A tour of Maryland's Calvert County is a series of flashbacks through a time machine.

You can go back 15 million years to the Miocene Era, when the land was covered with a warm, shallow sea teeming with sharks, whales, porpoises and crocodiles -- you can collect their fossil remains in the cliffs along the Chesapeake Bay.

You can go back to the 17th century when English settlers planted tobacco and paid a tax in tobacco to build the churches that stand amid the fields where the county's still most important crop is just about ready for harvesting.

Or go back to 1814, when Captain Joshua Barney, with a motley collection of ships called the Chesapeake flotilla, staged a heroic but futile attempt to stop the British Navy's advance up the Patuxent River to attack Washington. Or to 1863, when Isaac Solomon started an oyster- packing business on what is now Solomons Island, a center for pleasure boats, working watermen and charter fishing boats. Or just back to the 1930s when excursion steamers like the Emma Giles brought daytrippers from Baltimore to swim and ride the carousel at Chesapeake Beach and to eat seafood at Solomons.

"I've been on it. That's how I come to put this stairway here," says LeRoy ''Pepper"Langley, Master Shipcarver at the Calvert Marine Museum and creator of thousands of model boats, including the Emma Giles. "I knew there was a stairway there, but it wasn't in the prints. So I looked through some old pictures until I found it. The Emma Giles used to leave Baltimore about 9 in the morning. The younger people would get off in Chesapeake Beach and ride the carousel and the dippers, and the older ones would continue down to Solomons for seafood dinners. There was an orchestra on the boat and dancing all over the deck.'

