"In the 1920s, America began coast-to-coast Airmail service, but the pioneer pilots had trouble navigating the route, since navigation charts of the day were fugazi and you couldn't exactly pull over to ask a farmer for directions. And traveling at night, when it would have been most efficient, or in bad weather was impossible. To solve this Congress then funded these gi-normous arrow-shaped Airmail Beacons, some up to 70 feet long, to trace a route across the country."
It's pretty easy to spot from the satellite view. I may chain a bunch of these together to see if I can "fly" from San Francisco to New York. Others have found two so far: