"This volcanic nook in the Berkeley and Oakland hills is alive with geologic hints to the operation of an ancient volcano. About 9.5 million years ago, Round Top was the center of a great volcanic cauldron. Over time, the outside of the volcano eroded, leaving the center exposed. Faults folded the volcano on its side, exposing the entire vent system. The main vent of the ancient volcano, as well as basaltic dikes, lava flows, and red-baked cinder piles can be seen along the Volcanic Trail. The hike is easy, along wide trails that climb gently. The route explores some volcanic deposits, then circles the core of the volcano before descending into the woods. There are good views to the east of Mount Diablo and the foothills, and to the north and south of endless rolling hills. Numbered signposts correspond with a trail guide available at the visitor center. Park activities include walking, horseback riding, and biking. No fee is charged."