Lake Merritt
Geologic Tour of Oakland, California
Images on this page (c) 2003 Andrew Alden, licensed to About.com, Inc.
Lake Merritt is a salt lake, an arm of the sea controlled by dams. The low lands around it are former marine terraces, younger than 10,000 years old, from a time when the sea was higher than today (or the land was lower). Slightly higher are the slopes of old alluvial fans, laid down during Pleistocene time when the climate was more arid than today. Both of these, the terraces and the fans, are good ground although they aren't bedrock.
This view is from the roof of the Oakland Museum of California. Like the rest of the central city, it sits on an expanse of compacted marine sediment called the Merritt sand. Look at all of this on the geologic map below. The museum is at the southwest corner of the lake, across the street from the courthouse. The lake's east tip is to the left of the lefthand pine tree, below the rocky knob on the skyline called Round Top.
Part of the Oakland geologic map showing Lake Merritt and surrounding geologic units: Qms, Merritt sand; Qmt, marine terrace; Qpaf, Pleistocene alluvial fan; af, artificial fill.
The highlands beyond the lake are the Oakland Hills, part of a long ridge running down the East Bay just east of the Hayward fault. The fault itself is behind a lower ridge, covered with houses. The two ridges have very different sets of rocks, brought together here by some 12 million years of sideways motion on the fault. On this side are mixed Mesozoic rocks of the Franciscan complex, while the far side is former oceanic crust of the Coast Range Ophiolite, marine sediments of the Great Valley complex, and some much younger volcanic rocks. Round Top is largely volcanic, although tectonic forces have tilted the lava beds nearly sideways. (Start a side trip along Oakland's section of the Hayward fault at Lake Temescal.)
The museum roof has a fine garden on it, plus some art works and geologic specimens. One of those is a large boulder of serpentinite, and another is the large petrified log below, its end polished to show off the lovely agate the wood has become. (See a close-up on the Petrified Wood page.)
The next stop is at Adams Point, where there's a rare remnant of Oakland's original shoreline.
Download a 2400 by 2400 geologic map of Oakland (2 MB) for all the details.
