"Scenic view from atop Twin Rock" by U.S. National Park Service , public domain
Florissant Fossil Beds
Florissant Fossil Beds Petrified Forest National Park Service U.S. Department National Park Serviceof the Interior U.S. Department of the Interior Florissant Fossil Beds SiteNational Name and Designation Monument A tall forest grew in the valley behind the visitor center 34 million years ago. Some of the trees survived as fossils that are visible on the trails today. Scientists have studied Florissant’s fossil wood to understand changes in climate and forest composition and the process of petrification—turning wood to rock. Why are only stumps left? Are there more stumps underground? Heavy rain or rapid snowmelt can sweep rock and ash from volcanos into thick mudflows called lahars. Lahars can rush downslope at up to 120 mi/hr (190 km/hr). One lahar from the ancient Guffey Volcano flowed through the Florissant stream valleys 34 million years ago. This volcanic mudflow buried the forest there under more than 16 ft (5 m) of debris and killed the trees by preventing oxygen from reaching their roots. The lahar encased and protected the lower trunks, which are preserved as fossils. The roots and tops of the trees decayed or broke off. Researchers have searched for undiscovered stumps at Florissant in several ways. Ground-penetrating radar, which measures reflections from electromagnetic pulses, has not been effective because the upper soil of the park is rich in clay. A recent study shows promise for detecting stumps with a magnetometer instead. This is because the local magnetic field is weaker above the silica-rich stumps than above the surrounding volcanic rock, which contains the magnetic mineral magnetite. How does a tree petrify? Artistic reconstruction of the Eocene forest at Florissant. What kinds of trees lived here? Most fossil stumps at Florissant are redwoods similar to the coast redwoods now living in California and Oregon. Some of the petrified wood comes from hardwoods, including Hovenia (related to Japanese raisin trees), Koelreuteria (golden rain tree), Robinia (locust), Zelkova (related to elms), and Chadronoxylon. When mineral-rich water penetrates wood, it deposits silica on the plant cells. As the wood decays and water continues to seep in, more silica minerals (opal, quartz, and a quartz with microscopic crystals called chalcedony) form inside the cells. Most of the silica in the stumps at Florissant probably came from volcanic rock and ash. Certain types of wood, like redwood trunks, are more durable than others, which may make them more likely to petrify. Experiments show that wood can petrify in tens to hundreds of years in ideal conditions, but it likely took much longer for the stumps at Florissant to turn into rock. Some plant tissue remains after wood petrifies, which helps preserve the tree structure in cellular detail like that seen in this fossil wood of Koelreuteria (golden rain tree). Image magnified. Do the stumps have growth rings? Scientists cut thin sections of petrified wood to study growth rings and other plant features under a microscope. During petrification, minerals form in the spaces of organic tissue. This process can preserve individual plant cells. Some of the petrified redwood stumps at Florissant show clear growth rings. The Florissant fossil tree rings are wider than those of modern California redwoods, indicating a better growing season in the past. A technique called tree ring cross-dating matches the patterns of thin and thick rings among different trunks to see if the trees lived through any of the same drought or wet periods. Petrified Florissant trees have the same patterns, so it is likely that all the trees in the forest died at the same time. A single lahar probably covered the entire valley in a day. Colorful Petrified Wood You can see many colors in the petrified stumps. Cream-colored fossil wood usually contains quartz. Dark brown or grey indicates organic material, often in opal. Iron minerals make other colors, including black and dull red. Where can I see petrified stumps? Big 3 Stump N The Ponderosa Loop and Petrified Forest Loop trails pass more than a dozen petrified stumps (map at left). Some of the best stumps lie right behind the visitor center (stop 1). On the Ponderosa Loop trail, a modern forest surrounds the fossil one, and living pines grow directly on top of petrified stumps (stop 2)! The Big Stump (stop 3) on the Interpretive Pit (Seasonal Access) Petrified Forest Loop trail is completely excavated. The petrified trees are the largest fossils at Florissant. Please help protect them by staying behind railings. Law prohibits disturbing or collecting fossils in the national monument. Stump 1 Redwood Trio 0 2 Pines 0.1 mi 0.1 3 Big Stump 0.2 km Petrified Forest Loop 1 mi (1.6 km) Boulder Creeek Trail & Hans Loop Visitor Center 1 Ponderosa Loop 0.5 mi (0.8 km) Sawmill Trail 2 Hornbek Wildlife Loop Geologic Trail Explosive Excavations Early settlers knew of the stumps near Florissant, and tourists arrived with the railroad in 1887. Collectors removed dozens of exposed stumps by the turn of the century, sometimes by wagon loads. In the 1920s, two commercial sites excavated stumps on the land. One of these private operations used dynamite, which likely contributed to the cracks visible in the stumps under the shelters by the visitor center.