part of left over "historic" military installations in the marin headlands....right along the "hawk trail"...no idea what it could have been.. if you ever visit san francisco...go to the other side of the golden gate bridge and check out some breath taking vistas... http://en.wikipedia.org/wiki/Marin_Headlands
Sabine Stetson © 2013 to 2017 - Exemptions gladly given