When the moon comes up over Sotterly, an 18the century mansion on the Patuxent River in Maryland's St. Mary's County, the ghost of the fifth George Plater steals mournfully out of the secret closet where Sotterly's residents used to stash their valuables before river pirate raids. Clicking the dice with which he lost the mansion in a crap grame in 1822, he roams the grounds, observing, no doubt, that little has changed at Sotterly in the last century or two. Using Sotterly's unhaunted guest cottage - which we rented for a bargain $30 a weekend - as our base, we explored St. Mary's County, where little has changed since Maryland's first settlers landed here in 1634.

Descendants of these pioneers grew rich on tobacco. In the early 1700s, this planter class built great houses, like Portobello, Mulberry Fields, West St. Mary's Manor, and Sotterly. Many of the great houses are still there, but only Sotterly, begun in 1711, is open to the public on a regular basis (daily, June to September, 11 to 5; admission $2 for adults, 75 cents for children).

Docent Lisette Day led us on a detailed tour of the mansion pointing out the spectacular Chinese Chippendale staircase and the elaborate shell niches in the drawing room. Both were hand carved by Richard Boulton, an indentured servant at Sotterly who, when set free, built many of the area's fine churches.

"It's wonderful that we can go right into a room rather than being roped off," said a man with a group of visitors from Johns Hopkins University.

Sotterly is owned by a foundation, although Mrs. Mabel Ingalls, who donated her family mansion to the foundation, is occasionally in residence, which probably contributes to Sotterly's comfortable, live-in look. So do the frequent luncheons, receptions and meetings held here.

At the dock at the end of the dirt road down which the planters used to roll hogs-heads of tobacco, a boatload of visitors arrived, picked up the phone, and requested a ride up to the mansion - a service provided free if you pay the price of a tour.

Maryland's founders also arrived in St. Mary's by boat, touching land near the mouth of the Potomac on a small island they christened St. Clements. We stood on the porch of the St. Clements Island - Potomac Museum looking about a mile across the water at the small island and thinking that it probably hasn't changed a bit since the colonists landed. Each September - this year on the 24th - a blessing of the fleet commemorates the landing and visitors are ferried to the island. Groups who want to visit the island at other times can arrange it by calling the museum at 301/766-2222.

The colonists' boats - the 400-ton Ark and the 40-ton Dove - didn't linger long at St. Clements but followed a trader named Henry Fleet to a site on the St. Mary's River, where they founded St. Mary's City.

We made the trip to St. Mary's City by car - pausing briefly to see the 400-year-old Milestone Oak which was already a big tree when the colonists arrived. But we learned that a replica of one of the original colonists' boats - the Dove - is due to sail into St. Mary's City in October. The reconstructed Dove will tie up the main wharf and become a permanent tourist attraction.

The reconstruction of the Dove is a project of the St. Mary's City Commission, which is painstakingly and accurately recreating enough of the original settlement to give visitors a feeling for it. It is not a Williamsburg, though it almost became one. When the Rockefellers were looking around for a colonial city, they came first to St. Mary's City, according to local residents. As a condition for developing it, however, the Rockefellers insisted that all buildings be reconstructed on their original sites. The problem was that a graveyard occupied the original site of the state house. The local people refused to move the graves, so the Rockefellers moved their money to Williamsburg.

The old statehouse was built in 1674-76 at a cost of 300,000 pounds of tobacco, the local currency of the day. The statehouse was rebuilt about a hundred feet from its original site in time of Maryland's 300th anniversary, in 1934. This summer, in addition to being open for tours, the reproduced State House will host a series of plays by candlelight, selected to fit the setting. (For information about this series or about the series of Baltimore Symphony concerts to be held at St. Mary's College, adjacent to the state house, call 301/863-8522.)

On the lawn of the statehouse, overlooking the quiet St. Mary's River, the commission has created a microcosm of St. Mary's City frontier life for summer visitors. Every Saturday and Sunday afternoon actors will farm tobacco, run a print shop and probate office, and raise razorback hogs. The sets of this living theater lie just a potsherd's throw from the site of the commission's archeological dig - Aldermanbury Street, the probable main drag of 17th-century St. Mary's City. The site is lying fallow now, while archeologists sift and analyze the findings. But a completed dig at a house named St. John's can be viewed through picture windows.