Unfortunately, you can no longer dance your way to Solomons on the deck of an excursion boat, but it's a good place to begin your tour. It's a traffic-free, 60-mile drive from Washington down Route 4, which is what Pennsylvania Avenue turns into, through low hills topped with tobacco barns. Isaac Solomon's oyster-packing house is gone, but the seafood dinners that drew excursionists from the city are still there. At SOLOMON'S PIER RESTAURANT, you can eat local seafood and gaze either at the Patuxent River or, through August 22, at a production of the Fantasticks (curtain time: 9 p.m.). There's an ice-cream parlor attached to the restaurant. BOWEN'S INN, a hotel long popular with customers of Solomons charter boat fleet, features similar fare but without the view. Bowen's best feature is its bar in a separate building where you can order a gin-and-tonic and carry it out to the terrace overlooking SOLOMONS HARBOR. Also on the harbor is Solomons newest restaurant, the DRY DOCK, part of the Zahniser Marina complex, which used to be a mom-and-pop boatyard with dogs on the lawn, but is now as fancy as anything in Annapolis. If you don't have your own boat, you can go fishing with one of the SOLOMONS CHARTER BOAT CAPTAINS (a list is available from the Tri-County Council for Southern Maryland, P.O.Box 301, Waldorf, Md. 20601, (301) 645-2693), or rent a small fishing boat at the H.M. WOODBURN AND CO. DOCK. If you just want to cruise, take a ride on the WM. B. TENNISON, an 1899 Chesapeake Bay Bugeye operated by the Calvert Marine Museum in Solomons. The Tennison, a nine-log chunk canoe converted from a sailboat to a powerboat, used to cruise among the oyster boats, buying the catch and hauling it to packing houses. Now it takes visitors on one-hour afternoon tours of Solomons harbor or on more extensive three-hour evening cruises. (Sailing times: 2, Wednesday through Sunday, at 4, weekends; fares for the one-hour cruise: $3.50 adult, $2.50 child; for the three-hour cruise: $8.00 adult, $5 child.) "This is one of the last boats still operated by ropes,'' says Captain Jim Tallant, a retired Navy officer, showing a child how ropes attached to the rudder respond to a turn of the wheel. From the wheelhouse, Tallant points out the sights: Isaac Solomon's old home; the old M. M. Davis shipyard, birthplace of elegant old wooden yachts; Molleg's Island, where dead sailors were often washed from their shallow graves; the J.C.Lore & Sons seafood-processing plant, now being retrofitted by the museum for an exhibit on the history of commercial fishing in the area. The museum's dock, where the Tennison is headquartered, also serves as the site of one of the area's most bizarre boating events -- races of a fleet of radio-controlled MODEL SKIPJACKS, many of them made by members of the Solomons Island Model Boat Club under the direction of Pepper Langley. The club meets at the museum the first and third Wednesday of the month at 7:30; the next race takes place at 1 on August 22. Inside the CALVERT COUNTY MARINE MUSEUM (open 10 to 5 Monday through Saturday, 1 to 5 Sunday; admission free), there's a little bit of everything, from fossils to Indians, birds and boats. One exhibit displays artifacts found by underwater archeologists excavating the scuttled 1814 Chesapeake flotilla. In the newest exhibit on the Chesapeake as an estuary, a disembodied human hand reaches down into a tank with a small net. "We'll put him over in the big tank," says biologist Ken Kaumayer, owner of the hand, who is setting up the estuary exhibit scheduled to open in late August. "We're still trying to guess which fish get along. Last night one of the toadfish ate one of the sheepshead minnows for dinner." The museum also exhibits works of such local artists as photographer August Selckmann, who has chronicled the death of the Cedar Point Lighthouse at the mouth of the Patuxent River. Another lighthouse, the DRUM POINT LIGHTHOUSE, escaped a similar fate when it was moved to the museum grounds in 1975. Now restored, the screw-pile, cottage-type light is open to museum visitors. "There's an elderly lady, Anna Weems Ewalt, who was born in this bedroom," says guide Bill McGilvery. "Her grandfather was the lighthouse keeper. She decorates the lighthouse every Christmas and she launders the antique linens on the bed. The furniture is all original to the period but not to the lighthouse. The only original furniture we have is this metal chair. It belonged to John Hansen, the last lighthouse keeper." Hansen, who retired when the lighthouse was decommissioned in 1963, will lecture at the museum at 7:45 pm on August 21. To see a working lighthouse, drive north on Route 4 and east on Route 497 to COVE POINT. The first time we visited, the foghorn was blasting and the beacon atop the 51-foot tower was flashing. A perfect time to visit, we thought, but the Coast Guardsman on duty explained very politely that visitors were welcome anytime except when the fog horn was blasting and the beacon flashing. Returning on a sunny afternoon, we climbed the classic spiral staircase of the 150-year-old brick lighthouse. The view from the top made even the scary last few feet up a ladder and through a trap door worth braving. If you want to gather your own shark's teeth, ray teeth, fossil scallops and look for ancient whale jawbones, stop at CALVERT CLIFFS STATE PARK, a few miles north on Route 4. The cliffs rise 120 feet out of the Bay and hold the fossil remains of the creatures who lived here millions of years ago when the sea that covered much of Calvert receded. There's a two-mile walk down to the beach where fossiling, but no swimming, is permitted. MIDDLEHAM CHAPEL, on Route 4 north of the park