Created solely by English surveyor William Smith, it is based on his discovery that sedimentary rock across southern Britain contains layers that are arranged in a set sequence, from oldest rocks to youngest. What's more, even if sheets of rock from different time periods are of a similar colour, each contains distinctive fossils that can be used to identify the layer.
Smith used an innovative shading technique for his map. Painting with watercolours, he chose darker tones for bottom layers of rock, which gave the impression of three dimensions when surrounded by the paler colours of more recently-deposited rock.
The map was intended as a reference tool for mineral exploration, land drainage and agriculture. It was of interest to land owners at the time because it helped determine whether their property was likely to contain deposits of coal, which was in high demand.
Today, the oil industry still uses Smith's fossil correlating method to distinguish between rock layers. The technique has even proved useful on other planets – it was recently used to build up a new geological map of Mars.
Ceurstemont, Sandrine. 2015. “Map that changed the world has its 200th birthday”. New Scientist. Posted: January 15, 2015. Available online: http://www.newscientist.com/article/dn26803-map-that-changed-the-world-has-its-200th-birthday.html#.VM40umTF_d0