At the bottom of Cobbler Mountain, jutting from the foothills of the Blue Ridge about 50 miles west of Washington, a turning point has been reached in a long-fought development battle that may foreshadow bitter controversy over some of Northern Virginia's richest farmland and lavish country estates.

Real estate developers are in retreat. Advocates of farmland preservation are displaying new political strength. And Cobbler Mountain, dappled splendidly in russet autumn foliage, has assumed a political and philosophical meaning.

"That is the best farmland in Fauquier County," says Hope Porter, a leading conservation advocate. "It should never be developed -- ever, ever, ever."

"It's been a disaster," says Jane Mellott, a Cobbler Mountain developer. "We just gave up completely . . . . We're selling out and moving out of the county."

For more than a decade, government planners, landowners and developers in Fauquier -- a still largely rural enclave of dairy and cattle farmers, millionaires' estates, horse breeders and fox hunters -- have watched the steady spread of Northern Virginia's suburbs. Counties closer to Washington, such as Fairfax and Prince William, have boomed. Fauquier's population is rising. Some suburban-style development has already occurred, mainly near Warrenton and other Fauquier towns. And I-66, whose Fauquier stretch was opened this year, has moved the county 10 minutes closer to Washington.

At the same time, however, the political climate has shifted toward preservation. A key election two years ago brought in a County Board majority favoring a slower growth strategy. This year, the board tightened curbs on development through a controversial zoning measure. Preservation groups have stepped up pressure. And the nationwide real estate slump is taking an increasing toll.

"There's no boom town out here," says Richard E. McNear, the county's planning and zoning director. "That's not what you'd call growth mania."

Among those who favor the stricter controls on growth are many farmers, some wealthy owners of large estates, and others who have chosen Fauquier for its rural ambiance. Their arguments range from a deep-seated concern for agriculture to fears of higher taxes and urban ills. Opponents of the recent zoning measure include real estate developers and investors, other business operators and some farmers, worried that their lands' value may decrease. They view the regulations as infringing on property rights.

Nowhere is the preservationist trend more evident than at the foot of Cobbler Mountain, which stands above rolling farmland near the Fauquier town of Marshall and an I-66 interchange on what once was U.S. Chief Justice John Marshall's property. The mountain overlooks hundreds of fertile acres that have been a testing ground for the conflicting land-use forces embroiling the county.

When Jane and A.W. Mellott, transplanted Arlingtonians, started selling off their first 326 acres of land near Cobbler Mountain in the mid-1970s, they encountered no opposition. They called the development Lake Athlone. Today, all this land has been sold, and suburban-style houses are still going up on its 5- and 10-acre "farmette" lots near the Mellotts' artificial lake. "We grossed a little over $1 million out of it," Jane Mellott says.

By 1978, when they began their second Fauquier venture, prospects had changed. Protest mounted. "People just didn't want it. They said it was prime agricultural land," Mellott recalls. "Yet it had been on the market for two years. Any of those people could have bought it if they wanted to keep it prime agricultural land."

Eventually the developers skirted their opponents by settling for a scaled-down project -- one that did not require approval by the county's elected officials. They named it Athlone Hills. Instead of 20 or more home sites, the Mellotts split their land at the foot of Cobbler Mountain into just 14 lots, ranging from 5 to 25 acres. Today, only six lots have been sold and four houses built.

The Mellotts' third venture marked a reversal. It was to have been built on a second portion of the same 388-acre site on which Athlone Hills is situated. It was to have been called Athlone Mews. Instead, what ensued was a political battle and a lawsuit. At one point, some official records mysteriously disappeared from a county office, prompting an apparently futile state investigation. Several months ago, the Mellotts conceded defeat.

"Everything we did was within the letter of the law," Jane Mellott charges. "It's taking the right to someone's land away." They will be lucky to realize $1 million on the second and third ventures together, she says -- some $700,000 less than they once anticipated.

