BAY TODAY, GONE TOMORROW / S.F. region's defining feature is just a transitory puddle in geologic time

If you had planned to stroll to Ocean Beach 17,000 years ago, you would have been well advised to pack a lunch. And maybe dinner and the next day's breakfast, too.

That's because the coast was very far away. From where San Francisco sits, the beach was 26 miles west, about six miles past today's Farallon Islands.

The islands themselves, of course, were not islands at all -- they were peaks.

And San Francisco Bay? Well, there was no bay. And that would remain the case for several thousand years.

The site of the present bay was a series of broad valleys, each with a tributary stream that poured into a mighty, sediment-swollen river that originated in the Sierra Nevada. This river drained through the Central Valley, the Carquinez Strait, Raccoon Strait and the stony ramparts that are now the Golden Gate.

A bay, in fact, is something of an anomaly for the San Francisco region.

"During the past 600,000 years, the bay has only existed during three brief periods totaling about 15,000 years," observes Ken Lajoie, a senior geologist with the U.S. Geological Survey in Menlo Park who counts the Bay Area's geology among his specialties.

"The present bay has existed near its (current) size only for the last 4,000 years," he says. And if the past is any indication, it will be around for only another 1,000 years or so.

5,000-YEAR LIFE-SPAN

The geological history of San Francisco Bay is really the story of several bays -- each of which lasted only about 5,000 years -- and of the tens of thousands of years between, when the land supported big rivers and lovely valleys.

It is a story that involves the cataclysmic raising and lowering of the Pacific Ocean, stupendous volcanic eruptions, the creation of mountains through the grinding, compressing and upheaval of tectonic plates and the inundation of entire landscapes by vast floods.

It is also a story of life -- of primeval forests of cedar and pine, of great Pleistocene mammals such as mammoths and giant ground sloths, and of the people who hunted them with nothing more than flint-pointed spears.

The very dimensions of the story can't help but give one a certain perspective, observes Lajoie, who notes that even the most dramatic of the Earth's features -- bays, rivers, mountains -- are ephemeral in the context of geologic time.

'NOTHING IS PERMANENT'

"We tend to think that the Earth's features don't change, but geologically speaking, the fact is that nothing is permanent, or even around very long," he says.

Today's bay formed when the last ice age waned. At the height of the last glaciation about 17,000 years ago, large amounts of water evaporated from the oceans and fell as snow, not rain, compacting into huge continental glaciers. Sea level lowered by about 300 feet, and big expanses of the present continental shelf were exposed.

As the glaciers retreated, sea levels began rising, with rates ranging from three feet per century to 15 feet per century. About 10,000 years ago, the ocean began sneaking through the Golden Gate, forming the nascent San Francisco Bay.

"The bay only reached its present size within the last few hundred years," says Lajoie."

Even while the melting glaciers worked to form a large bay, other factors conspired to restrict its size. "The bay would be about twice its present size if sediments hadn't partially filled the subsiding basin it now occupies," Lajoie observes."

But to understand the forces that shaped the bay, you have to go further back in time -- close to a million years further.

The sedimentary record indicates that the interior of California didn't always drain through the bay basin, as is now the case. Minerals from the Sierra Nevada began appearing in the basin somewhat less than 600,000 years ago -- a blink in the context of geological time.

Prior to that, a vast inland sea called Corcoran Lake occupied much of the Central Valley, draining through the Salinas River into Monterey Bay.

VOLCANIC ERUPTION

About 760,000 years ago, a tremendous volcanic eruption occurred in what is now the Bishop area. A great caldera was created, and massive amounts of volcanic ejecta were deposited in the lake.

But this ash didn't make it into the bay until tectonic shifting in the Earth's crust caused the Bay Area to subside and the south end of the lake to rise about 560,000 years ago.

This caused the lake to spill over the ridge that separated it from the bay basin. The flow carved the Carquinez Strait and drained the lake. It could have happened so fast that it would have been a single, catastrophic event, says Lajoie -- a great gush of water roaring to the sea.

Since then, tectonic activity -- upthrusting of the Earth's crust -- has plugged the Central Valley's outlet through the Salinas River. Now, everything that flows into the Sacramento and San Joaquin valleys ultimately pours out the bay.

Another eruption, this from Mount Lassen and much smaller than the Long Valley Caldera near Bishop, also contributed sediments to the bay.

"This occurred about 435,000 years ago," says Lajoie. "(Sediments from the event) are called the Rockland Ash, and can be seen clearly in the sea cliffs at Fort Funston on the San Francisco coast."

As the ice from the last glaciation melted, the Sacramento and San Joaquin rivers became great, braided streams choked with sediment.

