Another half mile of Potomac River shoreline near Leesburg has been preserved under a scenic easement granted to the Northern Virginia Regional Park Authority last week by a former Associated Press correspondent and his family.

The easement on 66 acres of wooded hillside and river frontage, overlooking the C&O Canal and Sugarloaf Mountain across the Potomac, has been donated by Vinton Liddell and Robert S. Pickens and their two daughters, Jane Church and Cornelia Suhler.The perpetual easement protects the property against any future development.

The park authority last year acquired a nearby 67-acre farm, with 200-foot bluffs and another half mile of river frontage.

Opened this summer as Red Rock Wilderness Overlook, it includes a 19th century farmhouse, barns and three miles of nature trails. The park authority's 10th park, Red Rock, was sold at half its appraised value by Frances Valiant Speek and acquired with the help of a $132,500 matching grant from the state Commission on Outdoor Recreation.

Algonkian Regional Park, with 516 acres and 15 miles of Potomac River frontage, is located near the Pickens' property. Like Red Rock, it was acquired under the park authority's long-range program of preserving Virginia's Potomac shoreline from Harper's Ferry to Great Falls. The Maryland shoreline is all within the National Park Service's C&O Canal National Historical Park.

In 1974 the Pickens family offered to sell the park authority the whole 280-acre farm, which dates to a 1719 land grant from Lady Fairfax, but "when it became clear that it (the park authority) couldn't afford to buy and we couldn't afford to give away the property, we started working out an easement that would protect as much of the land as possible," said Vinton Pickens, an artist. The easement on the 66-acre tract is valued in excess of $100,000.

The Pickenses bought the land in 1935, named it Janelia Farm in honor of their two daughters, and raised cattle and a breed of horse known as Percherons. The land abounds in wildlife, with park authority naturalists counting 145 pieces of birds and at least 37 mammals, including deer, fox and wild turkey. The land is posted against hunting and will be accessible to the public in about two years when the park authority builds a trail to it.