Millions of years ago, a forest covered the land. Trees fell in the swamp and silica replaced the wood leaving a perfect replica of those trees. Unfortunately, treasure seekers removed a lot of the petrified wood but what is left is still impressive.
The Agate Bridge is reinforced with concrete
It was late in the afternoon, the low sun permitted us to be in the photo as shadows.
This had been a tall tree at some point.
These hills (there were many) are called "teepees".