Parkfield lies along the San Andreas Fault, one of the longest and most active faults in the United States, which appears in the town as a seasonally dry creek bed. The fault marks the divide between the North American Plate and the Pacific Plate (see plate tectonics). There is a bridge across the creek with piers on either side that have shifted more than five feet relative to one another due to aseismic creep since the bridge was constructed in 1936. Google satellite images show that the bridge, which Simon Winchester described as "bent" and having cracked and patched pavement, has been replaced since Winchester's book was published, and a sliding joint installed; however, the west piers of the bridge remain displaced to the north. The bridge sports signage announcing to drivers that they are crossing the plate boundary.