As a child, I loved stories that featured secret passages and hidden rooms. I cherished the suspicion that there was a different world beneath this oneor around some corner where it was waiting to meet me. C. S. Lewis, H. P. Lovecraft, even Thoreau fed this feeling. Then I discovered geologic maps and felt something of the same thrill.
Consider the glimpse beneath San Francisco in the image above. Telegraph Hill stands out in the upper right corner, a near-island of old bedrock surrounded by artificial fill (Qaf). Once upon a time, North Point was really a point. What is that odd river of purple, marked by dotted lines? Isn’t that big purple blob Potrero Hill? Isn’t that orange patch Mount Sutro? The rocks in the two places must be completely different! And that light color marked “Qd”that must mark the old sand dunes that got turned into Golden Gate Park and paved over everywhere else.
The map’s explanation provides all the letter codes (and yes, Qd stands for Quaternary dunes) and keys to the symbols. It won’t tell you everything; even all this detail is just a sketch. But it will give you a set of clues, in the form of literature citations, that you could pursue in a university library. And of course the map patterns are another set of clues that you could pursue in the field. Slowly, with the map’s help, you can learn to perceive the hidden rooms of the landscape.
If this kind of thing excites you, then you owe yourself a visit to the Map Sales Room on the U.S. Geological Survey’s campus in Menlo Park. Yes, most of these maps are available online at store.usgs.gov, but (1) the USGS store includes products from the state geological survey too, and (2) there is still something about well-selected inks on strong paper.
One map worth starting with is the “Geologic Map of the San Francisco Bay Region” poster. It’s packed with explanation to help you get the most from the map. You can peruse the whole poster online in PDF or Google Earth formats and explore it directly by zoom and pan, but for seven bucks the paper version is a bargain.
Below I list a bunch of USGS geologic maps for the Bay Area that are mostly online-only.
USGS County geologic maps
Alameda (bedrock): Open-File Report 96-252
Alameda (young sediments): Open-File Report 97-97
Contra Costa (bedrock): Open-File Report 94-622
Contra Costa (young sediments): Open-File Report 97-98
San Mateo: Open-File Report 98-137
Santa Clara (southern part): Open-File Report 97-710
Santa Cruz: Open-File Report 97-489
Parts of Marin/Sonoma/San Francisco/Contra Costa: Miscellaneous Field Studies Map MF-2337
Northeastern Bay Area: Miscellaneous Field Studies Map MF-2403
Oakland area: Miscellaneous Field Studies Map MF-2342
Point Reyes: Open-File Report 97-456
Southern San Francisco: Open-File Report 98-354
West Sonoma/Southern Mendocino: Miscellaneous Field Studies Map MF-2402
Finally, the USGS helps you explore the whole state’s geology (and every other state) at the Geologic Maps of U.S. States site.
I also highly recommend the state geologic map and database I keep on my iPod: Geograph_CA