They're having a party Saturday down by Pierce Mill. Rock Creek Park is 90 years old this month, and there'll be clowns, balloons, carriage and pony rides and things to eat, from hot dogs to the dishes of 24 countries. It's called a birthay party, but the craggy valley is thousands of years old, the remains of a mountain range you can still see in some places. Algonquins hunted and fished there long before September 27, 1890, when Congress named it a park.

Now it's a haven for dreamers and, more than anything, a playground, with tennis courts, a golf course, bike and bridle paths, an exercise course, picnic groves and hiking trails. It has an amphitheater, an art gallery, a trove of biology and botany materials and a working mill; and a formal garden, Dumbarton Oaks, and hundreds of plots where people can plant anything they like. All in the heart of the city.

And it's a worldly little place, a favorite of visitors and the virtual back yard for one of the biggest collections of diplomats in the world. So Saturday there'll be Barbados' sweet cocoanut bread, ginger beer and the heady drink, mauby; Indian puri bread with chickpea curry and samosas stuffed with spiced potatoes and Pakistan's variety with spicy minced meat. Bolivians and Chileans will bring empanadas, turnovers of olives, raisins and onion-flavored ground beef and the like wrapped in thin wheat pastry; milhoja, a sweet pasty of about 30 thin layers with sweet creamy filling, and humitas, a cornmeal mixture boiled in corn leaves.

Thai classical dancers will follow a Pakistani fashion show and Chilean folk dancers; crafts, jewelry, ceramics and such will be shown and sold; an American Indian dance group will powwow in ceremonial costume and skaters will do roller-disco. A hot-air balloon will sail if the wind's right, and the old mill wheel will show how grain was ground in the 1800s.

The salute will go on from 1 to 5 Saturday near the Art Barn. The Park Service will have a free shuttle bus from Carter Barron, 16th and Kennedy Streets NW; Tilden Street will be closed to traffic, but hiking, biking, jogging and skating are encouraged.