Surveyed in 1799 to mark the 31° North Latitude, this line charted the first southern boundary of the United States, separating the U.S. from Spanish Florida. The line was marked at one-mile intervals by earthen mounds approximately fifteen-feet square and three-feet high with a charred lighter-pine post at the center, hence the name Mound Line. Jointly surveyed by Major Andrew Ellicott , U.S. Commissioner, and Esteban Minor, Spanish Commissioner, to determine boundaries as agreed in the Treaty of San Lorenzo in 1795. The line began at the Mississippi River, ran east along 31° North Latitude to the Chattahoochie River, thence eventually to the Atlantic Ocean. Stockton (Alabama) was divided by this line, with some residents living in the United States and some in Spanish Florida. Although Stockton became a "border town," U.S. law generally prevailed in the area.
Related to the Mound Line, is the more famous Ellicott Stone, marking the boundary between the United States and Spanish West Florida.
This map shows the northern border of Florida as the 31° North Latitude boundary line established by Andrew Ellicott.