Nobody notices those things. Or do they? In July 2017 I got an interesting note from a bridge player, Ken Rhodes, who knew well the answer because, providentially, he long lived in the answer,note 1 Virginia Beach. He explained:
Prior to 1963 the city of Virginia Beach was a narrow vacation strip extending along the Atlantic coast from Cape Henry (at the mouth of the Chesapeake Bay) south to Sandbridge, which is well short of the North Carolina border. It was a successful summer beach resort, but little else. In that year the state legislature approved a consolidation of the city with its county, Princes Anne County, and the city suddenly became a behemoth of sorts, encompassing approximately 250 square miles of land and an equal amount of water. The population jump from 1960 to 1970 thus occurred primarily on a single day in 1963.
Interestingly, that was also the year that Nashville, TN, experienced a similar jump (not in percentage, but in number of new residents) for the same reason — Nashville consolidated itself with Davidson County.
|note 1 After some reflection I believe it's possible to inhabit an answer.|