Chapter 6, the central one of the eleven in Deep Oakland, is about a feature I would bet most Oaklanders have never given a name: the distinct arc of low foothills above Foothill Boulevard and around the head of Lake Merritt. Its hills are so hidden by homes and trees these days that my favorite photo of them is this old one from 1876, looking across Lake Merritt from 12th and Webster.
Of course, my regular blog readers have seen me write about this arc for years under the name of the Fan. It’s an intriguing set of old gravel hills that lies above Oakland’s first level (the shore and flats) and below the third level (the bedrock hills of the Piedmont block and the Millsmont-Eastmont hills), the fourth level being the high Oakland Hills east of the Hayward fault.
The simplest way to illustrate the Fan is to show it on the bare topography (digital elevation model) of Oakland with the geologic map of the same area next to it:
I call these hills the Fan because their crescent shape on the map reminds me of a tattered Japanese folded paper fan. They’re shown in a darker yellow than the flats, signifying gravel and sand and clay that’s coarser, older and more consolidated than the alluvium around them.
This ragged swath of large sediment piles presents a geological puzzle. My solution to the puzzle of the second level combines all the stories of Oakland’s geology I’ve told so far.
I’ll come back to the geology in a bit. First, some history.
In the old days the Fan was just another part of the Ohlones’ rich world; they kept it mostly tree-free with regular mild fires and harvested in season the plants and small game it supported. But it stymied the first explorers from New Spain in the early 1770s, who tried to walk around it along the Bay side. The trouble was, that route dead-ended at the slough we know today as Lake Merritt, and they had to detour past the marsh through those hills which, as Friar Pedro Font recorded in 1772, “annoyed us very much with their ascents and descents.” Four years later the Anza expedition found and took the correct Ohlone trail, the same basic route that I-580 takes today. The NL and 57 bus lines are good ways to survey the Fan in comfort.
The first Europeans to settle in Oakland territory, the Peralta family, picked a spot in the Fan for their first hacienda, and ever since it’s been a desirable place to live. To the Peraltas it was good pasture, to the Americans that followed it was good land for farms and country estates, noted for its “picturesque scenery on every side,” and in the early 1900s it became Oakland’s most desirable streetcar suburbs.
I love taking long walks in the Fan’s charming ups and downs, although the deeply dissected hills around Lake Merritt (Adams Point, Grand Lake, Cleveland Heights) are more challenging than the rest.
One thing that’s significant for me as a geologist is that the Fan wraps around the Piedmont block, which is the chunk of blue colors on the geologic map representing bedrock. I can safely assume that both the block and the Fan that embraces it were uplifted together. And what did that?
As I said in chapter 1, the Hayward fault is Oakland’s prime mover. It’s been carrying this part of town north for millions of years. And if you run the fault backward in your mind, about a million years ago the Piedmont block was down by San Leandro, where the fault was forcing it past the big body of solid rock that makes up the hills there. When the irresistible force of a fault does this, the immovable objects it brings together have nowhere to go but up, and that’s what I think happened: the Piedmont block got hip-checked. As further evidence, I cite the odd presence of deep Dimond Canyon cutting through the Piedmont block.
It makes sense if powerful San Leandro Creek was flowing over the block while it was rising, carving this textbook water gap. That hypothesis — a mix of inference, deduction and corroboration — is my contribution to science. I feel pretty confident about it.
Oakland Geology 
26 September 2022 at 1:09 pm
I love that you figured that out – it makes great sense to me, not a geologist but a reader of some geology books. I did, years ago, come up with a name in my mind for what you call the Fan – I think of it as the East Oakland Plateau. My first residence in California was on it (E. 22nd St on Ivy Hill), and going back and forth to Mills College by bike and bus I sensed its (carved) plateau-ness immediately. Reading your work has made my wanderings over and through it a much richer experience.