In the archeological lab, scientist Henry Miller has about 300,000 bits and pieces the residents of St. John's threw out. From one of many file drawers, Miller produced a painstakingly glued-together storage jar. "This was made in the colonies. How can you tell? Look at the red specks. English clay didn't have them. This was probably made by Morgan Jones, a potter on the Virginia side of Potomac, about 1670."

At Toddy Hall Pottery, a 10-minute drive from St. Mary's City, potter Michael Olson is reproducing brown-glazed cups that were in everyday use in St. Mary's City in the 1700s. "I worked from the shards the archeologists found," said Olson, who displays his wares in a semi-enclosed studio built to resemble a St. Mary's tobacco barn. "The original cups were made in England, probably at potteries in Staffordshire."

Olson's reproductions sell for $3. He is one of several young artists and craftspeople who have settled in St. Mary's County and form its embryonic art colony. Headquarters for the group is the St. Mary's County Creative Arts Forum Gallery in Lexington Park. The exhibitions change often, but you can almost always see paintings and drawings by promising local artists Charles Merritt and W. Scott Broadfoot.

Right across the road from the gallery is the Patuxent River Naval Air Station, the county's largest employer. The station is normally closed to the public, but on Sept. 9 the gates will be thrown open to welcome visitors to the annual air show, which will feature the Blue Angels, the Navy's performing jet aces.

In stark contrast to the jets are the horse-drawn buggies used as the sole means of transportation for the county's Amish population. On any Wednesday, county roads echo with the clip-clop of wagons heading for the Farmers Market in Charlotte Hall, where Amish farmers sell shoo-fly pie as well as fresh produce. The market also contains dozens of stalls with antiques and second-hand furniture and places to buy fresh fish and live ducks and rabbits. The Farmers Market is open on Saturdays, too, but most of the Amish stay home that day.

Founded by Catholics who were overthrown by Protestants in 1689, St. Mary's County has dozens of historic churches, most of them either Catholic or Episcopalian.

Ivy-covered Trinity Episcopal, adjacent to the reconstructed State House, was built from the bricks of the original State House, which was torn down in 1829. The Catholic congregation in the early days of the colony was St. Inigoes, whose successor church is located a few miles south of St. Mary's City. The tiny brick country church, built in 1785, is used for Masses only occasionally, but St. Francis Xavier Church on Newtown Neck, a narrow point shared by farms and seagulls, is still an active parish. Built by Jesuits in 1766, St. Francis claims to be the oldest Catholic church in continuous use in English-speaking America. St. Andrew's Episcopal Church, also built in 1766, was designed by Richard Boulton, former indentured servant by Sotterly.

If you're in St. Mary's County on a Sunday, by all means go to church. If you happen to be there on the first Sunday of the month, drop on visitors' day at the Harry Lundeberg School of Seamanship at Piney Point.

"I'd like to go to the Far East," said 20-year Frank Bender, one of the students assigned to show visitors around the school, which is run by the Seaman's International Union. Bender is currently learning about the Far East in a course called Ports of Call, held on a refurbished Potomac River excursion boat. On other boats tied at the school's dock, the students learn the basics of the engine room and the deck. They also take a turn in the school's cafeteria, prepping for galley duty. At the end of the 12-week course, the union gets the graduates jobs on American merchant ships.

Visitors to the school may see not only the ships used as classrooms but a mini-museum of Chesapeake Bay craft, including skipjacks, bugeyes, and log canoes.

Right across a bridge from the Lundeberg School lies Evans' Crab House, a reminder that the waterman's trade is alive and well in St. Mary's County. In an unassuming setting, Evans serves up the catch of local waters - oysters (in season), crabs and fish. Groups of ten or more who reserve in advance can order the "family-style dinner," a feast that may include soft clams, hard clams, oysters, scallops, fish, oyster fritters, crabs (served Southern Maryland style, with a spicy vinegar-based dipping sauce) crab cakes, fried chicken, cole slaw and hush puppies. The cost: $8.95 per person.

Water - the Potomac, Patuxent and St. Mary's rivers, the Chesapeake Bay, and countless creeks - is what St. Mary's County is all about. Unfortunately for visitors, most waterfront land is privately owned. A glorious exception is Point Lookout State Park, at the county's southern tip. A narrow point located where the Potomac River runs into Chesapeake Bay, the park has sandy swimming beaches, fishing areas and campsites. The park was once the site of a less happy camp, for Confederate prisoners of war. More than 3,000 Confederate soldiers died uner the unsanitary conditions at the camp and are commemorated in a memorial near the park's entrance.