Fossil Wood and Petrified Stumps

Topic Overview
A tall forest grew in the Florissant valley 34 million years ago. Some of the trees survived as fossils that are visible today. Scientists have studied Florissant’s fossil wood to understand changes in climate and forest composition and the process of petrification—turning wood into rock.


The Big Stump is one of the main attractions at Florissant Fossil Beds National Monument in central Colorado. The base of this redwood tree was buried in a volcanic mudflow. 

Topic In Depth: Why are only stumps left?
The bases of the redwood trees that likely stood as much as 200 feet tall in central Colorado were buried by a lahar or volcanic mudflow at the end of the Eocene. Lahars form when heavy rain or rapid snowmelt sweeps rock and ash from volcanic slopes into a thick mudflow. Lahars flow downslope very quickly—up to 120 miles/hour (190 km/hr). The lahar that flowed through the Florissant valley at the end of the Eocene buried the forest under more than 16 feet (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 fossil stumps. The roots and treetops decayed or broke off.

What kinds of trees lived in Eocene Florissant?
There are approximately 120 species of fossil plants preserved in the Florissant lake beds represented by leaves, fruits, seeds, and flowers. In contrast, the diversity of fossil wood is much lower with fewer than 10 species.

The large fossil stumps for which the monument is well-known are redwoods or an extinct species of Sequoia. Extant redwoods can be found in coastal California and Oregon.

The late Eocene angiosperm (flowering plants) woods from Florissant are usually ring or semi-ring-porous with distinct growth rings (see diagrams below). An exception is the extinct genus Chadronoxylon (order Malpighiales) which is diffuse-porous. In diffuse-porous woods, the vessels (part of the xylem or water-transporting tissue) are evenly scattered throughout the trunk and not arranged in rings. Two of the ring-porous woods have affinities to the Ulmaceae or elm family. There is also wood with anatomical similarities to Koelreuteria (golden rain tree; a genus in the Sapindaceae or soapberry family), Robinia (locust; a member of the Fabaceae or legume family), and Hovenia (Japanese raisin tree; a member of the Rhamnaceae or buckthorn family).
 

Certain characters of dicotyledonous woods can be correlated to climate. For example, ring-porous woods suggest a seasonal climate because trees produce larger vessels in the spring when the conditions are wetter and more favorable.
 

A cross section of fossil wood of Koelreuteria (golden rain tree) showing the vessels. Vessels, which are exclusive to angiosperms, transport water and nutrients though the plant.
 
Are there more stumps underground?
In short, yes there are more stumps underground. Some were previously uncovered, but reburied by the National Park Service in an effort to help preserve them. Researchers have searched for other 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. Clay has a high water content. Water is highly reflective and inhibits the effectiveness of ground-penetrating radar. 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.
 

This smaller stump is still partially buried underground.
 
How does a tree petrify?
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-rich minerals (opal, quartz, and chalcedony—a microcrystalline quartz) 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, 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 anatomy in cellular detail. The anatomy of wood, including fossilized wood, is studied by examining three different sections or views under a compound microscope. 
    
Do the stumps have growth rings?
Some of the petrified redwood stumps at Florissant show clear growth rings, whereas others are hollow in the center. The Florissant fossil tree rings are wider than those of living coastal redwoods, indicating a better growing season in the late Eocene. 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.
 

This stump is currently part of an active research project. The National Park Service is monitoring the weather conditions, including snow accumulation, at this stump to develop a long term conservation plan. The stump has a hollow center.

Colorful Petrified Wood
If you visit Florissant Fossil Beds National Monument, you will see many colors in the petrified stumps. Tan and cream-colored fossil wood usually contains the mineral quartz. Dark brown or gray indicates organic material, often in opal. Iron minerals account for other colors, including black and dull red.

Explosive Excavations
Early settlers knew of the stumps near Florissant, and tourists arrived with the railroad in 1887. Collectors removed dozens of exposed stumps and loose pieces of petrified wood by the turn of the century, sometimes by the wagon load. 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 some of the stumps.
 
 

A close up view of some of the cracks in the petrified stumps.
 

The “Trio” is one of the main attractions at Florissant Fossil Beds National Monument. Each stump represents a clone that sprouted from the base of a parent tree that later decayed. The metal bands near the top function to hold loose pieces of petrified wood in place.
 

This is one of the larger diameter stumps in the park. It was likely excavated by dynamite which contributed to the large number of cracks visible. The metal band near the top is helping to hold many of the loose fragments in place.