The stone over time ages and turns a dark grey/black. The people who created the petroglyphs noticed that when you chipped away the outer part of the stone what was underneath was lighter in color. Through this chipping process they created pictures. Crosses are usually attributed to the first hispanics that moved up. Other works are not as easily to interpret since each group may have had a different meaning for the same symbol.
There are also two volcanoes on this park but unfortunately I didn't have the time to check them out.
The Visitior center is a good stopping point and they can point you to hikes from 5 minutes in length to several hours. If you want to get out of the city and get some exercise and fresh air, this is a great spot to start.

