Many sidewalks in Oakland are stamped with information about when and by whom they were made. By looking at a variety of sidewalk stamps in an area, you can sometimes get an idea of the age of the houses in that area, or at least the age of the sidewalks.

There's a rad blog appropriately called Oakland Sidewalk Stamps that documents interesting & unique sidewalk stamps in Oakland. It's run by Andrew Alden, who also runs the Oakland Geology blog.

Some sidewalk stamps from around Oakland. Photos CC SA-BY Our Oakland: