Arrows Across America

I read recently about these huge concrete arrows that were laid out every 10 miles from coast to coast across the USA in 1924. Their purpose: to help guide the postal planes on their long journey. A beacon was also attached to a tower to ensure the arrow's visibility from high altitudes.
Read more at http://www.messynessychic.com/2013/11/15/the-forgotten-giant-arrows-that-guide-you-across-america/ - with thanks to DeAnna for this one!