Portola Redwoods is set in a natural stream basin of mixed evergreen redwood forest, featuring old-growth and second-growth trees. On sunnier hillsides the redwoods give way to hardwoods and shrubs. The dense under-story is mostly huckleberry and Tan Oak. Big Leaf Maple, ferns and other shade-tolerant plants are found along the creeks. The park is also home for a surprising variety of wildlife including an diverse bird population. Pescadero, Peters and Slate creeks flow along fault lines in the earth where oil seepage and marine fossil deposits hint at the complex geologic history of the area.