"That's a water-rolled rock," says Mike Johnson, Fairfax County's prehistory archeologist, kicking at a rounded stone imbedded in the ground, "so this must have been a riverbed at one time. And over there," he says, pointing to a gully, "was a road.

"Historical," he sniffs.

We are standing in the middle of a woodsy hill somewhere in Mason's Neck State Park at the southern tip of Fairfax County, along with the county's history archeologist Ed Chatelain, a couple of college interns and a class of 20 teenagers from the county's enrichment program. We're supposed to look for Indians or "evidence of prehistoric occupation," as the archeologists put it. Actually, we're trying to get our bearings on a contour map -- and on a time map that runs in the archeologists' heads.

"Up here, we're more likely to find historical stuff -- the colonists liked to be able to see," Johnson explains. "But the Indians never lived too far from water -- why carry water? -- so they'd be down in the ravines," he says, heading downward.

This survey of the park is part of Johnson's ongoing effort to map the prehistoric occupation of Fairfax. Since 1978, he and Chatelain have mapped about 10 percent of the county, finding more than 350 areas of Indian occupation.

That's nothing compared to Johnson's teacher, American University Anthropology Professor Charles McNett. Along with Catholic University's William Gardner and George Washington University's Bob Humphrey, the professor has chalked up 1,200 prehistoric sites along the Potomac Valley.

And more will soon be unveiled, McNett says. "There are artifacts scattered all over the city of Washington -- if you dig nearly anywhere that hasn't been disturbed, you'll probably find a few. In fact, one of our foreign students was sitting on the bench at a soccer game in our playing field and kicked over some pottery and a projectile point with his foot."

Feet seem to be an important tool in the "dirt archeology" of the U.S. Unlike the findings of "Raiders of the Lost Ark" fantasy, most artifacts in America are still quite close to the surface -- especially in Washington. "It rains a lot here," Johnson observes with classic understatement, "and the artifacts are either uncovered or washed down."

A shout from George Mason University intern Dave Taft sends us scampering up the hill. Taft, a geology major, has found an "outcropping of cobbles" (archeology-speak for a "bunch of rocks") of the type that Johnson says attracted Indians.

What that means in Virginia reads like a Monty Python menu -- quartz, quartz, quartzite and quartz, with an occasional soapstone thrown in for making bowls, and a piece or two of flint or obsidian imported from the mountains, says Johnson.

The pile Taft shows us reveals a few stones chipped by nature -- nothing authentically Indian. But if you go up that hill, across the top, down another ravine and across a summer stream, you'll find a quartzite cutting tool waiting for Johnson's return. The edge has been chipped to a lethal edge, the back is still rounded and easy to hold, and it's just lying there on the ground.

"We don't have to take every artifact," says Johnson, casually. "We just have to know where they are." He marks it on the contour map, whose squiggly lines are becoming less certain as the morning wears on.

Similar cutting tools were used by nearly all the area's Indian occupants. Stone tools and flakes have been found along the Potomac that date back at least 12,000 years to the hunting and gathering peoples who entered the area. "We all assume they were here before that," McNett says cheerfully, "but nobody's found anything older -- yet -- in the valley."

One reason may be the changing landscape -- much of the area occupied by native Americans more than 7,000 years ago is now underwater, Johnson says. "So finds that are older than that tend to be in the uplands, or above the fall line upstream of Great Falls."

The people who were in this area that far back were not so much tribes as small groups of extended families, says Wayne Clark, staff archeologist for the Maryland Historical Trust, and the man who probably knows the most about occupations along the Maryland side of the river. Their numbers were small, and the land was plentiful enough for these Indians to subsist on hunting and gathering -- an existence calling for highly skilled toolmakers.

Many archeologists have attempted to duplicate these tools in order to figure out just what was involved in their making. Johnson's Annandale lab has a shelf-full of arrows, spears, weights, axes and catapults that he's made in his spare time.

"We go out to Tysons Corner to the meat locker where hunters store their deer and get the leftover deer legs," he says, pointing to a spearhead attached by deer tendon to a two-part shaft.

The upper third of the shaft lifts off, so the spear can convert to a knife, and the spear itself is couched in a kind of wooden crook that Johnson uses as a slingshot. It's the sort of contraption that gives rise to the legend of American inventiveness, and Johnson is obviously proud of his work.

