Standing on the edge of glistening Pohick Bay, paralegal Laura Glass gazed doubtfully at the six-inch-square board before her, and the massive drill in her hand. Her job: Drill holes so that long nails could fasten the board to another for a retaining wall. But first things first.

"How does this start?" she asked Doni Croson, a Northern Virginia Regional Park Authority carpenter.

"Pull the trigger," Croson advised.

Glass looked at the drill again.

"Where's the trigger?"

Glass was among 110 enthusiastic--though sometimes inexpert--volunteers who showed up yesterday at Pohick Bay Regional Park in southeastern Fairfax County to lend a hand on several badly needed projects at the 1,000-acre site.

The event was part of National Public Lands Day, an annual sprucing-up of Uncle Sam's yard, from national parkland and forests to city- and county-owned properties. Since its start in 1994, with nine sites in three states, Public Lands Day has mushroomed: This year there were 190 sites and 30,000 volunteers.

In the Washington area, people fanned out over a dozen sites, building trails at the C&O Canal National Historical Park, repairing bridges at Greenbelt Park, spreading wood chips at Accotink Bay Wildlife Refuge near Fort Belvoir and helping to remove a deteriorating dock in Prince William Forest Park.

The get-together provides "a hands-on educational experience to people in the community on what the public lands are and what their roles are in preserving them," said Derek Young, a spokesman for the National Environment Education and Training Foundation, the D.C.-based nonprofit group that spearheads the event.

At Pohick Bay, a cleanup sponsored by the Bureau of Land Management, the volunteers included 60 people from a local singles group, who spent most of the day in warm sunshine building a retaining wall down by the park's marina, raking a field near its scout camping area and repairing trails that wind through the steep, wooded terrain.

The 60 singles were from the D.C.-area chapter of Single Volunteers, an Internet-based group founded by two Washington area women in 1997. The organization has since grown to include chapters in 13 other U.S. cities as well as in Australia and Canada.

"It's a win-win situation," said Vicki Steele, 38, of Alexandria. "Organizations that need help but don't have the money get help, and we are able to meet nice people, interact and do community service."

Her friend Monica Schaefer, 37, a systems analyst, was more blunt.

"I'm doing it to meet people--single men my age," she said, looking around at the dozen twenty- and thirty-something men who, along with a dozen or so women, were busy raking the field so grass could be planted. "So far, so good," Schaefer said.

Park Authority staff members, many of them working on their day off, concentrated on showing volunteers what to do and offering some gentle instruction.

Maurice Jackson, 58, a maintenance worker at Pohick Bay golf course next door, told the earnest group of rakers that volunteers didn't need to pick up every stone and pebble. "The grass will grow around them fine," he advised.

"You don't like to say too much," Jackson added as the volunteers continued to stoop down for small rocks, tossing them into a wheelbarrow. "You don't want to make any criticism . . . but if you give them a little pointer . . . the work will go faster."

At the marina, Glass and a half-dozen other volunteers were finishing a retaining wall started last year on National Public Lands Day. The park's four-member staff had been unable to get to it in the meantime.

Croson, the park's carpenter, demonstrated how to use a router to sand the edges of the long slabs of wood to give them a more finished look when stacked.

"Wait a minute," he said to Glass as she aimed her drill bit at the "X" marked on the wood. "It's going backward."

After the direction of the drill bit was reversed, Glass pressed it into the wood and then withdrew it. "Yea," she said, smiling at her handiwork.

CAPTION: Volunteers and members of the Northern Virginia Park Authority help move pieces of timber that will become a retaining wall at Pohick Bay Regional Park.