The Arlington County Board last week approved an agreement with the state to begin construction next September of a three-acre pedestrian park atop the I-66 underpass in Rosslyn.

County officials say that Rosslyn Plaza Park will be a green oasis in an otherwise stark urban setting and that it will dress up one of the gateways to Virginia when it is completed in 1986. No traffic will be diverted during construction.

The park, to be built in two levels, will be in the shape of a rectangle located between the eastbound and westbound lanes of Lee Highway and North Lynn and North Nash streets.

Fort Myer Drive will divide the park at street level, but plans include a 40-foot-wide deck over Fort Myer Drive with ramps coming from the two sides of the park and connecting to the deck and the existing Rosslyn skywalk. An entrance for the handicapped will be available through the RCA building on North Moore Street, which is connected to the Rosslyn skywalk.

Other walkways will connect the park to the Rosslyn North Building and the property of the Key Bridge Marriott Hotel.

Rosslyn's office workers and hotel guests can expect a large landscaped park with trees, benches and fountains, where they can relax amid the steel and concrete skyscrapers and the whirl of highway traffic.

Plans call for a natural amphitheater, with room for hundreds of people, to be recessed into the park ground.

County recreation division officials plan to schedule art exhibits, concerts, lectures and festivals for the park.

An elaborate landscaping plan calls for more than 150 large trees, such as flowering cherries, maples, honey locusts and lindens, more than 3,000 juniper, yew and holly shrubs, and more than 12,000 ground-cover plants, mostly ivy.

In addition, there will be three fountains, including one designed as a horseshoe-shaped waterfall, a quarter-mile of wall seats, 25 benches, rest rooms and a 10-foot brick sidewalk around the park.

The park will be built with $15 million in state funds. Ninety percent of the money comes from the Federal Highway Administration's trust fund, using tax receipts from gasoline sales, said Henry S. Hulme Jr., Arlington county's public works director.

Funding for the project came sooner than state and county officials expected, the result of people buying more gasoline, Hulme said.

Almost $7 million of the $12 million needed for the park's structural support has been spent on concrete beams and columns that motorists can see as they drive through the I-66 underpass.

For its part, the county expects to spend about $175,000 to furnish the park with the fountains, seating sculpture, benches, trash cans and bicycle racks.

The county, after assuming a 99-year lease of the park from the state, also will be responsible for its security and maintenance.

Key Bridge Marriott Hotel general manager James Fullerton said that the park will benefit tourists and business in Rosslyn. "The I-66 highway separated this hotel from Rosslyn for over 20 years," Fullerton said.

"It'll be good to be part of the Rosslyn community again," he added.

The National Park Service plans to beautify the area between the park and the entrance to Key Bridge, where the trolley used to turn around, Hulme said.

"You'll see sort of a grand entrance into the state of Virginia," Hulme said.