As a casual hiker, I used to take weekend walks through the wind-tickled, hawk-swiped hills of the East Bay—that open, landscape stretching east from Oakland and south from the Sacramento River to the Livermore Valley—and not think for a moment about water. Enchanted by a wrinkled country of curves and miles of sky, I thought of the land as a finished tableau, the grassy tint of its hills shifting rhythmically, year after year, from September-blond to March-green. The only water I thought about was maybe a creek, or the sloshing in the plastic bottle inside my pack.
It is only now, years later, that I see water everywhere in those hills—water that preceded them, created them, and to this day sculpts them into what they are. Salt water. Fresh water. Rain water. Where I once saw only an oak-freckled knoll, I now see rumpled remnants of an old sea floor, its flaxen sandstone rounded by rain and camouflaged by grass. Where I once saw a sun-baked ridge ringed by vultures, I now see marine rock crammed with clamshells and other fossils. On some days even the hills themselves appear to undulate, rising and falling like green oceanic swells. On such days, it doesn’t seem so outrageous to claim that crossing this same terrain several dozen million years ago would have required a boat.
The entire East Bay was once a young sea, which was itself preceded by an older, ancestral ocean. For millions of years, the future Bay Area lay under an open ocean. Its landscape was a seascape: no oaks, no bobcats, no kestrels, no globe lilies. Just fish-filled, clam-filtered salt water. Then, beginning about thirteen million years ago, islands began to appear. Slowly, the land rose out of the ocean.
Or, you might say, it buckled, crinkled, and shoved itself up from the water, millimeter by millimeter, earthquake by earthquake, driven by the same forces that today create tremors along the Hayward Fault: the bumping together of tectonic plates. This tectonic pressure also created California’s Coast Ranges, of which the East Bay’s sedimentary hills form a part. The slow but inexorable grinding of the oceanic Pacific Plate past the continental North American Plate is the main geologic story in the region. However, a slight (about 10 degrees) change in plate motion some three million years ago added a component of compression that caused the uplift and folding of the East Bay’s rolling hills. Today this compression continues, and the East Bay is still generally rising, even as gravity works its surfaces down through landslides, wind, and rain.
What distinguishes the East Bay’s rounded hills from the older rocks of the Coast Ranges (which underlie much of San Francisco and Marin) is that an inland sea persisted in the area long after the Coast Range rocks to the west had risen. Sediments eroding from the emerging land gradually heaped up onto the floor of this younger sea to form sandstones (coarser-grained deposits) and mudstones (finer-grained siltstones and shales).
Although the prehistoric sea has long since disappeared, water—in the form of rain—is still shaping those sandstones, siltstones, and shales. The more resistant sandstones, those that are better cemented or more tightly compacted, are often found holding up the higher ridges. But the mudstones are easily eroded. That smooth shape that makes you want to run up those hills results from the fact that the hills themselves want to run down. Their loose, erodible soils can easily become saturated with rainwater and run down all at once, or sometimes gradually, in landslides. According to geologist Doris Sloan at the University of California, Berkeley, geologists call the result “melted-ice-cream topography” because the slumping earth comes temporarily to rest looking much like a warming scoop of rocky road. During any wet winter it’s a good bet the hills are going to slump and flow somewhere, as though to hearken back to the time when the sediments themselves were under water.
For more than a century, rainwater and cattle have also been molding the slopes in a gradual way, terracing the hillsides into giant, curvaceous washboards. You might think a farmer plowed the slopes, leaving deep furrows along each contour. In fact, these “terracettes” are caused by cattle traveling over the rain-saturated soils and can be seen anywhere they graze, such as Briones or Black Diamond Mines Regional Parks. The “corrugating,” as Sloan describes it, is especially visible in winter, when grasses have died back to reveal the stair-stepped earth as it is slowly being trampled downslope. An intriguing question remains: Did such terracing exist before the advent of cattle, when large herds of elk trampled these same hills?
This land of hills bequeathed by the sea is not a landscape of extremes. The hills are not as flat as the Great Central Valley, not as steep as the Sierra Nevada, not as wet as the northern coast, not as dry as the Mojave. The winds that sweep across them are not hurricanes, the air is tempered more by fog than frost, and the light that strikes them is not unrelenting. Unlike a flat plain, the gradual slopes create shade each morning and evening. The daily course of sunlight across such slopes is a study in subtleties—a diurnal choreography of retreating and advancing shadows.
