Sibley Volcanic Regional Preserve

East Bay Regional Park District2950 Peralta Oaks Court, P.O. Box 5381 Oakland, CA 94605-0381 1-888-327-2757 (1-888-EBPARKS) www. ebparks.org Rev. 7/17

To Reach Sibley Volcanic Regional Preserve:From Highway 24 just east of the Caldecott Tunnel, take the Fish Ranch Road exit and go north, uphill, for 0.8 miles to Grizzly Peak Blvd. Turn left and go 2.4 miles to Skyline Boulevard. Turn left on Skyline and proceed to the park entrance, on the left. PUBLIC

TRANSIT: The closest bus line is AC Transit #18, which runs to the Montclair Transit Center in Mont-clair. From 12th Street BART or 19th Street BART, take AC Transit bus #18 to the Montclair Transit Center. Call AC Transit to confirm transit information: dial 511 (TDD/TTY: 1-800-448-9790).

Massive basalt was removed from this major 5 quarry pit. The north wall shows a set of thick lava flows tilted on edge, nearly vertical. The well-defined layers near the top of the face are jointing-units resulting from shrinkage caused by cooling. They are analogous to the basalt pillars of Devil’s Postpile in the southern Sierra.

6 This is a sequence of basaltic tuffs (ash) lying on top of a dark lava (far left), all tilted steeply to the east. The base of the tuffs was baked red, probably because the lava below was still hot and steaming when the ash landed. At the top of the sequence there is another lava flow (far right, to the right of the post) that baked the underlying tuffs red.

7 Before you is a basalt flow, massive at left, rubbly to the right. The lower parts, coming into contact with cold land surfaces, harden early and are often jumbled by turbulence and drag. The rubbly part of the flow is filled with vesicles caused by gas pockets, which were later filled with chalcedony, opal, calcite, zeolites, and sometimes green celadonite.

8 This huge, reddish-brown block of lava fell from the cliffs to the upper right. The lava cooled and hard-ened while it was still moving, resulting in dramatic frag-mentation, known as “autobrecciation” (self-broken).

9 See hard lava to the right and left, and soft, easily eroded tuff between, all tilted eastward almost to vertical. The lava to the left baked the top of the tuffs brick red. This lava looks as if it was probably 100-150 feet thick. These flows were not fluid, but contained enough silica to be more viscous. They probably moved at a slow walk, with glowing blocks of lava tum-bling down a steep front and setting vegetation on fire.

10 The brick-red knobs in the foreground, and rug- ged outcrops of the same color on the skyline, are made of cinder that flew from Round Top and landed while still hot, so the pieces welded together.

11 The major valley to the north across Highway 24 is Siesta Valley. It coincides with the axis of the Siesta Syncline, a great fold that has lifted up thousands of feet of rocks on both sides. You are standing on the southwest limb of the fold, which includes all the rocks in Sibley.

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Tilden Regional ParkRedwood Regional Park

Sibley VolcanicPreserve

Claremont Canyon Reg. Preserve

Huckleberry Botanic Reg. Preserve

BOTANIC GARDEN

Roberts Regional Rec. Area

Temescal Reg.Rec. Area

Crown Beach & Crab Cove

McLaughlinEastshore State Park

POLICE, FIRE, MEDICAL EMERGENCY ...............9-1-1 EBRPD HEADQUARTERS .................... 1-888-327-2757 PARK OFFICE ........ 1-888-327-2757, option 3, ext. 4554TRS RELAY FOR THE HEARING IMPAIRED ........7-1-1

WELCOME! Please enjoy the Regional Parks safely, and help us protect and preserve your parklands by complying with park rules and regulations.SAFETY and ETIQUETTE• Stay on trails. Taking shortcuts can be dangerous and causes erosion. • Wading and/or swimming in undesignated areas may be dangerous and may harm the watershed.• Carry and drink plenty of water. Dehydration is a leading cause of injuries on the trail. • Be prepared for sudden changes in weather conditions. • Trails can be slippery, rocky and steep. Proceed carefully at your own risk. • Wildlife may be present on the trails at any time. Feeding or approaching wildlife is dangerous and illegal.• Bicycles are permitted on designated trails only. Horses have the right-of-way on trails. • Keep the parks beautiful. Pack out what you pack in.RULES• Dogs must be leashed 200 feet from any trail or park entrance. Dogs must be leashed in parking lots, picnic areas, developed areas such as lawns and play fields, and on some trails. They must be under voice control at all times.• State law requires that all bicyclists under age 18 wear an approved helmet while riding on trails and roadways. Equestrians and bicyclists are also strongly encouraged to wear helmets at all times.• Smoking is prohibited, except in overnight campsites.• Drones are prohibited. Visitors are responsible for knowing and complying with park rules, available online at www.ebparks.org/rules.

