"Hey, I think we've got something," Mike Johnson said. He bent down - and there, in the red clay furrow of a cornfield behind Friendly Village of Dulles, a mobile home park, was a prehistoric spearpoint, its quartz body sparking in the sun.

In a three-hour walk through the furrows, Johnson found a handful of points and other hand-worked pieces of rock, including, possibly, a hide scraper. By studying how the rocks are shaped Johnson can identify their approximate age.

Some of the artifacts dated from 4,000 years ago. Some went back 10,000 years.

To Johnson and other amateur archeologists, the cornfield behind Friendly Village is one of at least 60 sites in Fairfax County where the imprint of prehistoric man can be found.

Although the sites may hold information that can illuminate a generally dark period of this area's past, the archeologists fear that many of them may be irretrievably lost as Fairfax attracts more new subdivisions, shopping centers and the streets and public services that the new residents require.

A case in point is a site at Nutley Street and Lee Highway (Rtes. 29-211) which is being bulldozed for a shopping center. Asked by Johnson to do what he could. Fairfax Supervisor James M. Scott (D), whose Providence district includes the area, contacted developer Cyrus Katzen, who, Scott said, has given permission for an archeological survey to be made and for artifacts to be removed.

"It's something," Johnson said, "but that doesn't mean the site will be preserved."

Scott said it's probably too late to preserve the site or any part of it. "If we knew about it a year ago, maybe we could have done something," he said.

The Fairfax County Board of Supervisors and the county's History Commission, however, are trying to preserve the sites that remain. The supervisors unanimously endorsed a resolution Monday calling on the county staff to explore how archeologically interesting sites can be protected from development.

The county's History Commission has formed a resources study committee that hopes to get professional and amateur archeologists interested in identifying sites before developers decide to build houses or parking lots on them.

Already, according to Johnson, who represents the Northern Virginia chapter of the Virginia Archeological Society on the resources study committee, many sites have been lost. Until recently hardly anyone knew that Fairfax was such a rich repository of evidence of early man - Indians who hunted and fished centuries before English settlers came.

One of the richest areas, according to Johnson, is the Accotink basin, which runs from just north of Fairfax City in the slightly southeasterly direction to Gunston Cove below Ft. Belvoir.

Accotink basin contains some of the densest development in the county, including the area arounf Fairfax Circle. But while some identified settlement sites have been lost - Circle Towers, for example, was built on one - others have survived.

Jon Brazier, a 15-year-old archeological colleague of Johnson, found a fluted spearpoint from the paleotithic period (10,000 to 12,000 years ago: not far from Fairfax Circle.

The ravages of time - primarily the highly acidic soil in Fairfax that dissolves even bones - have destroyed everything but stone artifacts and pieces of pottery. But even these remnants have a story to tell.

The different shapes of the spearpoints, for example, generally can locate the time of settlement. Johnson said acidic soil had long since obliterated wood dwellings at other Northern Virginia archeological sites but chemical stains indicate where posts were placed. From all the stains together, the pattern of living arrangements could be inferred.

While Fairfax's colonial heritage has received considerable attention, and numberous efforts have been made to catalogue and preserve it, little has been done, until recently, to document the county's prehistoric period.

"We're fairly in advance of the bulldozers out here," Johnson said during a break in his survey of the cornfield near Friendly Village. "We have some breathing space. But they're on their way."