Northern Virginia's newest public golf course opened last week, a rugged 18 holes created on the Pohick Bay Regional Park on Mason Neck at a cost of $2.5 million.

The course is built on the hilly grounds that surround a 1740 plantation. The property was acquired by the Northern Virginia Regional Park Authority in 1967 to prevent a housing development, and a study concluded that a golf course would be the best way to preserve the property and make it available to the public. It is the first public course in southern Fairfax County.

The 460-acre plantation was bought for about $1.2 million, at $2,600 an acre. Construction of the golf course cost about $1.3 million.

The brick plantation house, now the clubhouse, was known as Lebanon because of its many cedar trees. It was built about the same time as Mount Vernon and just before George Mason's nearby Gunston Hall.

The regional park authority's only other golf course, Algonkian, was acquired in 1974, also to forestall development and to preserve the Potomac shoreline in Loudoun County.

Unlike Algonkian, which is a relatively flat and easy course, the Mason Neck course is so hilly the park authority suggests most golfers will want to use golf carts instead of walking the course. Park officials have provided powerful gasoline-engine golf carts (electric carts can't negotiate the hills) which rent for $7.28 for nine holes and $12.48 for 18. Green fees at the new course are $4.25 for nine holes, $7 for 18 holes on weekdays and $1 more on weekends. Weekday discounts are given players under 15 and over 60.

The new course still has an unfinished look since landscaping and upstairs restoration of the plantation house are incomplete.

There are no spectacular views overlooking Pohick Bay since the course is laid out through woodlands that have grown up around the plantation house. But there is a small man-made pond beside two holes.

Park authority officials said that even if golfers cannot see the river they may occasionally see bald eagles overhead, as well as ospreys and the numerous marsh birds that nest on Mason Neck and feed in the upper reaches of the bay near the old plantation.

The course was inaugurated Wednesday when John F. Herrity, chairman of the Fairfax County Board of Supervisors, took swipes at several golf balls--he replaced his divots--for an audience of 50 persons.

Retiring Alexandria City Council member Nelson Greene was in the first foursome and won a preliminary putting contest. Greene was a member of an Alexandria golf course search committee that more than a decade ago identified the site as an ideal spot for a golf course.