Archeologist Stanley South isn't worried about the security of his spectacular new discovery. It's being guarded around the clock by U.S. Marines.

"Usually," South said yesterday at a press conference held to announce his find, "we'd be vague about exactly where we were digging. But with those Marine guards, we don't worry about people coming with metal detectors or trying to enlarge their own collections of old Spanish pottery."

South's discovery - the location of the oldest known fort in the United States - will involve the rewriting of history books and the revision, somehow, of an embarrassingly incorrect monument. The 400-year-old Spanish military fortifications were found, aptly enough, on the Marine base at Parris Island.

So far, the excavations have been restricted to the rough area of the golf course on the Marine base, and South said that "we're going to try to avoid the fairway."

He added that several golf balls had come flying near the excavators while they were digging in the first week of July. "We may have to get hard hats on the next expedition." he said.

Joseph Judge, associate editor of the National Geographic, which will help support further excavations, calls it "one of the major historic sites in the United States."

"The people who went to boot camp there will be amazed," he said. "They never knew there was a golf course, for one thing."

What has been discovered is the location of Fort San Felipe, which was well known from historic documents but had never been pinpointed geographically until now. From 1566 to 1587 the fort, the city of Santa Elena which it protected, and a later fort called San Marcos, were key elements in the bitter struggle between the French and the Spanish for footholds in the "Florida" territory. At that time, the name of "Florida" was given to all the land from the Gulf of Mexico north to the polar regions and from the Atlantic Coast west to what is now New Mexico.

In its heyday, Santa Elena was a bustling metropolis of 400 people, and it alternated with Saint Augustine (which was founded a year earlier) as the capital of Florida.

At the time, annual shipments of $10 to $15 million in gold were going to Spain from the New World through the Florida Straits, and France was trying to establish bases on the American mainland from which it could raid this shipping. Until now, it has been generally believed (though some scholars challenged this theory) that the old fort on Parris Island had been a French settlement called Charlesfort. On July 4, South's excavations uncovered the 14-foot moat that had surrounded the fort and some pottery that identified the spot as a Spanish settlement - not Charlesfort, but Santa Elena.

"Now that we have identified this site, we will still have to look for Charlesfort," said Robert Stephenson, director of the Institute of Archaeology and Anthropology at the University of South Carolina, which sponsored the first excavation and will co-sponsor further work.

At the press conference at the National Geographic building, Gen. Robert H. Barrow, commandant of the Marine Corps, said that the service would support the archeological work as much as possible. Still to be found are the town's plaza, houses and church, as well as the remains of other fortifications that were erected at various times in its short life span.

"But if you want to go and dig on the fairway," the commandant told the archeologists and other scholars, "we will negotiate what you can do." CAPTION: Picture, Stanley South with pot neck from Parris Island, by Douglas Chevalier