Geologic maps may be the most concentrated form of knowledge ever put on paper, a combination of truth and beauty. Here's how to understand them.
The map in your car's glove compartment doesn't have much on it beyond highways, towns, shorelines, and borders. And yet if you look at it closely, you can see how hard it is to fit all that detail on paper so it's useful. Now imagine that you want to also include useful information about the geology of that same area.
What's important to geologists? For one thing, geology is about the shape of the land—where the hills and valleys lie, the pattern of streams and angle of slopes, and so on. For that kind of detail about the land itself, you want a topographic or contour map, like those published by the government.
Here's the classic illustration from the U.S. Geological Survey of how a real landscape on the top translates to the contour map beneath it. The shapes of the hills and dales are depicted on the map by fine lines that are contours—lines of equal elevation. If you imagine the sea rising, those lines show where the shoreline would be after every 20 feet of depth. (They could equally well represent meters, of course.)