Picket Post Mountain
"A landmark and lookout point during Indian wars, site of outpost of Camp Pinal which was located at head of Stoneman Grade to the east. Soldiers protected Pinal City and the Silver King Mine from Apache raiders. It was the home of Col. William Boyce Thompson, mining magnate and founder of the southwest arboretum at the foot of the mountain."
The plaque is located just east of the entrance to the Boyce Thompson Aboretum which can be seen in the lower part of the photo looking past the marker toward Picket Post Mountain.