A spectacular park site on the bluffs of the Potomac River near Leesburg was deeded to the Northern Virginia Regional Park Authority yesterday by its 85-year-old owner who said her "chief interest in life is to preserve the beautiful, wild riverfront."

Frances Valiant Speek, who has lived at Red Rock overlook for 30 years made a gift of half the $265,000 value of the 67-acres tract just above the confluence with Goose Creek on Loudoun County.

The Virginia Commission of Outdoor recreation provided the park authority with a grant to pay the other half.

Park officials said the tract, to be named the Red Rock Wilderness Overlook at Mrs. Speek's request, will be opened to the public in two or three years. It will be reserved as a wilderness area, as specified in the deed. Hiking andnature trails will be develops, but there will be no facilities for picnicking, camping or more active patimes that would damage the area's sensitive ecology, officials said.

Mrs. Speek, the widow of a Labor Department economist who emigrated to the United States from Estonia after the 1905 uprising against tha Czar of Russia, said yesterday that it had been "my dream to preserve this special site on the river."

She said she and her husband had first seen the site with its 100-foot bluffs over the Potomac in the mid 1900s when the house was a shell and the area was overgrown with briars.

"We fell in love with it but it was three or four years before we were able to buy it," she said. The couple then restored the house and grounds and developed walks through the woods.

"I felt last year that I was unable to care for it as I always have and realized I couldn'tkeep it up the way I wanted," Mrs. Speek said.

Mrs. SPeek, the park authority and the state then worked out the gift-sale of the park, which commands a sweeping view of the river, the Maryland shore and distant mountains.

Park officials said the heavily wooded property abounds in deer, fox, wild turkey, heron and egrets among nearly 40 species of animals and 145 species of birds.

They described it as "ecologically fragile" land, wwich includes about five acres of flood plain rising into cliffs. Most of the alnd is rolling and hilly with several steep ravines cutting through it.

Officials at the regional park authority, praising Mrs. Speek's generosity, said that she had deeded the land fully to the authority, retaining only a life-time privilege of use of the property.