EVERY WEEKEND the cars pour across the Bay Bridge headed for the ocean beaches, bumper to bumper in their haste to put all or most of Maryland behind them. The sandy stretches of the Atlantic shores are the target and, perhaps because of this, the southern part of the Free State remains on overlooked watery world sleeping in the shadow of the better known Eastern Shore.

St. Mary's County, mother of Maryland, is a land designed not for swimming but for boating. Its shores, innocent of vacation resorts, are dotted with plantations once owned by towering early-Maryland figures and its past is heavy with the history of the state's beginnings. This is the land of the crabber, the oysterman and farmer, evoking a simpler era. If you do not know St. Mary's County, you have missed a whole fascinating world.

The first capital of Maryland was here in St. Mary's City, which is not a city at all but a tiny cluster of buildings. Leonard Calvert and his little band of Englishmen landed here in 1634 in the Ark and the Dove, seeking to establish a colony where Catholicism would not be frowned upon. A replica of the Dove rides at anchor at the foot of the bluff, a reconstruction of the first State House has been built on the original site, an ancient cannon points its muzzle over the harbor. Close by, Trinity Church, a johnny-come-lately built in 1828, lifts its spire to the sky above an old graveyard. That's St. Mary's City.

But the fun of if is the way in which the history of the place reaches out to involve the visitor, spilling out from the handful of simple frame houses without walls built on the grassy land beyond the State House door.

Stroll out behind the State House to look at the cannon and, like as not, a young man in Colonial costume will pluck your elbow.

"And what do you be thinkin' about the price of tobacco this year?" he will inquire. And there you are, transported right into the thick of 1685. You might be asked to help find an indentured servant who has run away, or even to serve as a juror in the trial of John Coode, a Protestant rabble-rouser accused of collaborating with Canada and the Indians. No history book will put you in touch with the 17th century so quickly.

You can also be a voyeur when Mary Clocker, out of patience with her husband, Dan, steps down to the tavern, another structure without walls, to bring him home and help her with the farm work. Up the road lives Chancellor Phillip Calvert, whom you see has his hands full with a new wife, half his age, as the mistress of his home. You meet them all -- you can hardly help meeting them as they argue it out in the tavern or the kitchen of one of the open frame buildings used to suggest the settlement.

Down on the river below is the replica of the Dove where Captain Tom Doyle, a retired member of the Harbor Police, will show you over the vessel. Everthing here is as neat as a pin and kept that way with the help of Explorer Scouts who will be manning the tiller when the Dove makes a two-week visit to Baltimore June 30th. The Dove is not very big and you wonder how it must have been to ride her in heavy seas.

The summer sun shines bright over St. Mary's City and the perfect antidote to the unrelating glare lies round the bend of the road on Chancellor's Point. Here the Nature Conservancy maintains a self-guided trail which leads you along the shores of the pretty St. Mary's River and through a leafy forest preserved for us forever wild and safe from the onslaught of concrete highways. This was land purchased by Chancellor Phillip Calvert, into whose domestic life you have peeked back at St. Mary's City. From here you can get a lovely view of the Porto-Bello plantation across the river.

Cross Manor, Cellar Hill, West St. Mary's Manor, Tudor Hall -- St. Mary's offers a bouquet of handsome plantation homes. But perhaps the jewel of them all is Sotterley. Just 60 miles downtown Washington, Sotterley sits on a hillside near Hollywood, Md., overlooking the Patuxent River, a graceful survivor from the 18th century. Sotterley is open to the public, and a stroll through is like a visit to a country home whose host is absent, perhaps because it has always been lived in and used since its beginning in 1727.

For almost three centuries Sotterley's owners have looked down from the long veranda on the Patuxent River and a flock of sheep like those which still graze on the hillside below. Country hams hang in the smokehouse as they always have, and you can buy some slices or a whole one to take home. In the corn crib a succession of cats have been born, the last still strolling the paths underneath the rose-covered trellises.

James Bowles, an Englishman of means, built Sotterley. His widow married George Plater, in whose family it remained for several generations until, as legend has it, the last of the Platers gambled Sotterley away in a dice game in the red-walled library. For the next nearly 100 years Sotterley belonged to the Briscoes, whose ancestors arrived on the Dove and its sister ship, the Ark. In 1910 it was bought by Herbert Satterlee, who restored it to its original elegance.

It was the nearly Platers who turned the house from a simple plantation home into a mansion. Most of the work was done by indentured servants, especially by Richard Boulton, who carved the famous Chippendale staircase and the shell alcoves in the drawing room, which to this day is only candelit. One story has it that on the day his term as bonded servant was over, he picked up his tools and walked off a free man, leaving behind a tiny piece of moulding in the staircase still unfinished.

