Fossil Forests of Yellowstone
from the book, "Petrified Lightning"

Petrified Lightning

Fossil Forests of Yellowstone

Of all the wonders in Yellowstone National Park, one of the most fascinating is usually passed over by tourists. In the Amethyst Cliff area of the park, exposed to view on the steep slopes, is a 2000—foot—thick section of layered rock that records the birth and death of 18 successive forests.

During the ancient Oligocene epoch, in the area now known as Yellowstone National Park, numerous explosive volcanoes caused huge amounts of ash to shower down on the land, burying large forested areas. Instances of ash "showers" were interrupted by periods of dormancy, often long enough for the forest to regenerate. So a new forest would grow and thrive on top of the layers of ash that buried earlier forests. Rock layers were deposited one on top of the other. One layer devoid of any remnants of plant life would be overlain by the remains of a younger forest destroyed by a volcanic shower of ash.

What happened in prehistoric Wyoming is written very concisely in the rocks. In the million or so years of time recorded there, huge forests developed, only to be disrupted when the area's dormant volcanoes came to life. Daylight was turned into sudden night, the air choked by black clouds of eruptive dust crisscrossed by violent lightning bolts. When the ash settled out of the atmosphere, where beautiful forests had once stood was now a sea of dead, burned trees, most blown over by the volcanic blasts. They resembled huge matchsticks strewn in the direction of the blast; nothing appeared to be alive.

As in all major disasters, however, some life always manages to survive. Eventually, after the blackened sky cleared and with hot volcanic ash a thing of the past, survivors began to stir. In relatively short periods of time new buds pushed through the soil covering, and a new forest was born. This rebirth, repeated over and over again during at least a million years of geologic time, is pictured in Amethyst Cliff.

Faulting has uplifted the areas of fossil trees and, aided by recent erosion, has laid bare the sides of several hills, exposing the successive forests, one on top of another. Doubtless several layers of buried forests still lie in the subsurface, not yet exposed by erosion. Many of the trees exposed to view on the sides of the hills are truly unique, for they stand upright in the same position in which they lived and died—over 40 million years ago.