Lizard Rock, Coyote Hills Regional Park, makes a fine photo vantage point.

      Two parks of interest to geology buffs are Sibley Volcanic Regional Preserve, in the Oakland Hills, and Mt. Diablo State Park. Sibley Preserve contains an extinct volcano, Round Top (1763’), which, along with three others nearby on private property, erupted around 10 million years ago, spewing lava, rock fragments, and ash. There is a self-guiding tour into the volcanic area, and an excellent brochure available at a small visitor center. (See the route description for “Round Top Loop.”) Mt. Diablo (3849’), the highest point in the East Bay, resembles a volcano but was actually formed when a large, rocky mass pushed up through layers of sedimentary rock and soil, sometime between one and two million years ago, twisting the layers and in places turning them upside down. You can see interesting rock formations at Rock City, on South Gate Road about 1 mile past the entrance kiosk.

      Plant Communities

      California has a rich diversity of plant life. Some species, like coast redwoods, date back to the dinosaurs, whereas others have evolved within the past several thousand years. Roughly 30 percent of the state’s native plants grow nowhere else. These endemics, as they are called, include many types of manzanita (Arctostaphylos) and monkeyflower (Mimulus). Botanists divide the plant kingdom into several major groups: flowering plants, conifers, ferns and their allies, mosses, and algae. A plant community consists of species growing together in a distinct habitat. Here are the principal plant communities you will encounter along the trail. (The common names for plants in this book are mostly from Plants of the San Francisco Bay Region, by Eugene N. Kozloff and Linda H. Beidleman.)

      Oak Woodland

      No tree symbolizes the East Bay better than the oak, a sturdy, long-lived tree whose leaf makes a fitting symbol for the East Bay Regional Park District, and whose name echoes in cities throughout California. Oak woodlands are found generally at low elevations on gentle slopes; foothill woodlands, where trees such as California buckeye and gray pine accompany oaks, occupy steeper or higher ground. If the trees have considerable room between them, making the terrain seem park-like, the area is called a savanna. Park visitors with an interest in plant identification will soon learn to recognize the six common East Bay oaks—three deciduous and three evergreen or “live”: valley, blue, and black, and coast live, canyon live, and interior live. Oaks are islands of life: they produce acorns that are eaten by animals and birds (and until recently, by Native Americans), and provide both shade and shelter in a sea of grass. More than 100 species of birds are associated with oak woodlands in California.

      Mixed Evergreen Forest

      Mixed-evergreen forests contain oaks and other species, usually California bay and madrone, and perhaps California buckeye and bigleaf maple as well, in a habitat that is cooler and wetter than the one occupied by oak and foothill woodlands. The understory often contains shrubs such as toyon, blue elderberry, hazelnut, buckbrush, snowberry, thimbleberry, oceanspray, and poison oak. Carpeting the forest floor may be an assortment of wildflowers, including milk maids, fairy bells, hound’s tongue, and western heart’s-ease.

      Oaks, such as this one in Black Diamond Mines Regional Preserve, symbolize the East Bay.

      Riparian Woodland

      Riparian, or streamside, woodlands often contain large, deciduous trees such as western sycamore, bigleaf maple, Fremont cottonwood, and white alder. Growing with them will be willows and perhaps California bay, California buckeye, hazelnut, and blue elderberry. Other streamside plants include snowberry, creek dogwood, vine honeysuckle, and California wild grape. This type of habitat provides the best display of fall colors in the East Bay.

      Redwood Forest

      At one time coast redwoods blanketed the Pacific coast from central California to southern Oregon. These giants are the world’s tallest trees and are among the fastest growing. Commercially valuable, they were heavily logged, especially in the Santa Cruz Mountains. All of the East Bay’s virgin redwoods are gone, most having been logged between 1840 and 1860. A few pockets of second-growth redwoods still exist in Redwood and Anthony Chabot regional parks, and in the City of Oakland’s Joaquin Miller Park. Tall redwoods, with their extensive system of needle-covered branches, shade out most other species. Often western sword ferns are the only plants able to grow beneath these towering giants. Near streams in a redwood forest, where some light penetrates from above, look for evergreen huckleberry, thimbleberry, and hazelnut.

      Chaparral

      This community is made up of hardy plants that thrive in poor soils under hot, dry conditions. Chaparral is very susceptible to fire, but some of its members, such as various species of manzanita, survive devastating blazes by sprouting new growth from ground-level burls. Although chaparral foliage is mostly drab, the flowers of many species are beautiful, with some blooming as early as December. The word chaparral comes from a Spanish term for dwarf or scrub oak, but in the East Bay it is chamise, various manzanitas, and various species of ceanothus that dominate the community. Other chaparral plants include mountain mahogany, bush poppy, toyon, and chaparral pea.

      Main Marsh, in Coyote Hills Regional Park, offers opportunities for photography and nature study.

      Grasslands

      Where we see green, rolling hills in East Bay parklands, the botanist sees “disturbed” areas of nonnative plants and weeds which show the effects of civilization—farming, grazing, road building, and burning. Before humans intervened to alter the landscape, the grassland community in the East Bay contained mostly native bunchgrasses and a wide variety of wildflowers, and supported large grazing animals such as tule elk and pronghorn. Today those grazers are gone, replaced by cattle, and most of the grasses we see here, including wild oats, Italian rye, and fescue, are aliens from Europe and the Middle East. Also noticeable are invasive nonnative thistles that often border the trail or dominate an entire hillside. In spring the East Bay’s grasslands are beautifully decorated with bright wildflowers, some of the most common being California buttercup, California poppy, red maids, and shooting stars.

      Coastal Scrub

      Among the plants that make up coastal scrub, also called soft chaparral, are coyote brush and poison oak, found almost everywhere, along with California sagebrush, coffee berry, bush monkeyflower, black sage, and yerba santa.

      Salt Marshes

      Around the edge of San Francisco Bay you will find salt marshes—wetlands exposed to tidal flooding but protected from the high winds and waves found along ocean beaches. Three of the most characteristic salt marsh plants are cord grass, which grows in the lowest marsh zone and gets a twice-daily soaking from the tide; pickleweed, a middle zone plant which can tolerate some salt water; and salt grass, an upper zone resident, out of reach of all but the year’s highest tides.

      When the first Europeans arrived here in 1769, San Francisco Bay contained more than 300 square miles of marsh; today only about 20% of the original marshland

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