University of Edinburgh literature professor David Farrier adapts his book Footprints: In Search of Future Fossils for the BBC:
If cities have a geological character, it begs the question of what they will leave behind in the stratigraphy of the 21st Century. Fossils are a kind of planetary memory of the shapes the world once wore. Just as the landscapes of the deep past are not forgotten, how will the rock record of the deep future remember Shanghai, New York and other great cities?
The main components of a modern city have their origins in geology and are therefore, in their different ways, highly durable. The majority of the world’s iron ore formed nearly two billion years ago. The sand, gravel, and quartz in concrete are among the most resilient substances on Earth. These hard-wearing materials once existed in natural deposits. But where before it was only water, gravity, or tectonic activity that moved them, now it’s a combination of human initiative and hydrocarbon fuels.
At first [a] car will simply rust but, as iron dissolves well in anoxic water, once the oxygen level decreases its metal components will begin to dissolve. Or perhaps a part of the chassis will mineralise, reacting with sulphides to form pyrite. The iron in steel beams or embedded in reinforced concrete, kitchen implements, or even tiny quantities of iron in the speaker of a mobile phone will all acquire a glittering sheen. Even whole rooms – a food court kitchen fitted with stainless steel worktops – might be transformed into fool’s gold.
Of course, if humans continue to live in a technological society far into the future (and why not?), modern cities might vanish by our hands. Won't that confuse future archaeologists.