Agate hunters could get run over by earth-moving machines if they continue trespassing on a St. Croix River bridge construction zone, the Wisconsin Department of Transportation warned Friday.

Frustrated by growing numbers of rockhounds digging in overturned soil where a Hwy. 64 extension is being built in St. Croix County, Wis., the road contractor has asked for help from law enforcement to keep people away.

"They're not driving around golf carts," WisDOT spokeswoman Chris Ouellette said of workers operating bulldozers, dump trucks and other large vehicles. "We're just really concerned about the safety of people. They need to stay out of there. They're on our property in this work zone, doing a very dangerous thing."

Ouellette wasn't sure how the word got around, but she said agate hunters first appeared on weekends and then began prospecting on weekdays as well, causing alarm that they could be hurt or killed.

"Nobody wants to come across somebody when they're driving a bulldozer; that would be tragic," she said.

WisDOT started warning agate hunters three weeks ago, but the warnings were ignored and now the St. Croix Sheriff's Office has been asked to enforce new no-trespassing signs, she said.

David Rusterholz, president of the St. Croix Rockhounds geology club, said any fresh-turned soil in the St. Croix River valley will attract people in search of Lake Superior agate, deposits that glaciers once carried south from the big lake. His club has a standard of ethics about staying off private property, he said, but many other people are hobbyists who flock to construction sites.

"I doubt there's anything special about that location," he said.

Agate is a semiprecious stone that's commonly used in making jewelry, but some people collect it because they like the looks of it, said Rusterholz, a chemistry professor at the University of Wisconsin-River Falls. Valued pieces, he said, can sell for $500 or more.

"You show agate to someone who doesn't care and it means nothing," he said. "It's like looking at a flower. It's something absolutely beautiful and it's made by nature."

Another member of the Rockhounds, Jim Swanson of Stillwater, said the "word of mouth" quickly spread about road work in St. Croix County because "there are a lot of people interested in rocks and gems."

He's heard that agate hunters also have searched the bridge construction site on the Minnesota side of the river. On both sides of the river, they probably didn't understand they were entering private property, he said.

"It's kind of stupid to be around when there's equipment," he said.

A housing construction site in Lake Elmo was a recent hot destination for agate hunting, Swanson said, because any fresh dirt will become the latest promise of a good agate find.

"Just look for people with pails," he said.

Ouellette said WisDOT owns the area where Hwy. 64 is being built, but road contractor H James and Sons would be liable if anyone got hurt. "They are very concerned about the safety of people wandering around on the site," she said.