Work on such toolmaking occurred wherever there was a plentiful supply of rocks, so at every outcropping we pause -- and put the kids to work. Twenty trowels come out of 20 back pockets, as the teens scurry around like experienced chickens with their teacher urging them to "keep scratching."

"What are you looking for?" I ask, nudging some leaves aside cautiously with my foot.

"Anything," says one. "Points, maybe."

"What have you found?" asks Johnson.

"Dirt," moans another.

The history archeologist has admitted that he's lost, and the adults gather for a map-reading consultation, while their free labor brings out canteens of cold water and sunflower seeds to munch.

It's a snack the area's earlier occupants would have approved of: According to McNett, the first plants of human cultivation in this area were probably "Jerusalem artichokes and sunflowers." That was more than 4,000 years ago, when the weather had warmed up and the human population increased, making agriculture a more reasonable lifestyle.

Other easily grown crops began to make an appearance until, by the time of Christ, the corn, beans and squash we traditionally associate with the Indians played a big part in their culinary lives.

The sedentary life caused other technological and social changes. Soapstone bowls gave way to pottery -- "crude, mud-like things at first," says Clark, "that looked like the sort of thing your kindergartener might make." The Indians progressed to decorated items tempered with sand, crushed rock or shells. Mason's Neck has already yielded a few pieces of this more sophisticated work, Johnson says, and shards of it can be found all over the Potomac Valley.

By the time the native occupants were decorating their pottery, they were also associating in definable language groups and settled areas."Nobody's going to roam around carrying a load of pottery on their backs," says McNett.

Outlines of houses and forts have been found in such places as the water treatment plant near Sibley Hospital, along the George Washington Parkway and at the upper end of the C&O Canal. These belonged to at least three separate groups -- the Algonquin speakers, the Susquehannocks (who spoke a type of Iroquois), and the Nacotchtank, also called the Anacostians, who lived where their name suggests.

This last group, according to McNett, set themselves up during the colonial period as "middle men for the fur traders, buying cheap and selling dear. They dealt with nearly everybody, but nobody liked them -- it's the banker syndrome."

It was not so much the appearance of the white man but the start of the fur trade that wreaked havoc with the Indians' delicate social balance, McNett feels. "It made 'em fight each other," he says succinctly.

Whites reacted to the Indians in a variety of ways already familiar to most Americans: They used the Indians as allies in the French and Indian War (the Algonquins sided with the French and the Iroquois-speakers with the British), and they pushed them out of their native lands (the Susquehannocks left almost immediately after John Smith arrived in the early 1600s, running from both the whites and the Iroquois). The Piscataway, a group of Algonquin-speakers in southern Maryland, were hustled onto reservations in the Zekiah Swamp and Nanjemoy Creek in the 1670s, according to Clark.

By the 1690s, however, the Indians had had enough and went north to join the Iroquois in New York -- the same place they'd descended from hundreds of years earlier, Clark believes. A dozen or so families stayed behind, though -- ancestors, possibly, of at least three different groups in southern Maryland now claiming to be Piscataway.

These groups are none too pleased with the prospect of archeologists "picking over their ancestor's bones," says Steve Potter, archeologist for the national Capital Area Park Service. "You can understand that. But they have this image of archeology as it was in the '30s and '40s," he says,"when amateurs in this area left sites looking like the down side of an Army mortar range."

But most recent archeology in this area has not been full-scale excavation but "site location and testing," says Johnson. "Development is going on so fast that if we spent time excavating one site, we'd lose another 100 sites to the bulldozer." A federal law requires an archeologist's okay before a VA/FHA loan can be granted, and state and local archeologists spend much of their time surveying land to be developed, as well as public parks.

The time frame in the park we're in has suddenly jumped ahead a few thousand years, as the natural woodland gives way to a tangle of historic-occupation-type plants -- honeysuckle, strawberries, jonquils. Large trees give way to brush and vines; the students, hacking their way through, are sure there's a house nearby.

Here, then, is an important hint to potential future archeologists: Historical archeology takes you into once- cleared land now full of nearly impenetrable brambles, whereas prehistorical archeology takes you on digs along wide-open defunct streambeds -- an excellent reason to stick to the Indians. LOCAL DIGS -- Mike Johnson takes out volunteers almost every weekend from March through October and works with them in the lab during the winter months. For information, call the Archeology Department, Fairfax County Planning Office, 642-5807.