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SALT LAKE CITY (AP) -- Utah state geologists and a group of intrepid volunteers say they've finally answered a nagging geological question: What's the longest natural arch in the world?
The answer, they say, is Landscape Arch in Arches National Park in southeastern Utah.
Of course, it all depends on how you measure it. But members of the all-volunteer Natural Arch and Bridge Society spent years developing a standardized measurement that looks at the widest horizontal opening in each arch -- not the arch itself.
Using that yardstick, they found that Landscape Arch spanned about 290 feet. Kolob Arch in Zion National Park came in at 3 feet shorter.
Grant Willis, a geological mapper at the Utah Geological Survey, said the group's survey -- which ranks the top 10 arches in the world -- finally offers definitive answers about the arches with the longest spans.
"In my mind, the question has been answered," Willis said.
There are others that are taller or may otherwise be found to be larger using a different set of criteria, but, at least in this survey, Landscape Arch is king.
Landscape and Kolob have for years been mentioned in the same breath when it came to discussing the world's biggest sandstone arches, which form when softer rock in the middle erodes to leave behind an arch out of remaining harder rock.
Shortly after the Natural Arch and Bridge Society formed in 1988, talk turned to how to best size up the arches in a definitive, standardized way. It took years for members to agree on a complex mathematical definition. The laymen's version emerged as "the maximum horizontal extent of the opening," said Jay Wilbur, of Austin, Texas, a founder and current vice president of the group.
They started in 2004, first just looking for updated measurements of some of the most well-known arches, including Landscape, Kolob and Rainbow Bridge, which is also in Utah.
Eventually they turned their lasers and other measuring equipment to arches in Arizona, Colorado and New Mexico. Some of the previous measurements were fairly close. Some were off 30 to 40 feet, Wilbur said. "We have a lot of technology available today that wasn't available in the 1980s," Wilbur said.
Landscape and Kolob emerged as clearly the longest. But another, Fallen Monarch in Arches National Monument, likely would have exceeded both had it not collapsed several hundred years ago, Willis said.
An arch in China called Tushuk Tash, whose length hasn't been measured yet, may rival some of the longest but won't eclipse Landscape for the biggest span, Wilbur said.
At Arches National Park, home to more than 2,000 arches, officials have been careful never to claim Landscape is the longest, said Sharon Brussell, who is part of the park's interpretive ranger division.
"It's always been up to such debate," she said. "There are so many different ways to measure, different people measuring, different measuring techniques."
She said the park will continue to refer to Landscape Arch as among the largest, without making a definitive statement. "We're still going to play it safe," she said. "We're just glad to have it."
After Landscape and Kolob, other arches on the group's top 10 list are:
- Aloba Arch in the Sahara Desert
- Wrather Arch in Arizona's Paria Canyon
- Morning Glory Natural Bridge in Moab, Utah
- Rainbow Bridge at Rainbow Bridge National Monument in Utah
- Sipapu Natural Bridge in Natural Bridges National Monument in Utah
- Stevens Arch near Utah's Escalante River
- Outlaw Arch in Dinosaur National Monument in Colorado
- Snake Bridge in Sanostee, N.M.
(Copyright 2009 by The Associated Press. All Rights Reserved.)
