Fairfax County supervisors yesterday approved a plan to convert most of the Lorton prison complex into a 2,500-acre park and keep a small portion for a residential neighborhood, preserving one of the largest remaining tracts of open land in the region.

At a marathon meeting, the supervisors also were debating early today the future of Evans Farm, the subject of bitter wrangling between a developer and McLean residents, many of whom hoped to preserve the 24-acre parcel that had served as an oasis for families who came to feed the ducks and pet the horses and other animals.

And the supervisors again delayed approval of Cox Communication Inc.'s purchase of Media General Cable, expressing frustration with Cox's refusal to respond to customer concerns.

The Lorton prison, operated by the the District on 3,200 acres of federally owned land along Interstate 95 north of the Occoquan River, is scheduled to close in 2001. The question for county officials, developers and those who live near the site in southeastern Fairfax has been what happens next.

Yesterday's vote sets the stage for a complicated land deal with the federal government to transfer ownership of most of the property to the county. Supervisors hailed their action as a victory for advocates of open space and parkland.

"We hear a great deal of concern from our constituents about growth and development," said Supervisor Gerald E. Connolly (D-Providence). "Here's a wonderful opportunity to be able to do the right thing."

The 9-to-1 Lorton vote, which creates a new area of the county to be called Laurel Hill, could transform the area in the next five years if all the pieces fall into place. Those pieces include swapping part of the Lorton land for some private land on Mason Neck that the federal government wants.

The vote came after an hour-long public hearing during which more than a dozen speakers urged the board to approve the plan. Casting the lone "no" vote, Supervisor Elaine N. McConnell (R-Springfield) said she wanted more revenue-generating commercial and industrial uses for the property.

But Andrew Nicholls, a representative of the Sierra Club, praised the board for making a commitment to land preservation.

"Lorton is one-fifth the size of Arlington County," he told supervisors. "It represents a remarkable opportunity. Things like this don't come along very often."

Carol Bowie, president of the nearby Newington Commons town house community, which recently came under a barrage of errant bullets from a Lorton firing range, said the area's "long-suffering citizens" are eager for the plan to be implemented. "I urge all of you to transform one of the major blights in Fairfax County into a wonderful area," she said.

The plans approved for Lorton yesterday--which foresee as many as 1,500 homes on the property, including a 450-unit residence for the elderly--are a far cry from previous proposals. In the 1980s, a massive project was envisioned that included 10,000 homes and several high-rise offices. As recently as a year ago, a citizens group proposed to allow as many as 3,900 homes and two large town centers.

County officials and activists wanted to limit development because of concerns about adding more traffic to the I-95 corridor.

Even with the board's approval yesterday, the transformation of Lorton is far from certain. First, there is the issue of cost.

The federal government is willing to sell most of its land at a reduced price. Even so, the county estimates it may have to pay as much as $16 million for the land.

The county then would have to rehabilitate or demolish buildings now on the site, most of which need new air-conditioning and sewer hookups. And there is the question of who pays to clean up environmental problems on the property, which was used as a dumping ground for years.

Development of Lorton also depends on a complicated land swap required by Congress. Yesterday's vote makes possible the swap, in which a private landowner on Mason Neck would trade 900 acres of undeveloped land there for about 235 acres of the Lorton site.

The federal government would use the Mason Neck land for an educational facility for its wild horse and burro program. In exchange, the family of landowner William Lynch would get the Lorton property, with permission to develop up to 1,500 homes there.

Lynch and the federal government are still negotiating details of that swap.

On the cable television question, the supervisors' one-week delay in voting on the Cox-Media General deal comes less than a month after they directed their staff to negotiate with Cox about lowering the county's cable rates and improving customers' equipment and viewing choices.

Supervisors criticized Cox yesterday for not addressing those issues.

"You have made no commitments," Connolly told company executives.

Supervisors also were critical of agreements that lock owners of apartment buildings, hotels and businesses into contracts with Media General for the life of the county-approved franchise. Supervisors offered, then withdrew for a week, a proposal that would force Cox to rewrite those agreements.

The county's attorneys warned supervisors that such an action would probably be illegal, and Cox executives reacted angrily.

"It's disappointing," said Claus Kroeger, Cox's vice president for operations. "We're ready to invest $200 million to $300 million to upgrade this system. That is being held up by private interests."