The farm is on a spectacular strip of land across the river from Mount Vernon, and the corn grown there is the corn of the 18th-century settlers. Called Virginia Gourdseed because its kernels are shaped like gourds, it's the starchy, mealy, nonsweet maize the Indians raised. It grows 14 feet tall, and except for a tractor breaking up the soil once a year, it's cultivated by the broad hoe. Weed control is by pumpkin leaves.

Penned in by zigzagging split-rail chestnut fences, the cattle and the sheep, the horses and the pigs are modern breeds. But the pious hope is that their promiscuous breeding will throw back the varieties that lived two hundred years ago.

The project of recreating a typical, mid-18th-century farm of tidewater Maryland may take 50 years, says Dr. David Percy, director of the National Colonial Farm in Accokeek, which is a joint venture of the National Park Service and the Accokeek Foundation, a private conservation group. "Every year we add two or three crop varieties," he says. "We now have nine different crops as close to the l8th-century varieties as possible. Our aim is to approximate the 130 to 150 varieties that made for self-sufficiency in those days. We are looking for the closest approximations of the plants and livestock in use in colonial America."

Called "the nation's first living historical farm project," the National Colonial Farm was founded 25 years ago. The workers wear period costumes, and the farm is open to the public every day except Monday. Its three main crops are corn (the primary staple of the time), tobacco (the main cash crop) and wheat. The winter wheat is a variety called Red May, unchanged from the 18th century; the seeds came from the Department of Agriculture's seed bank.

The fields are on both sides of the cedar lane that leads from the gatehouse to the main buildings. The only historic structure is a tobacco barn, built around 1715, donated by Anne Arundel County. The other buildings are careful recreations: the main barn, which houses fodder for livestock and exhibits tools used by colonial families; the smokehouse, which is in regular, pungent use; the outkitchen, with displays of utensils, dried herbs and vegetables; and the privy, then called The Necessary, which is not in use.

The herb garden offers more than 50 varieties used by the colonists for cooking, medicine and cosmetics. Next to it is the kitchen garden, which produces such colonial favorites as Champion of England peas (for drying) and Cornfield beans. Many of the vegetable seeds come from Cornell University's historic garden and from gardeners affiliated with the nonprofit Seed Savers Exchange in Princeton, Missouri.

The sheep -- Hog Island and Dorset Horn varieties -- roam freely, and the hope is that they will cross and cross again, and eventually, say in 20 years, yield the original colonial sheep known as the Maryland Rat-Tailed, which no longer exists. Similarly, Chincoteague ponies and various types of cattle and swine are being crossed with other varieties, in the hope that somehow the nondescript common breeds of the colonial period will re-emerge.

The dozen or so turkeys are the original domesticated variety, known as Virginia Golden -- a breed that will soon disappear, says Dr. Percy, giving way to the modern turkey bred for white meat.

Now spread over 151 acres, the National Colonial Farm is part of a nationwide network of such farms -- "and some 50 of them are worth seeing," according to John Schlebecker, secretary-treasurer of Washington's Association for Living Historical Farms and Agricultural Museums.

Schlebecker says the National Colonial Farm is one of the best. "Our aim is to farm as people farmed in the past," he says. "We play-act as farmers of previous centuries, and we save plants, animals and ways of doing things."

But the motive is not only nostalgia. "There are today 15,000 cultivars (varieties) of corn," says Percy, an agricultural historian. "But our corn hybrids are based on six varieties. Which means that we are on a narrow genetic pinnacle. If a disease or insects attack two or three of those six varieties, we'd be in serious trouble. The same goes for all our main grain crops in the United States -- which is dangerous if not foolish. That is why the maintenance of heirloom seeds is extremely important."

Heirloom seeds are those seeds that have proved their value over time, but have lost out to modern hybrids that promise uniformity in size and color, better yields and disease-resistance.

"We are preserving the Virginia Gourdseed corn," Percy says. "This is the actual eating corn of the 18th century, the most important variety. This is really the Indians' corn, not the colored corn which was used for ceremonial functions. Our farm is the primary source of Virginia Gourdseed for the country. We get 60 bushels per acre -- which may not sound much if you compare it with the up to 300 bushels you get in Iowa. But it's not bad compared with the 90-bushel average in southern Maryland."

No money is spent on chemicals. "Perhaps because we don't use chemicals, our bird count is tremendous," Percy said, citing last year's Audubon Society count of 90 species. "The birds cut down on our insect population. Still, we guarantee one worm per ear of corn. That worm doesn't do much damage, just nibbles on the tip. Believe me, our corn is a great corn." NATIONAL COLONIAL FARM Take Indian Head Highway south from the Beltway (Exit 3A) 10 miles; turn right on Bryan Point Road. Open 10 to 5; closed Mondays and holidays. 301/283-21133