Anybody ever heard of these? They came up on another forum that I frequent. They are large concrete arrows the USPS put in to guide postal airplanes after WW1. Some, most, have disappeared, but some still exsist. They can be located by their coordinates. Digger9 came to mind as there was one just west of Marengo.
There's background on them at: www.messynessychic.com/2013/11/15the-forgotten-giant-arrows
The list of them: http://surveymarks.planetzhanna.com/airway-beacons-list-western-u-s
Looks like there were 20 across Iowa total.
With the history behind them it seemed like a cool place to put some caches.