Pirates are said to have attacked Sotterley, so handy from the river, planning the attack for the daytime when they thought the men would be at work in the fields. But a hunt breakfast was in progress at the mansion and the riders routed the brigands. Secret loose panels and a passageway hidden in a closet attest to the fact that a hasty escape was never very far from the occupant's minds.

The bell in the William Cummens clock on the landing is thought to have been made by Paul Revere, all the windowpanes, at least in the library, are the 18th-century originals. The rugs in this room are Jail Agra, rugs Queen Victoria ordered to keep Moslem prisoners busy while serving sentences for refusing to grease cannon balls with pork fat.

Sotterley could do with a coat of paint and the floors undulate gently with the warp of centuries, but these things seem only the charming foibles of an aging beauty. The house is open and cool, always filled with flowers from the truly magnificent cutting gardens that stretch endlessly in the sunlight. To this day Sotterley is the setting for elegant parties, for though Mabel Ingalls, Herbert Satterlee's daughter, deeded it over to a private foundation, she returns occasionally with friends for a country houseparty.

St. Mary's City, Hollywood and Solomons, at the tip of Calvert County across the Patuxent, from a triangle that can be managed in a day's trip from Washington, though an overnight stay in one of the many budget motels makes it a more leisurely junket. In any case, don't miss Solomons, where the best seafood restaurants of the area are and where you can get a bit of a feel for the biology of the bay in the Calvert Marine Museum.

Take the new Governor Thomas Johnson Memorial Bridge across the river, well-marked-off Rte. 4 below Hollywood, and turn right off the bridge for the museum. You can't miss it because of Drum Point Lighthouse, clearly visible from the road and looking for all the world like a cozy cottage on stilts.

In the year before the bridge, Solomons was an isolated community of watermen and farmers at the tip of the peninsula. At one time its school bus was a boat, the old James Aubrey, which chugged along the shoreline daily collecting students for the high school. The door beyond the museum lobby celebrates this old boat, with a recreation of her hull complete with port and starboard lights still hanging.

The history of oystering in the bay is all laid out here, a tale of an industry sadly diminished from the old days when the J.C. Lore oyster packing house was a mainstay of the community economy. The museum has plans to restore and re-create the old outfit, but in the meantime you can study the progression of harvesting from the old hand tongs to the automatic dredge, which is now outlawed in Maryland except on Monday and Tuesday. You can also study crabbing methods and the models of old schooners, skipjacks and bugeyes. And you get introuduced to many of the marine inhabitants of the bay, not very unlike those who lived there 20,000 years ago.

They say the ghost of a young girl inhabits the cupola of the lighthouse, keeping company with the lens upstairs and occasionally opening doors that were left locked the night before. If so, it must get windy and lonely up there, and noisy when the 1,400-pound bell rings, which it does occasionally for the edification of visiting school children. This crew-type lighthouse is one of only three such cottage types remaining on the bay, and you can see a six-minute film devoted to the monumental job of moving it here to the museum from its original location at Drum Point.

One of the very nicest things you can do in Solomons is to take the river cruise in the Wm. B. Tennison. She's a bugeye whose bottom was built from only nine mammoth logs, somewhat in the fashion of a dugout canoe, though she has a comfortable super-structure and an awning to keep the sun off of part of the deck. She steams out to the bridge at 2 p.m. for an hour's ride each Saturday and Sunday, and nothing could be more pleasant than watching the shoreline slip slowly by as she moves out like a respectable mother hen among the fly-by-night small pleasure craft.

A representative from the museum goes along to answer questions and point out the osprey nest where a feathered brood of youngsters is being fledged. The nest looks like an untidy pile of sticks thrown into the channel marker where it is perched, but the mother looks down with enormous dignity on curious passersby. The departure is from the museum dock and, if you want a three-hour trip, you can get that later in the day.

The big treat of a southern Maryland adventure is of course the fish, which has to be as fresh as any you will ever find. Pier I is close by the lighthouse and the dock, and serves every variety of finny fare in a nice dining room overlooking the estuary. You can eat your soft-shelled crab and watch a family of geese making their way across the water in hopes of a handout, mother and father convoying the goslings with sober solicitude. The mallards here are as greedy as city pigeons and swarm around the harbor police when its crew docks for a restorative cup of coffee.

If you have time, stop on Saturday morning on the way down at the Farmers' Market at New Market on Rte. 5, just north of Mechanicsville. This is the most wonderful hodgepodge of kitch, antiques and produce imaginable, where you can buy anything from fresh-churned Amish butter to a larger-than-life plastic bust of Elvis Presley. You can also eat broiled pigs' feet and cornbread, or browse among the quite large selection of golden oak furniture, Victorian drawings of children and kittens, old shoes, pressed glass, cast-off clothing and junk.

P.S. You'll know you've left the city behind when you see that the High's down the road from the market features live bait.