from these volcanoes were less explosive than that of the Mount Princeton region, they did erupt, spewing lava and ash. Since the eruptions were smaller however, the ash and rocks which did become airborne often traveled only a short distance, often little further than the immediate slopes of the volcano. The loose material tended to absorb water and, once saturated, turned into a thick, unstable sludge.

About 34 million years ago (34.1), one of the ash and water accumulations on the slopes of the Guffey volcano came loose and began to flow down the mountain, in what is known as a lahar, or volcanic-debris flow.  Following the bed of the ancient Fourmile Creek, it flowed into the valley below.  In its path was a forest of large redwood trees. The lahar flowed around them, burying everything on the forest floor to a depth of fifteen feet. Subsequent lahars dammed the valley south of the forest, trapping the waters flowing in.  The waters which filled the valley covered the earlier forest-burying lahar and became the ancient Lake Florissant, in size about one mile wide by twelve miles long.  The tops of the redwoods rotted away, but the fifteen feet buried in mud was preserved as stone, leaving a small petrified forest.  Dissolved minerals in groundwater had  penetrated the cells and tissues, replacing the organic structure with silica.

Lake Florissant would suffer additional accumulations from subsequent eruptions.  Ash fell into the lake and occasionally lahars brought more mud, which preserved a large number of plants and insects.  The formations at Florissant National Monument have yielded 1,700 species of fossils, from among more than 50,000 specimens dating from the late Eocene Age of 34 million years ago.

Suggestions for further reading.

Thomas W. Henry, Emmett Evanoff, Daniel A. Grenard, Herbert W. Meyer, and David M. Vardiman, "Geological Guidebook to the Gold Belt Byway Colorado, Gold Belt Tour Scenic and Historic Byway Association, (Gunnison, CO 2004).
Ralph Lee Hopkins and Lindy Birkel Hopkins, "Hiking Colorado's Geology," The Mountaineers, (Seattle, WA 2000).
Kirk R. Johnson and Robert G. Raynolds, "Ancient Denvers: Scenes from the Past 300 Million Years of the Colorado Front Range," Denver Museum of Nature & Science, (Denver 2003).
Andrew M. Taylor, Ph.D., "Guide to the Geology of Colorado," Cataract Lode Mining Company, (Golden, CO 1999).