HOW MANY VOLCANOES? Besides Round Top, there are smaller ones outside the Preserve to the north and southeast. Another, of rhyodacitic composition, underlies the Lawrence Berkeley Labo-ratory and Little Grizzly Peak in Tilden Regional Park. About 9.8 million years ago it was erupting beside Round Top. Subsequently it was shifted about three and one-half miles northwest by movement along Wildcat Fault. That makes a total of four volcanoes.THIS PRESERVE is named to honor Robert Sibley, a founder and director of the East Bay Regional Park District, and president of the Board of Directors from 1948 until his death in 1958. The original 227-acre Preserve was dedicated with Tilden Regional Park and Temescal Regional Recreation Area in October 1936, two years after the Park District’s formation. Vari-ous parcels have been added to the preserve over the years, including a 250-acre parcel in 2010.

At the southern park boundary is 240-acre Huckle-berry Botanic Regional Preserve. The Skyline National Recreation Trail, which connects Richmond-El So-brante to Castro Valley, traverses both of these parks.

Sibley VolcanicRegional PreserveOakland, Berkeley, Orinda

California poppies. Photo by Isa Polt-Jones

Photo by Jerry Ting

Photo by Michael Short

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Round Top Creek

San Leandro Creek

EBMUD WATERSHED.Hiking permit requiredexcept when hiking onSkyline Trail/Bay AreaRidge Trail. Call (510) 287-0459.

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REDWOOD REGIONAL PARK

HuckleberryStaging Area

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AC Transit #305

Fire St. #6

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Separate map available.

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Self-guided tourbegins here. See descriptions below.

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Skyline National Trail.DOGS MUST BE UNDER VOICE

CONTROL AT ALL TIMES.HORSES MUST STAY

ON THE SKYLINE TRAIL.

BICYCLES ARE NOT PERMITTEDIN HUCKLEBERRY

REGIONAL BOTANIC PRESERVE.

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Best access to the staging area is from Canyon Road in Moraga, then turn right on

Pinehurst Road.

AREA IN YELLOW SUBJECT TO CLOSURESee www.ebparks.org/

sibley for status.

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A SELF-GUIDED TOUR OF ROUND TOP VOLCANOESBy Stephen W. EdwardsFormer Director, Regional Parks Botanic Garden

This Preserve features a complex volcanic center that was the source, 10 million years ago, of most of the lavas that underlie the ridges from Inspiration Point in Tilden Regional Park to Moraga. Round Top stands out today because it was originally surround-ed by sedimentary rocks of the Orinda Formation, which have eroded away. Over the years, folding and erosion have exposed a cross section of a volcano.

Lava from the vent has been dated as old as 10.2 million years. Round Top’s basaltic dikes (feeders of the vents), tuff-breccias (ash containing a jumble of blocks and chunks of lava), lava flows, red-baked cinder piles, air-fall tuffs, and the major vent itself can all be seen in an easy hike. The numbered descript-ions below correspond to stops along the trail.

Walk up the paved road to the EBMUD water tank to see a dark basalt dike, a feeder of lava to the crater, that cuts through a sequence of tuff-breccias (grayish brown) and pebbly mudstones (light gray), inside and near the crater bottom. This pit was made by quarry operations in which massive basalt lava was removed. The pit exposes the interior of the Round Top volcano. You are stand-ing on bedded tuff-breccias, which filled much of the crater, settling at times into a small lake. The steep wall across the pit consists of lava that capped the crater after it was filled. Eventually the Round Top vent buried itself in basalt flows.

This roadcut exposes Orinda Formation river gravels, sands, and mudstones. The red (when moist) streaks and layers in these river beds were

caused by oxidation of iron in the sediments. Such varicolored “redbeds” sometimes contain fossils of plants and animals. Elsewhere in the Preserve, bands of more intense red are found at the tops and bottoms of lava flows, where iron was oxidized and reddened by baking and steam action; these bands are called “bake zones.”

Before you is a wall with basalt on the left and Orinda mudstones on the right. The bedding in the mud-stones gives the appearance of drag-folding resulting from relative uplift of the lava occurring during the past 10 million years. Alternatively, the disruption of the mudstones may have occurred earlier, at the time of volcanic activity. This site was close to, or was in, the wall of the volcano, and would have been subject to slumping, sliding, and plowing.Continued on reverse

Year opened: 1936 Acres: 928Highlights: Hiking, biking, horseback riding, self-guided interpretive tour, backpack camp, panoramic views.Did you know? This parkland was opened as Round Top in 1936 with the formation of the Park District. It was renamed in 1972 to honor Robert W. Sibley, a Park District founder and director.

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