I recently put up a new printable version of the US Geological Survey’s geologic map of the Oakland region. You can download it, and the separate page with the key to all the colors and symbols, from the map page. This is a small preview.

^{Adapted from USGS MF-2342}

I’ve always recommended this USGS map, published in 2000, for the general public to study and enjoy, but one shouldn’t fall in love with it. That’s because geologic maps, especially this one, are as much interpretation as observation, and they always simplify what you’ll actually see on the ground.

There are several things to beware.

First, the different fields of color look definitive but may instead be tentative assignments. A single color may mask a lot of variation, which is no surprise because the rocks within each field may have accumulated over millions of years or may consist of mixtures. Compare the same area in the 2000 regional map and the 1969 quadrangle map, each one produced by different geologists for different purposes.

^{Top image from 2000, bottom image from 1969. Learn more in this post.}

Perhaps a future researcher will find and analyze enough rock exposures to split a field into subdivisions, or lump it together with another one instead. Also consider that the 2000 map was compiled as a database, which required every pixel to be assigned to a field, so arbitrary choices and simplifications needed to be made. Any one field may have had exceptions inside it that had to be ignored, and blank spaces were not allowed. A close-up version of a smaller area may look quite different, even if it was compiled from the same set of field notes.

Second, the sharp lines have blurry meanings.

When lines separate two different color fields, they’re supposed to represent contacts, usually a fault. Most of them, though, are merely inferred: you find sandstone here and lava there, so somewhere between those two spots there must be a contact, and absent other evidence you’ll approximate it right in the middle. And it may not be clear what moved where along that fault, let alone what time it happened. What kind of line you’ll draw — solid, dashed, dotted — and what extra symbols you’ll add — triangular teeth on the upthrown side to signify thrust faults, or double ticks on the downthrown side to signify normal faults — are judgment calls. The 2000 map has very few solid lines on it.

Third, consider the part along the Hayward fault.

While the trace of the fault, where aseismic creep is currently active and the next ground rupture can be expected, is well known, it’s not marked on this map. Here the fault is a vaguely defined zone, right across the middle of this image, that’s covered by a set of small color fields. Over thousands of years, the active trace can meander within that zone. Over longer periods of time, the whole fault has been reorganized many times. Parts of it record only a few kilometers of displacement while other parts show much more. Some faults that split off the main trace, like the Moraga fault, used to take part in the Hayward fault but were abandoned and now run through the high hills without producing any earthquakes. The same is true of the San Andreas fault. They’re Frankenstein faults. Somehow the various blocks of the Earth’s crust rearrange themselves and the faults adjust into new straight lines, undoubtedly leaving splinters behind. Because I’ve seen so little bedrock along much of Oakland’s segment of the Hayward fault, I think of the fault zone as a totally separate color until proven otherwise.

Enjoy the map and let it tempt you, but trust reality first.

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