These arrows were once used as navigation for the U.S Air Mail pilots. These were active in the 19th century. The arrows would also have a light beacon beside them and a small metal hut for the people who worked at the location. The light beacons would have a small light in them to help guide the pilots during the night.
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The arrows are now considered as landmarks and were stopped being used when radar systems started to appear.
More information and history in the links below.