Oakland Geology

Like all true Oaklanders, I keep coming back to Lake Merritt. In this visit, I’ll muse about the many times in the recent geologic past when there was no Lake Merritt here.

If we assume, as I do, that the uplifted block of bedrock making up most of Lake Merritt’s watershed is about 1 million years old, then this little arm of the Bay has a fairly deep ice age history. A million years rather neatly fits the period of Pleistocene time after the Mid-Pleistocene Transition, when for imperfectly understood reasons glacial cycles changed from roughly 40,000 years to 100,000 years in length.

Let us then stipulate that this part of town has gone through about ten full glacial cycles. Each time the world’s ice sheets and ice caps grew, the sea level fell by a hundred meters or so — three or four hundred feet! Here’s a recently published set of sea level data for the last nine cycles. Various lines of evidence agree, though never in exact detail, about the timing and magnitude of the changes. That’s what paleoclimate science looks like.

^{From Rachel Spratt & Lorraine Lisiecki, 2016, A Late Pleistocene sea level stack, Clim Past 12(4)}

Each time the sea fell, all of San Francisco Bay slowly drained dry and the coastline withdrew out past the Farallon Islands. Every stream that could do so cut into the newly exposed ground, digging gulches, ravines and canyons into the young sediment where before they were prevented by the high sea level. That’s when Lake Merritt became temporarily Merritt Canyon, with Merritt Creek surging along its bottom.

Each time, Merritt Creek dug out all the gravel and mud that had filled the basin of Lake Merritt and shoved it straight out into the Bay, where the drainage ran south and then around the end of San Bruno Mountain into the Pacific. That’s right — the Golden Gate didn’t exist. Instead, the Bay area streams and the great Central Valley rivers drained through what’s called the Colma Gap.

Here’s an illustration from a publication I refer to often, Sandy Figuers’ “Groundwater study and water supply history of the East Bay Plain,” that shows the typical drainage pattern of those times.

^{Courtesy State Water Resources Control Board}

It’s very interesting: the river ran east of the Potrero Hills in Richmond, east of Yerba Buena Island, and all the way down to around San Mateo. And Merritt Creek, right in the center of that map, pushed a big fan of alluvium — river gravel and sand — into the basin that rivaled the fans of the other major East Bay streams. It went right under Alameda. That’s because Alameda wasn’t there.

This configuration of the Bay lasted from about 630,000 years ago, when the great freshwater Lake Clyde that once filled the Central Valley broke through the hills and cut Carquinez Strait, until the last warm interglacial period about 125,000 years ago. The sea at that time rose even higher than it is today, and motion on the San Andreas fault closed the Colma Gap. That’s when another gap opened up farther north on the fault, which became the Golden Gate.

So when the next glacial age began, the whole drainage pattern of the Bay shifted dramatically. Also the winds: instead of blowing south through the Colma Gap, the ice age westerlies carried huge amounts of glacial sand through the Golden Gate and across San Francisco, across the dry grasslands of the Bay and onto the East Bay slopes. That’s when the big fields of sand dunes accumulated in San Francisco and in three places on this side of the Bay: in downtown Oakland, in Alameda and on Bay Farm Island.

The latest time that Merritt Canyon formed, Alameda sat in its way all of a sudden, and I think Merritt Creek must have drained west, down San Antonio Creek (today’s harbor estuary), not southward as shown on this figure from the same report.

Here’s part of a third figure from that report, showing the depth to bedrock in Oakland as determined in boreholes.

Merritt Canyon really stands out underground: over repeated ice-age cycles, as the Bay floor has gradually subsided, the earliest versions of the canyon now lie 600 feet below Lake Merritt.

There should be a record of successive incisions and fillings of the canyon preserved down there. It would take a concerted campaign of core drilling and seismic reflection profiling to map and characterize it, and if I were a billionaire like some people I won’t mention, I’d spend the money even though it would be a huge hassle to get the permits.

This entry was posted on 27 September 2021 at 8:01 am and is filed under Oakland streams and water. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.