Oakland has some small areas mapped as marine terraces. They run about 20 to 40 feet above sea level and they’re very flat. Here’s a look down 5th Avenue toward the bay that shows the terrace well, beyond the dip in the road. I was standing at E. 20th Street, on the Pleistocene fan; the low point in the road is at E. 18th Street.
I name it Clinton terrace because it underlies the former town of Clinton. The terrace runs along the foot of the fan as far as Fruitvale. Here it is on the geologic map, unit Qmt, “Quaternary marine terrace deposits (Pleistocene).” I was standing at the asterisk.
In the map description, Russ Graymer correlates this terrace with other terraces along San Pablo Bay at Lone Tree Point and Wilson Point. Those have been dated at about 125,000 years old using uranium-thorium dating of oyster shells. That time was a well-known highstand of sea level, and coeval terraces occur up and down the Pacific coast. So we imagine the bay waves lapping against the older alluvial fan, nibbling off sediment and spreading it around to build a nice flat terrace. It was probably a lovely tidal marsh until the glaciers resumed, the sea fell and land vegetation moved in. Later, creeks dug into it along Park Boulevard, 14th Avenue and 23rd Avenue.
You can see there’s a bit more of the terrace under Lakeside Park. A final piece underlies the Valdez Street area beyond the upper left corner.