After more than half a century, Montana has a new official geological map, with more than triple the detail of the last edition.

The previous version of the state geological map, published in 1955, included 76 distinct geological units. The map unveiled Monday in a ceremony in the Capitol Rotunda features 324. A geological unit is a rock formation that is distinctively recognizable from everything else around it and extends over a distance, according to Ed Deal, state geologist and director of the Montana Bureau of Mines and Geology at Montana Tech in Butte.

"I never thought I'd get so old that the geology changed," joked Gov. Brian Schweitzer, who said a framed version of the new map will hang in the Montana Room in the governor's office.

Deal said the new map, roughly 50 by 80 inches and acommpanied by a separate legend and detailed comb-bound brochure with additional information, was authored by four mappers in the bureau.

"But they compiled the work of hundreds of mappers who did the more detailed work over several decades," he said. "In 55 years we've learned a lot more about the state, there's been more mapping and better accessibility."

Speaking to students from Helena High as well as the Project for Alternative Learning at the unveiling, Schweitzer said understanding what lies beneath the surface of the Treasure State has economic ramifications.

Schweitzer said he intends to ask the next Legislature to designate one particular formation, the Madison limestone, as Montana's official State Geologic Feature.

Evident in the Bridger Mountains, the Little Belts and in Lewis and Clark Caverns, Schweitzer said Madison limestone, the state's "most important" feature, is also valuable as an underground aquifer and as a source of trapped natural gas and oil. In the future, Schweitzer said the formation may prove beneficial as a storage place for carbon dioxide, or for "carbon sequestration," the trapping of the greenhouse gas beneath the ground to slow global warming.

"We have some of the most remarkable geology in the world," Schweitzer said. "The geology that's exposed today won't change during your lifetime, but it will change during the next thousands and millions of years."

It shouldn't be a half-century before the map is updated this time around. Deal said that unlike the previous edition, this map is stored electronically and printed on demand, meaning the computer files can be updated as needed to accommodate new information about the state's bedrock layers.

Copies of the map are available starting this morning for $60 (high-quality plain paper) or $85 (glossy) through the Bureau of Mines and Geology Web site at www.mbmg.mtech.edu.

Reporter John Harrington: 447-4080 or john.harrington@ helenair.com.