Petrified Wood: The Exhibition

Araucarian trees lived over 225 million years ago, during the time of the dinosaurs, once part of a great forest that extended from Texas into Utah. Today, these trees survive only in the temperate to subtropical parts of the southern hemisphere, just crossing the equator into the Philippines. They seem to have been eliminated from more northly areas by the same catastrophe that caused the extinction of the dinosaurs.

Some paleobotanists believe that the characteristic umbrella shape of many modern araucarians has been retained from the time when this shape served to protect their foliage from grazing by the gigantic sauropod dinosaurs, like Apatosaurus.

Above: An extensive forest of araucarian trees is found today growing in New Caledonia. Photograph courtesy of G.J. Watkins-Colwell.

Judging from the size of their trunks and the stature of their descendants, araucarian trees grew upwards of 150 to 200 feet (45 to 60 meters) in height, with trunks 4 to 5 feet (about 1.5 meters) in diameter.

Working on a 12-foot (about 3.5 meters) scaffold in the Great Hall, exhibit designer Laura Friedman painted the18-foot-high (about 5.5 meters) background image of an araucarian tree for the exhibition. “I would have liked to have simply projected the image of the trees onto the mural surface, and traced around it, but I was unable to do that as there was no space in the hall to set up a scaffold at the proper height at a sufficient distance from the wall (the dinosaur island is in the way). So instead a stencil was made in pieces by projecting a small drawing of the araucarian trees onto large boards taped to a wall in the studio. The enlarged drawing was traced on the boards in sections and then cut out into a giant, lace-like stencil. I taped the stencil pieces together onto the wall, traced it, and then filled in the silhouette with paint.”

After these trees died and were toppled by the forces of nature, they were transported by streams into swampy lowlands. As they tumbled along or jostled one another, branches, bark and small roots broke off or were worn away. Over time the logs were buried in silt, mudstone or volcanic ash. This blanket of sediment cut off oxygen to the logs, slowing decay.

Above: These petrified logs near Petrified Forest National Park, Arizona, are an example of what can be found strewn across the landscape of the Petrified Forest and surrounding areas today. Photograph courtesy of Ralph Thompson.

At the same time, groundwater containing dissolved silica seeped through the logs, encasing the original cell structure of the wood in silica that hardened into stone.

Above: This portion of a polished petrified trunk, Araucarioxylon arizonicum, weighs approximately 450 pounds (200 kilograms) and measures 2 feet (60 cm) in diameter.

The 450-pound (200 kilogram) petrified trunk is moved into position by Maishe Dickman and Eric Hoag of the Yale Peabody Museum’s Construction Shop.

Araucarioxylon arizonicum
YPM 45161
Chinle Formation, Petrified Forest Member
Late Triassic Period
(approx. 225 million years ago)
Near Petrified Forest National Park, Arizona

 

During this process of petrifaction mineral impurities mixed with the silica, producing this highly prized “rainbow wood,” such as this gem-like cross-section (above) displayed in the exhibition. The brilliant array of colors comes from manganese oxides (blues, blacks, purples), gypsum (white), and the iron oxides limonite (yellow) and hematite (red, rust).

The cross-section of rainbow wood being placed into the exhibition by Walter Brenckle and Eric Hoag of the Yale Peabody Museum’s Construction Shop.

 Araucaria mirabilis
YPM 28339, YPM 53182
Jurassic Period
(approx. 160 million years ago)
Cerro Cuadrado, Patagonia
Province of Santa Cruz, Argentina

 

Together with the petrified wood from these trees, you can also see foliage and cones that were once attached to these trunks before they were swept away and buried. On the right are 2 petrified araucarian cones from a petrified forest in Patagonia, Argentina. The cone below shows its outer face, consisting of individual cone scales. Seeds can be seen in the sliced and polished cone above.

Above, left: A modern, nearly ripe seed cone from Brazil. Note the cone scales that end in narrow points. As shown in this exhibition, paleobotanists must reassemble the whole tree from its often widely scattered parts.

Araucaria mirabilis
cones and stems
YPM 44607
Jurassic Period
(approx. 160 million years ago)
Argentina

 

This remarkable slab of hardened volcanic ash from the Jurassic Period in Argentina on display in the exhibition contains over 40 araucarian cones, many still attached to the branches on which they grew. Cones of Araucaria disintegrate within a few weeks of ripening, so this specimen preserves a moment in geologic time when a volcanic eruption tore down the forest canopy, burying these cones just as they had finished ripening.

From left to right: Ruth Lapides, Professor Leo Hickey, Exhibit Designer Laura Friedman and Yale Peabody Museum Director Michael Donoghue at the June 26, 2004 opening of Petrified Wood: Rainbows in Stone.