MEDORA (AP)- Officials at Theodore Roosevelt National Park say an out-of-the-way petrified forest is becoming a popular tourist destination.

"Petrified wood is probably the most common fossil found in North Dakota," said John Hoganson, a paleontologist with the state Geological Survey.

"More and more people are interested in ecotourism, nature tourism," Hoganson said. "In North Dakota, we are really trying to promote that from the area of paleontology. For the right group of people, these places are destination points."

In the past, North Dakota's petrified wood had been a nuisance to many area residents. It is so common that when Interstate 94 was built west of Dickinson, no one wanted a 120-foot-long petrified log construction workers unearthed. Ranchers in the area toss the wood-turned-rock into piles.

University of North Dakota associate professor Joseph Hartman said most of the state's petrified wood is in an area between Williston and Interstate 94, east nearly to the Missouri River.

"Sixty million years ago you would have been standing in a swampy lowland surrounded by huge trees, similar to the swampy areas of south Florida today," Hoganson said.

The conditions stretched from western North Dakota into Montana and Wyoming

"Eventually, there was a climate change," Hoganson said. "Basically, this area of the world became drier and cooler so these swamps dried up and these huge trees, these forests, died."

When some trees died, the trunks fell and rotted away, leaving stumps. Wet sediment covered the stumps and the few trunks that had not rotted.

"They have been hermetically sealed from the outside environment," Hartman said. "Nothing is going to happen if it is exposed to the air."

Mineral-rich water soaked into the buried wood, and over millions of years they became petrified.

"Some of those trees were huge," Hoganson said.

The biggest he has seen sits in the Long X Trading Post, a new Watford City information center. The stump, 9 feet in diameter and 8 feet tall, weighs 16,000 pounds.

"A lot of the petrified stumps are still embedded in the hills, in the buttes," Hoganson said.