Oakland Geology

Lake Merritt, subject of chapter 2 of Deep Oakland, is something of a yang to the Hayward fault’s yin: “Whereas the Hayward Fault perturbs us with energy from the Earth’s interior, Lake Merritt connects us to the world ocean, the world atmosphere, and the cosmic cycles of the solar system.” The lake is far more than it appears to be.

The lake’s human history is well known. First of all it’s not a real lake, but a freakish arm of the Bay — a slough — that reaches straight inland a couple miles, unlike any other shoreline feature in the East Bay. We’ve called it “lake” since Mayor Samuel Merritt dammed it at the 12th Street crossing in 1869, even though that only muffled the daily tides. For a long time it was the city’s open sewer. Then over the course of the 20th century, the slough was mucked out, its shore developed as city parkland and its waters cleaned up to make it the ornamental pool we love so much today, with real live sea life and shorebirds.

In the landforms around the lake, the geologist sees evidence of three different parts of the deep past. Two of them are related to the ice age, by which I mean the latest of dozens of ice-age cycles that we’ve been living through for the last few million years. The younger of these two landforms is the sandy platform that holds downtown Oakland.

Chapter 3, “Downtown,” gets deep into that, so I’ll just briefly say here that if you picture an ice age, it’s a cycle roughly 100 thousand years long of huge polar ice caps growing and melting. The ice ages are triggered by slow oscillations of the Earth’s orbit caused by the gravity of Jupiter and Saturn, and that’s the cosmic cycle I mentioned. Today we’re living during a warm peak, but not so long ago we were at the cold peak, with maximum ice and very low sea levels. The shoreline was way out by the Farallon Islands, and everything from here to there was fresh sand and gravel carried down from glaciers in the Sierra Nevada. That’s how San Francisco got covered with sand dunes, and so did downtown Oakland and the whole dry Bay in between.

Now this point is important: if you picture what must have been here at that time, it was dry land. In fact, the same streams that feed the lake today were digging a deep ravine here.

Then there’s the older of the two ice-age landforms, the flat ground on Adams Point that holds Lakeside Park (and a few other places around the lake).

This platform is a little lower than the downtown platform. It’s made of gravel, not sand.

It dates from the last time there was a warm peak in the ice-age cycles. The sea level was even higher than today, and the same streams that drain into the lake today were building terraces of sediment along that higher shore.

The whole ice-age thing, highs and lows and 100,000-year cycles, means that for most of recent geological time there’s been no lake here; Lake Merritt is a rare, temporary feature that reappears only during the warm peaks like today. But it keeps coming back because of what’s upstream: that block of hills near the Bay, in front of the high hills, that the city of Piedmont sits on.

All of the streams that drain into the lake come from that ridge, and when it rains hard enough for them to do some serious work, those streams combine to send a powerful flush of water through here. That’s why the deep ravine that periodically becomes Lake Merritt persists through cycle after cycle of ice ages. But that’s a story I get into in later chapters. For now I’ll just say that it’s connected to the third set of Lake Merritt’s landforms: the steep hills of the Adams Point, Cleveland Heights and Grand Lake neighborhoods.

Look for these hills and terraces next time you visit. Collect them all.

This chapter refers to the following blog posts:

The Merritt Sand
Adams Point alluvium
A walk around Lake Merritt in 2100, after sea-level rise

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