William Smith's kaleidoscopic 1815 geologic map of England was the bedrock of many modern land-based industries.
There's a good chance your last encounter with a geologic map was in 8th grade science class, and your memories of it are sandwiched between awful puns about minerals (I'm not gneiss, I'm full of schist).
But you should care about geologic maps, because they aren't merely arcane tools for those who study earth sciences. They're essential to anyone who uses the earth's resources or builds on it. By visualizing the age, makeup, and relationships of rocks along a given swathe of the earth’s surface, geologic maps are the best, simplest way decision-makers can "identify and protect valuable resources, avoid risks from natural hazards, and make wise use of our land," according to the Idaho Geological Survey. They are the reason your house isn't perched atop an ancient landslide area, and the reason Halliburton fracks where it fracks.
Wider literacy about their function and history is in order. Fortunately, a new exhibit at the Geological Society of London presents a great starting point: A rare, first-edition print of William Smith’s 1815 Geological Map of England and Wales, the first true geologic map of a nation ever produced.
Where previous attempts to map geology had only described rocks by type (here's gneiss, there's schist), Smith, a canal-digger-cum-"Father of English Geology," was the first to visually classify rocks according to their age and how they were deposited. His brightly colored, now-200-year-old document, which shows the stratigraphy of England, Wales, and part of Scotland, became the bedrock of geologic cartography—as well as of the modern world's industries around coal and oil, gold mining, highway systems, and railroad routes, as Simon Winchester describes in his 2001 book on Smith. Long thought lost, the particular copy now on display at the Society (both online and in the flesh) was rediscovered in the archives just last year.