Robert Sibley Volcanic Regional Preserve

The region has remnants of rocks from the Orinda Formation and the Claremont Formation.[1][2][3] From the Orinda Formation rocks include "river gravel, sandstone, and mudstone." The river gravel are red in color. The sandstones are around 65 million years old. From the Claremont Formation, rocks include marine rocks.[2]

The surface of the basalt found on the dike of Round Top are of the color "light brownish or rusty gray" and the interior of the basalt is of the color "gun steel blue."[2]

A bake zone is a region where heat from the lava of the volcano changed the color of the surrounding rocks to red. Such rocks include rhyolite tuff, lapilli agglomerate, and vesicular basalt.[2] Rhyolite tuff are remnants of ash from a northern volcano around 4 miles (6.4 km) away. Lapilli agglomerate are remnants of volcanic cinders. Vesicular basalt are remnants of gas bubbles from cooling lava.

The Stone Property is a restricted region of the park. The area consists of pure basalt around 100 feet (30 m) wide, as well as "autobrecciating basaltic flow" which occurs when lava continues to flow under a cooled surface.[2]

• East Bay Skyline Regional Trail, which is 31 miles (50 km) long, traverses the Sibley preserve on the ridgeline that separates Wildcat Canyon Regional Park and Anthony Chabot Regional Park.
• Round Top Road goes from the Sibley visitor center to the peak of Round Top. CLOSED
• Round Top Loop Trail circles Round Top's peak.
• Volcanic Trail, once a quarry haul road, contains most of the stops on the self-guided volcanic tour.
• Quarry Trail connects the middle of Volcanic Trail to a point lower down on Quarry Road.
• Pond Trail is a short trail segment that descends to a couple of ponds on the north side of the preserve.[1]

American vetch is native to California and can be found at Sibley Volcanic Regional Preserve.[1]

At least two man-made labyrinths exist within RSVRP. The first, and arguably most frequently visited, is known as the Mazzariello Labyrinth. Constructed in 1990 and donated as a "gift to the world" by East Bay resident Helena Mazzariello,[lower-alpha 2] it is a favorite destination for hikers who come to pray, meditate, and examine talismans left in the center by previous visitors.[9] In this labyrinth, she would walk her goats, stating that, "I can enter with a question, and inevitably, I will emerge with some insight. It is a powerful spot."[2] Notwithstanding the official gate hours listed below, Friends of the Labyrinth claims that the labyrinth has visitors 24 hours a day, 7 days a week.[8]

The next marker along the Round Top Loop Trail leads to the Volcanic Trail, which crosses to the left. At Post No. 4, the hiker can see a smaller, heart-shaped labyrinth. It is not nearly as elaborate as the Mazzariello Labyrinth, and apparently not as heavily used.[9]

There have been claims of other, earlier labyrinths hidden in the preserves, but Friends of the Labyrinth dismisses these as urban legends, as no evidence has been found. The organization reports that analysis of high-resolution aerial photographs show no trace of such activity.[8]