On May 30, 1974 the [Green Spring] district was declared a National Historic Landmark. On December 12, 1977, the United States Secretary of the Interior agreed to accept preservation easements for nearly half of the 14,004 acres (57 km²) in the district. This means that the NPS does not own any of the land in the area but merely owns the development rights, ensuring the continuing rural and agricultural nature of the area.

From Wikipeddia.