The stones marking the boundary of the District of Columbia were set in 1791 following the survey of Maj. Andrew Ellicott and his assistant, Benjamin Banneker, a free black astronomer and mathematician. Cut from Aquia sandstone, the markers were spaced approximately one mile apart. The intermediate stones originally stood two feet high, while the corner stones were three feet high. Of the total of forty stones marking the district’s 10-mile boundary, fourteen were in Virginia; Virginia’s surviving twelve are included in this designation. Their condition varies. Some preserve original inscriptions, others have been reduced to stumps. In 1915 the Daughters of the American Revolution erected iron fences around each of the markers. The Virginia portion of the district retroceded to the Commonwealth in 1846; the Virginia markers now roughly define the western boundaries of Arlington County.