Ellicott’s Stone, also known as the Ellicott Stone, is a boundary marker in northern Mobile County, Alabama. It was placed on April 10, 1799 by a joint U.S.-Spanish survey party headed by Andrew Ellicott. It was added to the National Register of Historic Places on 11 April 1973.It is the only known stone monument set by Ellicott when he surveyed the 31st parallel north latitude, which served as the boundary line between the Mississippi Territory in the United States and Spanish West Florida. The boundary line extended along the 31st parallel from the Mississippi River east to the Chattahoochee River, as set forth in the 1795 Pinckney Treaty, formally known as the Treaty of San Lorenzo. Ellicott's Stone is the initial point for all United States Public Land surveys in the southern region of Alabama and Mississippi. It is the point of intersection of what is known today as the St. Stephens meridian and the St. Stephens baseline.