6. District of Columbia South Cornerstone
A temporary cornerstone was placed here in 1791, the first of 40 stone markers to demarcate the 10-mile square of the Federal City. A permanent stone, erected in 1793, is visible in the seawall below the Jones Point Lighthouse. Alexandria was a part of the Federal District until 1847, when it was retroceded to Virginia. African American farmer, Benjamin Banneker, stayed in the base camp on the Point to perform the survey's mathematical calculations and maintain the accuracy of the astronomical clock. This exploit and his almanacs led to his public recognition as America's first black man of science.