There is a foundation comprised of two sections of cement on the south side of the arboretum path. Rising up from the soil like ancient English stone formations, it may have been a foundation for a farmhouse, but there is no confirmation of this. There are sections of the foundation that look like chimney supports. Even though the Pony Express usually stopped in Sacramento and the mail was transferred to steamboats sometimes a rider would pass by here on his way to San Francisco. There is a monument to this effect on the other side of the path to the southwest from the ruins.
At some point someone made a temporary labyrinth in the western half of the foundation. Comparing the photos it looks like the walls have broken down quite a bit in the time since the one with the labyrinth was taken.
further notes and photos
Some local high school students use it as a place to smoke weed and eat shrooms.
I heard once (but I can't remember when, or who I was talking to) that this place used to be water storage in the summer months when Putah Creek (which used to flow through what's now the arboretum) dried up in the summer. The walls never were very high. This was of course "back in the day".
Old waste water treatment facility?