"Giant 70-foot concrete arrows that point your way across the country, left behind by a forgotten age of US mail delivery. Long before the days of radio (and those convenient little smartphone applications), the US Postal service began a cross-country air mail service using army war surplus planes from World War I. The federal government funded enormous concrete arrows to be built every 10 miles or so along established airmail routes they were each built alongside a 50 foot tall tower with a rotating gas-powered light. These airway beacons are said to have been visible from a distance of 10 miles high."
Via Seth Dixon