On top of a small hill within the city limits of Walnut Creek, California in Acalanes Ridge Open Space is the remains of this aviation arrow.  Before our current GPS satellite & radar, airplane pilots who did transcontinental airmail routes at night would fly following these large cement arrows built high up on ridges or mountaintops, often accompanied by beacons with a light (though not useful on a day like this).  While there were over 1500 of them across the country back in the day, about 100 of them remain today, decaying & forgotten like this one.  This one is unique for having a double arrow, being the second arrow on the San Francisco-Salt Lake City route operating during the mid to late 1920s.  Pilots would follow the beacons & directions of the arrows using a compass to stay aligned w/ each arrow until they encountered the next one, directing them ultimately to Reno & then Salt Lake City.  Most of these sites are now gone, particularly during WWII as a means of making sure any enemy aircraft didn't use these features to scout out American soil.