Louisiana Purchase State Park is a state park located near Blackton, Arkansas, commemorating the starting point of land surveys made of the 1803 Louisiana Purchase. The park encompasses 37.5 acres (15.2 ha) of swampland, a landform which is regionally in decline due to agricultural development practices. The centerpiece of the park is a 6-foot (1.8 m) granite marker place in 1926. The park was designated a National Historic Landmark in 1993 in recognition for the location's importance in the surveying of much of the western United States.