As the sun journeys from east to west it strikes the face of some slopes directly, others hardly at all, leaving traces of its journey in the great mosaic of swaying grasses, brushy chaparral, and dark stands of oaks. Slopes facing southwest, which endure afternoon heat as well as direct sun, are mostly grass or chaparral, with few trees. Shady, northeastern slopes tend to be cooler and damper, encouraging the growth of oak woodlands and forests.
It is the thin-bladed grasses—those knee-deep, grain-tipped stems lining every hillock and valley—that announce an abundance of sun and paucity of water. Sprouting after the rains begin, the short-lived annual grasses bloom, set seed, and die by June. Each year, the rains of winter are literally sucked into billions of spiking stems, translating the hills’ precipitation into lush pelts of billowing green. Then, as moisture disappears over the late spring and early summer, so too does life recede in each stem, rendering the slopes as pale as the buff-colored coyotes roaming them.
Such a blanket of countless blades might seem beautiful and benign, stretching toward an endless blue sky. In fact, the grasses’ supreme presence betrays a disturbed and altered land. What we think of as the hills’ two quintessential colors—spring green fading to autumnal tan—are, from a distance, very like what you would have seen 300 years ago. But closer inspection reveals that the East Bay’s original wildflowers and native perennial bunch grasses have been largely replaced by hardy European annual grasses, brought as seeds by the Spanish with their livestock in the late 1700s. These introduced grasses have displaced indigenous, variegated oceans of purple owl’s clover, orange poppies, blue lupines, and bunch grasses, according to Steve Edwards, director of the East Bay Regional Parks Botanic Garden. Edwards has documented that before the Spanish arrived, some hills were ablaze in spring, “shining yellow and orange” for miles. Others were probably “solid pink” with clarkias in early June.
Although the composition of wildflowers and grasses on the hills has changed dramatically, the patchy beard of chaparral has remained relatively intact. Composed of drought-tolerant, impenetrable shrubs and dwarf trees like manzanita, scrub oak, buckbrush, and chamise, chaparral often grows on steeper, rockier slopes where plants can take root in fractures. In contrast to grasslands, which can survive on mudstone in the East Bay, chaparral is often found on the old sea’s sandstone, according to Edwards.
Unlike the chaparral, the East Bay’s oaks grow on a wide variety of soils—as long as there is water. Like palm oases in a desert, oak-covered folds and forested canyons flag the availability of water, defining in their green, leafy profiles the places where streams run, groundwater is shallow, soils retain moisture, or dew persists. The oaks’ haunting shadows are cast atop ridges, along draws, and upon valley floors, unifying the landscape in curving, muscular gestures that echo the rounded contours upon which they thrive. They are spherical, graceful trees for a spherical, graceful land.
More than a hundred years ago, writer Charles Nordhoff observed this powerful aspect of oaks when he described them in 1873 as “evergreen trees which nature has planted so that the finest park-like effects are produced.” Nordhoff was speaking of one particular species of oak—the coast live oak, which grows along the coast from Mendocino to Baja California and is the most common species of oak in Alameda and Contra Costa counties. The “live” in its name denotes the fact that it is an evergreen, remaining active in winter. The hills are also shaded by two other evergreen species, the interior live oak and canyon oak, as well as three deciduous species—the valley oak, blue oak, and black oak. According to botanist Bruce Pavlik, co-author of Oaks of California, the East Bay enjoys a rich diversity of oaks because it lies in a transition zone between the wet coast and dry interior. The long, dry summers of the interior East Bay hills favor oaks over other tree species, such as redwood and Douglas fir, that prefer the foggy coast.
Throughout the East Bay hills, water often determines where—and in what sort of grouping—an oak will grow. Closed-canopy forests (often including bay laurels, madrones, and buckeyes) occur in wetter areas, whereas open woodlands and grassy savannas are in drier locations. The blue and valley oak savannas east of the Oakland-Berkeley hills are “at the edge of a climate that supports tree growth,” according to Pavlik. The principal gardeners of such “park-like” savannas are climate and fire: the relative lack of water in the East Bay’s climate helps determine the wide spacing of oaks in a savanna; fire both culls and boosts oaks through cycles of burning and regrowth.
