This was kind of an interesting discovery at Bonny Doon. As I mentioned the hills are made of sandstone, and from my earlier photo, you can see that it is easy for people to carve graffitti into the rock. Another thing that has been done is carve these ridges into the rock to make it easier to get down off of the hills. The sandstone is very grippy, so one doesn’t slide down all that much, but on the steeper inclines these gooves have been carved so people can put their feet in them and, sort of, slide down.
Note from the future 10/19/2012: wow! Did I pull this explanation out of my ass, or what? Ha! It is kind of hilarious to read this many years later. A REAL geologist corrected me via the contact form and said this about the grooves:
I stumbled across your site looking for photos of where I live in Bonny Doon California. I saw the very nice pictures of the Moon Rocks you took.
Being a geologist, I could not resist pointing out that the grooves in the rock were(are) formed by water erosion not carved into by people for better footing.
Just though you would like to know.
Thank you for setting me straight! 🙂