Contours and topography are just the first part of a geologic map. The map also puts rock types, geologic structures and more onto the printed page through colors, patterns and symbols.
Here's a small sample of a real geologic map. You can see the basic things discussed earlier—the shorelines, roads, towns, buildings and borders—in gray. The contours are there too, in brown, plus the symbols for various water features in blue. All of that is on the map's base. The geologic part consists of the black lines, symbols and labels, plus the areas of color. The lines and the symbols condense a great deal of information that geologists have gathered through years of fieldwork.