temperature extremes increase. So in January, you might find it warmer in Berkeley than in, say, Concord.
Although our climate—average conditions over the course of a year—is mild, our weather—daily atmospheric conditions—can be exciting. Wind is perhaps the most unpredictable condition, sometimes blowing ferociously on an otherwise perfect day, at other times disappearing as you make a slight change in elevation or orientation. Strong winds can turn a pleasant hike into an ordeal, and can even be hazardous, knocking down trees and power lines. But wind can be a bonus too, bringing relief on a hot day or clearing the air after a winter storm. You can use a weather radio, available at Radio Shack, outdoor stores, and other outlets, to receive broadcasts from the National Weather Service. You can also find up-to-the minute weather information on the Weather Channel or on the Internet at www.weather.com.
California poppies, among the East Bays most common wildflowers, bloom from February through November.
Geology
The geology of the Bay Area is a complex story, written in stone, with a plot line constantly changing and an ending yet to be determined. The principal actors in this drama are the major fault lines, fractures in the earth’s crust, that run along the east and west sides of San Francisco Bay. It is the release of tension along these fault lines that we feel as an earthquake, a natural phenomenon both awe-inspiring and terrifying. Anyone who experienced the 1989 Loma Prieta quake felt in a mere 15 seconds some of the power of the geological forces that have been at work in the Bay Area for millions of years.
California’s most famous fault, the San Andreas, runs from the Gulf of California, near the Salton Sea, northwest to Cape Mendocino and the Pacific Ocean. In the Bay Area, the fault goes through San Mateo and Marin counties, passing San Francisco just outside the Golden Gate. Two major faults associated with the San Andreas—the Hayward and Calaveras faults—cross the East Bay from southeast to northwest. The Hayward fault starts in the southern Santa Clara Valley and passes through the hills of Oakland and Berkeley. The Calaveras fault, farther east, follows a stretch of Interstate 680, passing near Pleasanton and San Ramon.
San Francisco Bay, actually the flooded mouth of the Sacramento–San Joaquin river system, lies in a basin between the San Andreas and Hayward faults. Over the past several hundred thousand years, changes in sea level caused by waxing and waning ice ages filled and drained this basin many times, the most recent being about 5000 years ago, when water trapped in great sheets of ice that covered parts of North America was released into the oceans, raising sea level by hundreds of feet.
Rising astride the Hayward and Calaveras faults, and a network of smaller faults which crisscross our area, are the hills of the East Bay, part of the Coast Ranges of northern California. The Coast Ranges—a complex system of ridges and valleys that stretches from Arcata to near Santa Barbara, and inland to the edge of the Central Valley—were formed millions of years ago, as the floor of the Pacific Ocean was dragged under the western edge of North American continent. This process scraped material from the ocean floor and piled it higher and higher on the continent’s edge, in what is now California. The East Bay hills, built mostly from sedimentary rock and some basalt lava, were uplifted, folded, and eroded into their present shape by geological activity that began three to five million years ago and continues today.
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
