Forsooth, there is a molten intruder in the tower.

But before you boil the oil, read the placard. The intrusion dates from millions of years ago, when a band of volcanic lava squeezed between the tower's sedimentary limestone layers. And the knights that you'll see on horseback this weekend are friendlies, participants in a jousting tournament set against a backdrop that looks like a medieval castle.

The seven towers known as Natural Chimneys, the highest of which rises 120 feet above the lawn of Virginia's Natural Chimneys Regional Park, brood like a Gothic ruin with a scrub cedar on top in lieu of a heraldic flag.

Some 500 million years ago, the Shenandoah Valley was a vast sea. Sediment from the sea formed limestone layers, and cataclysms beneath the earth pushed these particular layers to mountain height. Erosion carved towers out of the rock layers, and an underground stream cut tunnels in them.

Today, the tunnels form the interior of the castle. Intrepid tourists trek through them, but may be put off by the sight of a dead bat or two. Better to take the overlook trail from the parking lot to an observation perch level with the highest towers. Here, you can be the princess in the tower or the dashing prince who has come to liberate her.

Modern knights -- male and female -- will be at the park this Saturday for the 162nd annual Natural Chimneys Jousting Tournament, said to be the oldest continually held sporting event in the United States. At the cry of "Charge, Sir Knight," competitors gallop an 80-foot course and aim a lance through a dangling steel ring. This kind of jousting, part of a knight's training program, dates from 17th-century France and is a modification of an earlier, deadlier form of jousting.

The ghostly Gothic towers will provide a suitable backdrop for the event. With all those lance-wielding knights around, no molten intruder will stand a chance. CHARGING TO THE CHIMNEYS Natural Chimneys Regional Park is in Mount Solon, Virginia, a three-hour trip from Washington. Take I-66 to I-81 and proceed south to Mount Crawford. Go west on Virginia Route 257 to Bridgewater, then south on Route 42. A right turn on Route 731 will lead you to the park. An alternative is to take I-66 to Front Royal and take Skyline Drive south to Elkton. From Elkton, take U.S. 33 to Harrisonburg and Virginia Route 42 to the intersection of Route 731 and follow signs to the park. Usually it costs $2 per car to view the chimneys, but on tournament day the car fee will be waived and there will be a $1.50 admission fee per adult, 75 cents per child. The tournament, one of a series that culminates in the national championship October 16 on the Mall in Washington, will start at 2 and continue until about 6. At 8 there'll be more jousting, against the dramatically lit backdrop of the chimneys. The event will also include food, bluegrass music and arts and crafts of the Shenandoah Valley. A queen, selected in a beauty pageant this Friday, will knight the winners. Overnight accommodations are available nearby in Harrisonburg. The park itself has camping facilities. For more information, call 703/350-2510. MORE MERRIE JOUSTING MEN MECHANICSVILLE -- This Sunday the tournament scene shifts to Horse Range Farm in Mechanicsville, Maryland. The joust, which benefits the volunteer fire department, begins at 1 and includes a parade, tours of a little red schoolhouse on the grounds, pageantry and refreshments. Admission is $2 for adults, $1 for children. To get there: From the Beltway take Maryland Route 5 south to Mechanicsville and turn left on Route 6 to the farm. It's about an hour from the Beltway. COLUMBIA -- Knights in armor will charge each other this weekend and every weekend through September 25 at the Maryland Renaissance Festival at Symphony Woods, adjacent to the Merriweather Post Pavilion in Columbia. This is a re-creation of the deadlier form of jousting in which the knights wore armor and often dueled to the death. The festival also includes craft exhibits, medieval food, music and a costumed cast of thousands. Gates are open from 10:30 to 6 and tickets cost $6.95 for adults, $2.50 for children four to 11. OTHER JOUSTS -- Other Maryland tournaments will take place August 27 at 1 at Christ Episcopal Church in Port Republic, Calvert County; September 11 at 12:30 at Clear Spring, west of Hagerstown; September 18 at 1 at the Tucker Road Community Center in Oxon Hill; September 24 at 1:30 at the St. Mary's County Fair in Leonardtown; on October 1 at 11, the state championships will take place at St. Margaret's Episcopal Church in Annapolis, and on October 16 at 11, the national championships will take place on the Mall, near the Lincoln Memorial.