For preservationists, the significance of the Mellotts' retreat has been underscored by a later event. A 683-acre farm near Cobbler Mountain was recently purchased for more than $1.5 million by a foreign corporation, reportedly representing an Arab investor.

Before the new zoning ordinance was adopted, a preliminary plan was submitted to split the land into approximately 10-acre tracts. After the new law took effect, the proposal was withdrawn. Joseph W. Trundle, a lawyer for the buyer, says he cannot explain the move, but preservationists view it as another victory.

"This is the primest of the prime," said Judy Almquist, a preservation leader, as she drove past the recently purchased farm on a tour of Cobbler Mountain's surroundings. She is secretary of the Cobbler Mountain Citizens Protective Association. "You can see what this area is. It's nothing but one farm after the other, after the other -- not at all the appropriate place for a subdivision."

Not only would development destroy productive farmland, she contends, but it would also result in higher taxes stemming from new demands for schools, roads and other public services. It would, she says, alter the scenic character of the area and bring about inevitable conflicts between farmers and suburban commuters.

"What you have is minibikes in your cornfield," remarked Almquist, whose family lives on a small farm. "It may sound trivial, but it really isn't to a farmer."

Some farmers elsewhere in Fauquier agree. On a big dairy farm near Opal in the southern part of the county, Cecil L. Lomax talked wistfully about nearby farmland on which a development sprang up several years ago. Teen-agers from the development have smashed his tractors' headlights and vandalized other farm vehicles, he said. He has gotten complaints about the stench of his 300,000-gallon manure tank. "They're not used to this sort of odor," he said. "They should have smelled before they bought. That's the price they pay for the good life."

On another dairy and cattle farm south of Catlett, R. Wayne Arrington recalled how a nearby development project led to arguments about long-established farm boundaries. "We have enough to do farming, never mind fighting off nuisances like that," he said. "Development and agriculture do not mix."

Not all farmers object to development. Giles Early, another southern Fauquier farmer, wants to sell his 578 acres and complains about the recent zoning restrictions. "They're taking my equity away from me," he says. "It could cost me $1 million."

Fauquier's development controversy differs markedly from preservation fights in more urban areas. In this pastoral county, a 5- or 10-acre tract is regarded as small. The issue is suburban sprawl, not urban blight. Development usually entails single-family homes, not apartment buildings, offices or industrial complexes. Preservationists say they do not object to new housing ventures within the county's principal towns; their opposition focuses mainly on suburban-style development in agricultural areas.

Nor is demand for new housing surging. The biggest housing development outside Warrenton is in receivership, with 132 lots still unsold. Melvin K. Helmick, its owner, blames high mortgage interest rates and the economic downturn. Only a few thousand commuters a day are using I-66 from Fauquier, according to preliminary estimates, though highway officials expect the number to rise. Population has grown only about half as fast as once predicted, reaching 35,889 last year, a 36 percent increase over 1970.

Fauquier developers themselves cherish the rural atmosphere. "Even though I'm a developer-broker, I love this county. I don't want rampant growth," says Christopher Anthon. And as Karen Cosner, another Warrenton developer, notes, some disagreements center on just what farmland can remain economically productive. Development, she argues, should not occur on "working" farmland, but it is a reasonable alternative for land used as a tax shelter or incapable of being profitably farmed in today's economic climate. Sometimes, she adds, development has "cleaned up land that's overgrown."

The recent shift toward preservation, reflected in the turnabout at Cobbler Mountain, is already stirring a reaction. A group called Legality in Land Use has been formed to oppose zoning restrictions. Two court challenges to the new ordinance have been filed, and additional legal tests are considered certain. "We're looking at a very fertile battleground," says Allen H. Olson, a lawyer for preservation groups. H. Ben Jones, who filed one of the suits against the zoning law, contends the county has exceeded its state-granted authority.

"We are accused of being no-growth in Fauquier County," says John B. Adams, chairman of the Board of Supervisors and a supporter of the controversial zoning law. "We certainly know that you can't stop growth. It's just that it should be directed in the proper places."