entrance, dates not from th west side of Route 4. A small, unassuming restaurant popular with county people, it has excellent homemade crabcakes (closed Sundays.) For something much more exotic, take a left a few miles up the road to VERA'S WHITE SANDS marina and restaurant. The restaurant -- pink, with portholes -- perches on a bluff overlooking beautiful St. Leonards Creek, but its spiritual home is Polynesia, or Vera's idea of Polynesia. Hawaiian music greets you at the entrance. Somerset Maugham or Paul Gaugin could be sitting in the booths shrouded by hanging glass beads; George Wood, wearing a straw hat and a Phillipine shirt, is sure to be at the piano bar, crooning golden oldies. Vera, in pink chiffon, a sequined hairband pulling her platinum hair from her tanned, time-weathered face, floats among the Easter Island gods, Egyptian goddesses, ersatz palms, fish baskets, Foo dogs and eclectic miscellany. Waitresses in halters and sarongs serve maitais, strawberry daiquiris and other tropical trouble. But the food menu, except for a Luau Platter at $12.95, runs to more conventional fare. Two crab cakes, with salad, vegetables and a slice of watermelon (they think pink, chez Vera) cost $10.75. At the Calvert Cliffs NUCLEAR POWER PLANT's Visitors Center -- a right turn about half a mile up Route 4 from Vera's -- you can travel from the distant past to the eerie future. There are the foundations of a house dating from the early 18th century and an excellent small museum in an old tobacco barn with exhibits telling the story of the area since prehistoric times. Then the theme shifts to energy; you can pedal a bicycle to see how much energy it takes to power a television camera, and play atomic tic-tac-toe on a computer. From there, it's an easy logical jump to the power plant itself. Since no visitors are allowed inside, there's an "audio-visual tour." Nothing, however, quite prepares one for the sight of the plant itself, seen from an overlook sheltered by a charming little wooden gazebo. It's unbelievably huge and ugly. As meaty a moral issue as nuclear power may be, a more immediate moral issue for the casual visitor to Calvert County is access to the waterfront. Most of the Cheseapeake coast is occupied by small cottage communities which, by might, right and threats to tow away your cars, bar visitors from the beaches. Fortunately there are a few places where, for a fee, you can get in on nature's good thing. The nicest beach we found was at MATOAKA COTTAGES (east on Long Beach Road in St. Leonard and follow the signs). The cottages, part of an old children's camp on a beautiful 40-acre preserve planted with vineyards, bamboo, holly and wild cherry trees, rent by the week. But for a fee of $1.50 per adult and 50 cents per child, we were allowed to change in an empty cottage and enjoy a lovely stretch of beach for a day. The owners, Larry and Connie Smith, rent swimming rafts, boats and crab nets and -- in case you can't find your quota of fossils -- sell shark-tooth necklaces. The beach is litter-free and noncommercial, so bring your own picnic. Bring some meat tenderizer, too, which helps relieve the pain of sea-nettle stings, a summertime fact of life in the Chesapeake. The Patuxent side of the peninsula, offering more sheltered achorages, draws fewer summer cottage builders and more working watermen. A typical watermen's community is BROOMES ISLAND, at the western end of Route 264, a few miles off Route 4. The island, attached to the mainland by a causeway, has the county's only oyster-processing plant, which operates in months with an R in them. Fishing a church bazaar and a southern Maryland chicken-and-seafood dinner, will take place here August 28 from 1 to 6. Admission is 50 cents. Trees with knees -- bald cypresses -- are the attraction at BATTLE CREEK CYPRESS SWAMP, a short detour off Route 4 on route 506. The knees, part of the trees' spreading root system, pop up in a marshy area teeming with birds and other wildlife, and may be viewed from a boardwalk trail. The trail and adjacent nature center are open 10 to 5 Tuesday through Saturday, 1 to 5 Sunday. Admission is free. Another beautiful old Episcopal church is ALL SAINTS CHURCH, at the intersection of Routes 4 and 2 in Sunderland. Built in Flemish bond with Palladian windows, it stands in a grove of hemlocks. (Sunday services, 8 and 10:30). From Sunderland, you can go west to the lovely unspoiled town of LOWER MARLBORO, an important port until the Patuxent silted over. Or travel east to CHESAPEAKE BEACH, which was "the beach" for Washingtonians before the Bay Bridge and the automobile made ocean beaches more accessible. The popular old carousel now has a new home in Watkins Park near Largo, but anAMUSEMENT PARK MUSEUM is in the old railway station at Chesapeake Beach. It's open Sundays from 1 to 3 or by appointment. Call 301- 855-6472. The town is now more important as a charter fishing center, and a head boat leaves at 8 a.m. daily from the dock at the ROD AND REEL RESTAURANT. It costs $15, plus $3 for a fishing reel and another $3 for bait. There's also a commercial beach at Chesapeake Beach and another -- with nets to keep out the nettles -- a few miles south at BREEZY POINT. At about the time Chesapeake Beach was enjoying its heyday, a country doctor named Chaney was seeing patients in his rambling old Victorian home on Route 4 in Dunkirk. Today Chaney's office home is a charming country inn called THE PENWICK HOUSE. Lunches and dinners are served Tuesday through Sunday (try the Maryland fried chicken with cream gravy and the chocolate pie), and there's an all-you-can-eat hunt breakfast on Sundays from 10 to 2:30 at $8.95 for adults, $5.95 for children.