On Saturday, October 10, 1767, astronomer Charles Mason and surveyor Jeremiah Dixon arrived at what is now the Mason-Dixon Historical Park. At the high point now known as Brown's Hill, their Mohawk Indian guides forbade them to go farther west because an existing treaty between William Penn and the Proprietors of Penn's Woods, later Pennsylvania, and the Iroquois Six Nations allowed passage only to that point, where the north-south Catawba Trail met the east-west Warrior Path along a crest between the Monongahela and Ohio Rivers. On Sunday, October 11, 1767, they set up their instruments on Brown's Hill and spent the next ten days completing survey observations. Mason recorded that at 233 miles, 17 chains, 48 links, they set a post marked W on the west side and heaped earth and stone around it. That post later rotted away and in 1883 was replaced by the Sinclair stone, which stands on the exact spot where they set their last marker in 1767. The Mason-Dixon survey had been commissioned after inaccurate grant descriptions and poor survey work caused an eighty-year boundary dispute between lands granted to William Penn by King Charles II and lands granted in 1632 to the Proprietors of Maryland under Charles I, a dispute settled in 1763. The Royal Society of England then nominated Charles Mason and Jeremiah Dixon to use the latest scientific methods to establish the boundary line. Beginning in 1763, they also surveyed the Pennsylvania-Maryland border, the Maryland-Delaware boundary on the Delmarva Peninsula, and made the first accurate calculations of the true lengths of one degree of latitude and longitude. Because their work relied on precise astronomical terms and references, it was called the first and longest accurate survey in history and a scientific feat of the highest order. At Brown's Hill the line was beyond the western edge of the Maryland grant, but at 233 miles it was still 23 miles short of completion by Mason and Dixon. After further negotiations between Pennsylvania and Virginia and a new treaty with Shawnee and Delaware Indians, the line was completed in 1784 by the American scientists David Rittenhouse and Andrew Ellicott.