Petrified trees are window to Yellowstone’s tropical past

2014-05-22T00:00:00Z Petrified trees are window to Yellowstone’s tropical pastStory By BRETT FRENCH Photos By CASEY PAGE french@billingsgazette.com The Billings Gazette

There is a ghostly redwood forest in Yellowstone National Park.

The trees are identical to the massive pines that grow 200 to 300 feet tall in California.

The difference is that the Yellowstone redwoods are petrified, not alive. They range in size from broom handle width to 4 feet wide. Stripped of their limbs and most of their bark, the largest stand 40 feet tall, their roots locked in rock. Although now lifeless, the upright logs stand as testimony to a tropical time in Yellowstone’s past, one that ended suddenly and violently.

Time travel

Imagine this: About 55 million years ago a vast inland sea that had once covered large portions of Montana was retreating to the east. Just south of where Mount Washburn now rises to 10,243 feet in Yellowstone’s north-central region, a large stratovolcano once stood. For comparison, Washington state’s 14,400-foot Mount Rainier is a massive stratovolcano — a tall peak with gentle slopes at the base rising steeply near the top.

Stratovolcanoes are known for explosive eruptions. Italy’s Mount Vesuvius, which blew in 79 A.D., was a stratovolcano. The eruption killed thousands of Romans, burying the towns of Pompeii and Herculaneum under a dozen feet of ash. It’s estimated the eruption produced flows of hot gas and rock that reached more than 570 degrees. The plume from the eruption shot an estimated 20 miles high, enveloping much of southern Europe under a cloud of ash.

Fifty million years ago, the Yellowstone volcano’s slopes would have been home to a variety of plant life much different from today. Some of the plants growing there are now found in Southeast Asia — like cinnamon and breadfruit. Also growing on the flanks of the volcano were pine, redwood and sycamore trees.

“Imagine yourself in a King Kong landscape,” said park geologist Cheryl Jaworowski. “It would have been a humid, tropical environment close to sea level.”

Rock trees

It is the remnants of these trees that are now found standing upright at different elevations for about 20 miles along the northwest end of Specimen Ridge in Yellowstone, in what is widely regarded as the largest fossilized forest in the world.

When they were described in a 1921 publication by geologist F.H. Knowlton, he believed the trees had been buried by volcanic debris over many years. Trees would grow, he theorized, then be covered by volcanic debris upon which a new forest would grow.

This theory was based on the fact that the petrified trees were found at different elevations along the hillside as it rises 2,000 feet above the floor of the Lamar River valley, and that most of the trees still stood upright.

Later scientific studies of the forest have poked holes in Knowlton’s theory.

One found that the petrified trees are so well preserved that scientists could examine the tree growth rings. By comparing the rings of trees at lower elevations with those higher up, scientists saw similar growth patterns indicating they grew through the same patterns of wet and dry years.

There was also no indication of layers of topsoil associated with the different levels of trees as one would expect to find if they had grown at the site.

Lack of scientific evidence to back up the multiple burial scenario has prompted creationists to assert that the trees were deposited during the great floods of Noah’s time, as described in the Bible.

Modern explanation

“One of the tenets of geology is that the present is a key to the past,” Jaworowski said.

So when the Washington volcano Mount St. Helens erupted in 1980, scientists had a living laboratory to study. Trees uprooted by the volcanic blast were found sunken upright at the bottom of Spirit Lake, below Mount St. Helens, with their root balls still intact.

The newer theory hypothesizes that Yellowstone’s now-petrified trees were swept off the side of the stratovolcano in a river of melted snow, ash, sand and rock called a lahar, or volcanic debris flow. Silica in the muck invaded the living tree’s cells, eventually turning them into a forest of stone. Twenty-seven layers of petrified forest have been identified, so geologists believe the deposition happened in several events.

“As far as we know, there were multiple debris flows, not just one massive event,” Jaworowski said.

The mass of sludge must have been enormous — a catastrophic deposition known to geologists as the Lamar River Formation — as the layer in which the trees are buried measures almost 1,500 feet deep. Most of Barronette and Abiathar peaks, which tower more than 10,400 feet above Soda Butte Creek in the northeast corner of the park, are also composed of this same Lamar River deposition. In fact, much of the northeast corner of the park has this formation as its base layer.

“It was a place where a lot of sediment accumulated over time,” Jaworowski said. “The Lamar River didn’t exist at that time, it was just sort of a valley bottom.”

See it yourself

After describing other areas with fossilized trees, the geologist Knowlton wrote that the ones found in Yellowstone were “the most remarkable fossil forests known.” William Holmes, who first described the fossilized trees in 1878, wrote that while riding horseback along Amethyst Mountain he saw “rows of upright trunks stand out on the ledges like the columns of a ruined temple.”

There’s a lone petrified tree at the end of a short turnoff between Mammoth Hot Springs and Tower Junction that is the easiest for Yellowstone visitors to see. The tree has been fenced off with wrought iron which, maybe purposely, is not unlike those found around some old gravesites. The high fence is an attempt to keep souvenir seekers from chipping off a chunk of the 50 million-year-old tree.

At the base of the short trail leading up to the tree is a plaque giving a brief explanation of the theory behind the tree’s existence. A historic photo taken in 1907 shows a second petrified tree, which is now gone thanks to thoughtless souvenir looters.

Hikers can see the petrified forest by traveling about 5 miles east from Tower Junction. Just before crossing the Lamar River bridge there is a service road to the south. Follow the trail from here, staying on the ridge west of the West Fork of Crystal Creek. The hike is steep. Ranger-led walks are sometimes taken to the area. Inquire at the Tower Junction Ranger Station or the Albright Visitor Center at Mammoth for more information.

“When you’re up there after that steep, strenuous hike having your lunch, look around and marvel that the tops of those mountains in the distance were once the valley bottom,” Jaworwowski said. “To me, that’s the amazing aspect of what we see in the northeast part of the park.”

Hikers can also take the 5.5-mile long 7-Mile Hole Trail near Canyon down to Sulphur Creek. It is near here that geologists have found the remains of the stratovolcano’s core.

While visiting the petrified trees on a sunny day, with the scent of pine wafting pleasingly through the air, it may be easier to imagine a more tropical Yellowstone — one covered with a redwood forest, ferns and exotic spice trees like cinnamon. But there’s even trouble in paradise, as the remains of the ancient volcano’s explosion have shown.

Copyright 2014 The Billings Gazette. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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