Other players besides water and fire help determine where oaks grow. Grasslands with no oaks might have clayey soils that are tough for oak seedlings to grow in, partly because of the soil’s texture, but also because the frequent slumping rips out trees and grinds up roots. A rocky outcropping on a hilltop might allow a few oaks to suck rainwater from its cracks. Clumps of oaks in a savanna might delineate where, decades ago, a parent tree dropped acorns around itself, creating offspring. Or a clump of oaks might be an old acorn cache forgotten by a gray squirrel, scrub jay, dusky-footed woodrat, or acorn woodpecker, all of whom carry off and store acorns in their own secret hiding places, uphill and down. When the cache goes uneaten, the acorns have an opportunity to sprout.
One other player figures prominently in the ecological dance of the hills. As Pavlik and others have pointed out, California’s native peoples have been residing in oak-dominated landscapes for at least 12,000 years. The East Bay’s Ohlone and Miwok tribes depended heavily on oaks as well as native grasses for food. Their acorn mortars, pecked into bedrock, can still be seen in places like the Sunol Regional Wilderness. The Ohlone and Miwok burned their lands regularly to encourage the resprouting of forage for game as well as edible plants for themselves, and to keep their acorn harvest abundant. (Among other things, burning reduces pests that take shelter in the leaf litter under oaks.) The park-like savanna Nordhoff described may well have been the last vestige of a complex, sustainable system of native fire management, through which people lived off oaks for centuries, along with grizzlies, elk, deer, squirrels, birds, salamanders, moths, wasps, mistletoe, lichens, and bacteria.
It was the arrival of the Spanish that brought great change to the landscape. In addition to bringing seeds of exotic grasses and other weedy species, the newcomers banned the burning of grasslands, set their livestock out to graze, and shot the elk, thereby depriving the indigenous villages of their resources and ending centuries of fire-based stewardship. The Spanish also cut down acres of oaks to plant crops and for use as firewood. In the footsteps of the Spanish came the Mexicans, and eventually homesteaders from Europe and back east, who continued these practices. Upon the rolling hills, one tide of life—with its seasonal shapes, rhythms, and colors—yielded to another in a geologic blink of an eye. Perennial bunch grasses and wildflowers were replaced by annual grasses; elk and grizzly were replaced by cattle; oak savannas were replaced first by walnut orchards, then eventually by backyard exotics such as sweet gum and acacia.
Today, patches of native flora still exist in the East Bay. Throughout the East Bay Regional Park District’s 92,000 acres of open space, you can find relatively intact communities of indigenous plants. The region’s overstory of long-lived oaks, gigantic bay laurels, buckeyes, and madrones are all remnants of the original flora. Near-pristine thickets of chaparral cloak the upper reaches of Mount Diablo (a state park) as well as portions of Morgan Territory and other parks. And although fields of native bunch grasses do not remain, the EBRPD is experimentally managing areas of bunch grasses at Point Pinole Regional Shoreline and Sibley Volcanic Regional Preserve using a combination of cattle grazing and fire. The original carpets of wildflowers are gone, but eye-widening displays still burst into bloom in some areas of the hills, including on Mount Diablo, and at Sunol and Black Diamond Mines.
A sense of loss is perhaps inescapable when roaming these hills, but it needn’t blind us to what remains or keep us from seeking to better understand the flux of life upon them—and working to keep intact what we have left. In the meantime, we can take a moment to lie on our backs in the grass to watch the hawks, vultures, and eagles. We can roll acorns in our palms like marbles, or try to crumble sandstone with our fingers. We can listen for the whisper of dry oak leaves as they sift down through branches. We can sniff a laurel leaf. Or, as my four-year-old son did recently, we can climb a coast live oak and refuse to come down. Not ever.
Most recent in Geology
A visit to Kehoe Beach takes you on a journey to one of the Bay Area’s most dramatic geologic sites, where you can see rocks that have traveled far through time and space to pause temporarily in the Bay Area.
In a newly published paper, scientists link groundwater depletion in the Central Valley to geologic uplift and maybe even earthquakes.
Some people swear there's earthquake weather. Some people swear there's not. So what happens when an earthquake strikes California during earthquake weather? We called the Berkeley Seismology Lab to get an expert opinion.