Yorba Linda / Santa Ana Canyon field trip, Part 2

This is part two of my report on Saturday's field trip to see the remains Anaheim Union Water Co. canals in eastern Yorba Linda. (Click here for part one.) Today's photos show some of the interesting, mostly unrelated things we saw along the way. (There will be more about irrigation in what will be the third and final post in this series.)
.
The photo above shows our group near the "trail head." The construction in the background replaces apartments that burned down during the recent fires.
.
The photo below shows a few trees from the large Valencia orange groves that still occupy some of this area. They've been mostly neglected for years, but there's a plan underway to revive them as organic groves and start selling the fruit again.
My surveyor friends may have to help me out with the photo below. Does anyone know what a metal disk marked "F P No. 1" in an iron pipe means?
It's been a long time since cattle roamed this land, but here's a brittle bone that turned up in an area where the brush had burned away.
At one point we came across a clearing near the railroad tracks with lots of stone fire rings. Some were old and nearly obscured by hard-packed soil. Others, like the one shown below, were newer and more obvious. There were about seven rings total. I pointed out that this was a lousy place to start a fire. Steve said the rings were probably used by "knights of the road." Then it hit me: We were standing in the middle of a genuine "hobo jungle." If only John Hodgman had been there to appreciate it. I hummed "Big Rock Candy Mountains" most of the way back to the car.

Thanks to Susan Faessel for the first three photos in today's post.