"All that glacial outwash was dumped in the Central Valley and the Delta," says Lajoie. "There was so much sediment that huge dune fields blew out of the river near Antioch about 15,000 years ago. The same thing happened where Oakland now sits."

Oakland, in fact, is built over a deposit of dune sand known as the Merritt Sand. There was no bay at that time, but those sediments ultimately covered much of the bay basin.

SOURCES OF SILT

A good deal of fine silt and clay still comes into the bay from the Sacramento-San Joaquin river system. But the lion's share of sediment -- mostly sand and gravel -- issues from Alameda Creek, which drains Livermore Valley through Niles Canyon.

"Many people find that surprising," says Lajoie, "because the biggest source of ongoing bay sedimentation isn't even (directly) adjacent to the bay -- it's Livermore Valley. There's a huge alluvial fan (of sediment) deposited by Alameda Creek that spreads out beneath the bay from the Coyote Hills to the shore of Palo Alto."

Other things have helped the bay become a prime sediment trap -- things of a tectonic nature. Mountains have built on both the San Andreas and Hayward faults, sharply defining the limits of the estuary. The process continues today, with the continuing uplifting of the Santa Cruz Mountains, which straddle the San Andreas Fault.

Simultaneously, the land just west of the Hayward Fault is subsiding; meanwhile, structural rock underlying the South Bay is slowly sinking and gradually tilting eastward.

As the Santa Cruz Mountains and the Berkeley Hills ascend, they are squeezing the zone of bedrock between them, known as the Bay Block.

"The interesting thing about the Bay Block is that it is basically free of faults at this point," observes Lajoie. "But as it continues to be compressed, it might eventually develop new fault lines."

CHANGES CONTINUE

The change has been dizzying -- and it won't abate. It will continue, regardless of the level of human activity. Erosion and plate tectonics will grind on inexorably. And another ice age is not merely likely, Lajoie observes -- it is inevitable.

Lajoie says there is increasing evidence to indicate that ice ages are triggered by perturbations in the Earth's orbit, subtle movements caused by the gravitational effects of Jupiter and our moon.

MASSIVE AMOUNTS OF DUST

"The planets formed by sweeping up massive amounts of dust and debris when the solar system formed, but a lot was left over," says Lajoie. "It appears concentrated in a disk around the sun."

The Earth's orbital plane tips through this debris disk every 100,000 years or so, Lajoie observes. The dust occludes sunlight, reducing the amount of thermal energy that reaches the planet's surface.

That probably isn't enough to start an ice age by itself, he says.

"The oceans distribute solar energy globally, but the margins are very fine," he said. "The orbital parameters can't do it by themselves -- but they're triggers. When the balance is finally thrown off in the oceans, glaciation can occur very quickly."

But what about global warming? Couldn't the ongoing atmospheric loading of heat-trapping gases like carbon dioxide forestall another ice age and guarantee the longevity of the bay, orbital wobbles notwithstanding?

Lajoie doesn't think so.

"I'd prefer that we maintain a cautious attitude about releasing greenhouse gases, but I don't think they could overcome the orbital signal," he says. "I think it's just too strong."

Our distant descendants, then, might well have to forgo bay views if they plan to live in San Francisco.

"When glaciation occurs, the bay drains, and everybody ends up walking to the Farallones," Lajoie says.

THE MAKING OF A BAY

Three geological processes have shaped San Francisco Bay: the rise and fall of sea level, the shifting of tectonic plates and the deposition of sediment from rivers. All work in concert to form an estuary that is in constant flux -- and that periodically disappears for tens of thousands of years.

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-- Sea Level Rise

During the height of the last Ice Age 17,000 years ago, sea level dropped by 300 feet, and there was dry land west of what is now the Farallon Islands.

Melting glaciers caused sea levels to rise, and the current bay began to form around 10,000 years ago. It has existed near its current size only for the the last 4,000 years. -- The Bay Block

Shifting tectonic blocks continue to shape the bay. The rising Santa Cruz Mountains and Berkeley Hils are compressing the Bay Block, a vast slab of Franciscan rock underlying the south bay and its sediments. Though the Bay Block is essentially free of faults, it is expected that this compression will ultimately cause new faults to form.

CORCORAN LAKE

About 760,000 years ago, much of California's Central Valley was a great freshwater inland sea known as Corcoran Lake. The lake's outlet was the Salinas River, ultimately draining to Monterey Bay.

Then about 560,000 years ago, tectonic uplifting allowed the lake to rise sufficiently to cut through the soft soils of what is now the Bay Area. The Carquinez Strait was rapidly carved.

The uplifting also plugged the Salinas Valley outlet, leaving San Francisco Bay as the Central Valley's only outlet. Sediment deposited by Central Valley rivers limits the size of the bay.