Why I Hunt for Sidewalk Fossils

These oft-overlooked records invite us to imagine what has been and what might be.

A paleontologist once told me that city sidewalks hold snapshots. If I trained my gaze toward my feet, he said, I would find evidence of all kinds of commutes: traces of hopping birds, the soles of humans shoes, restless leaves that fell and sank into wet concrete at just the right moment. I might see a smattering of little paw prints zigging, zagging, doubling back, evidence of important rodent business that didnt often overlap with mine.

These marks are too recent to pass muster with scientific sticklers, but in all respects except age, they are fossils. There are many ways to make one. Some form when a creature is entombed in sediment: Water percolates through, flush with minerals, and over time the mixture infiltrates the bones, where it settles and forms stone. Other fossils are casts, made, for instance, when a shell dissolves and leaves behind a mold that eventually fills with sediment, which hardens into rock. But not all fossils involve remains; some catalog movements. These are the kind that stipple our sidewalks nascent trace fossils, records of fleeting contact.