In the late 1400s, the great Leonardo stood at the foot of a cliff, looked at the layers, and scratched his head. There were shells, so it had to be something to do with the sea. But the layers, all those layers … how come?

Layer upon layer upon layer. Layers within layers. Some thick, some thin. Some hard, some soft. Some one color, some another. The interfaces sometimes bold, sometimes subtle. But no obvious order. How come indeed?

In the 1670s Steno confirmed the marine origin of many layers, and by the 1760s Lavoisier had distinguished between layers...

You do not currently have access to this article.