The big ol' flat rock just lay there, all 70 or so pounds of it. Sixty-year-old Johnny Nalls, his steamy breath swirling around him in the cold, eyed it suspiciously.

"I'm gonna tie that rascal down low," he said finally when he had decided where to place it. "Don't want to rassle with that."

For the last five months, Johnny Nalls has been rasslin' with stones from a tumbled-down stone fence that's at least as old as he is. In fact, it's probably a good deal older. He has been sorting and chipping and rebalancing stubborn globes of gray stone, one on top of the other, since October. And even though the fence is less than 100 yards long, he expects to be still at it come spring.

"It takes a long time to build a proper stone fence," said Nalls, who can rebuild about five feet of old fence a day. "It takes so long that you get to feel it's gonna start fallin' down behind you--catch up with you, you know--before you can ever get finished."

Nalls, a stonemason for 34 years, is one of a handful of local men who can still build stone fences the proper way: with smoothly tapered sides and each stone exquisitely balanced and fitted. He can make them track straight across fields or edge tightly against dirt roads. He can snake them around old trees in gentle arcs and he can make their ends come out neatly canted. And he can do it as easily without using concrete ("dry wall," they call it) as he can with concrete. It is a craft he learned from men who had learned from their fathers, rough-edged men all of them.

"They're all dead now," Nalls said. "Lee Brill, Edgar Hurst . . . All of 'em. Nobody's learnin' how to do it anymore."

Just why is no mystery.

"I remember one time Mr. Riggs--he owned Brook Hill farm near Middleburg--Mr. Riggs said to Harold Lovett, who I worked with, 'What's gonna happen when you old people are gone? Why don't any of the young men take up the trade?'

"Well, Harold Lovett looked at him and said, 'Because it's too much damned work, that's why.' " Nalls laughed. "And that's the truth."

There are hundreds of miles of stone fencing in Fauquier and Loudoun counties. It laces the countryside, gray and simple. Most of it was built before the Civil War, and a goodly portion predates the American Revolution. Almost all of it was built by poor farmers or the slaves of the rich. Immensely skilled they were, because so much of their work still stands despite season after season of repeated nudgings by frosts and livestock and groping vines.

There's not much that can be done to stave off the inevitable decaying--unless you have unlimited time to rebuild them yourself or Blue Ridge-sized mountains of money to bankroll the $15-to-$25-a-foot rebuilding cost. Now, if you wanted to build one from scratch--that means finding the right kind of stone, having it trucked in, having the foundations dug and laid--it would probably cost $50 a foot and up.

Because of the enormous cost, almost all of which is labor, usually only the very rich have their fences repaired with any regularity. Many of them are beginning to find that it's cheaper simply to have a bulldozer dig a trench and bury the old piles of stone underground. If they still want a fence, they can put up wood at about $5 a foot. But it is painful to see the stone fences come down.

"You have to think how much work went into them," said one prominent Middleburg realtor recently. "The settlers had to first clear a field. They cut down the trees. Then they had to pull out the old stumps. Then they began tilling the fields. That's when they'd start to find the stones. So they'd pile them up as they ran across them in the fields, until after a few more years they had uncovered enough to build a fence."

The real work began then. A three-foot-wide trench was dug two or more feet deep for the entire length of the fence. It had to be deep enough so that the foundation was below the frost line. Then the biggest stones were piled in the trench until they reached ground level.

Next, two stone walls, their bases about three feet apart, were built up and inwards from the ground. Small scrap stone was used to fill the middle. Large, longish rocks, like that which Nalls decided against hefting, were lain crosswise between the two outside walls so that part of them protruded out from each side. Ties, they were called, because they tied the two different walls together.

"When I rebuild a wall, I don't disturb the base because it's already settled down in the ground good," said Nalls, who is working on a fence on property owned by multimillionaire Paul Mellon near Upperville.

"It's a little more complicated workin' on fences than most people think," Nalls said as he tapped on a piece of stone with a battered hammer. "These old stone fences are made out of iron stone. You can't split it so it isn't good for building houses. These, you keep beatin' on 'em and all you end up with is gravel.

"It would drive you out to the bughouse if you let it, trying to get 'em to fit. But I love it. It's just like a jigsaw puzzle."

Nalls learned stone craft from a man named Cleave Ryan, dead now many years. "He told me that one time stonemasons could come in and have their own pattern, their own style, and you couldn't change 'em." But times got modern, and people started ordering the type of stonework they wanted in spite of the style of the craftsmen, which took some of the beauty and dignity out of the work.

"Well, they're payin' fer it, aren't they?" Nalls told Ryan.

"Well, gawd damn!" replied Ryan, "you ought to do all right in this business." Just then a truck slowed as it passed Nalls and the fence, which is about three-quarters done.

"That's the most work you done in six months," teased the driver as he leaned out of his cab. "If you don't slow down, you'll get finished by spring."

"Maybe," said Nalls, smiling in return. "Maybe."