How to Read a Geologic Map 3
Page 3, A Dispute Over Colors page 1 page 2
You could have a geologic map without using colors, just lines and letter symbols in black and white. But it would be user-unfriendly, like a paint-by-numbers drawing without the paint. But what colors to use for the various ages of rocks? When that question first arose in the late 1800s, the geologists of the world couldn't agree.
After several decades of intensive mapping in the 1800s, the big picture of the continents began to emerge. For the first time, a reasonable map of Europe's rocks—and America's too—was possible. Scientific organizations began to work out a consistent scheme of colors to use. This took a lot of discussion, because each nation's geologists had strong preferences. Nevertheless, by 1881 European geologists were about to agree on a color scheme for the first geologic map of Europe.
American geologists had different needs—North America has large areas of Paleozoic rocks, whereas Europe is largely younger rocks of Mesozoic and Tertiary age. The European scheme gave lots of colors to the younger rocks, but the Paleozoic was mostly gray. An American map using shades of gray for its wide areas of Paleozoic rocks "would offend good taste and be illegible," as Bailey Willis later wrote. "Moreover . . . the choice of colors remaining available for the Paleozoic is much too limited for satisfactory discriminations."
In 1881 John Wesley Powell, who had just become director of the U.S. Geological Survey, felt the need to publish a geologic map of the nation as fast as possible. To outmaneuver the Europeans, he issued a color standard ahead of them. That has since become known as the American standard, and the European colors evolved into the International standard. The colors used on the Texas state geologic map, shown here, follow the American standard. And I use these colors in my geologic time scale.
But every geologic map has its reasons to vary from the standard. Perhaps rocks of certain time periods are missing so that their colors are available for use elsewhere; perhaps the colors would clash badly; perhaps the cost of printing forces compromises. That is another reason why geologic maps are so interesting: each one is a customized solution to a particular set of needs, and one of those needs, in every case, is that the map be pleasing to the eye.
I've been adapting printed geologic maps for the Web for several years, but I believe paper will be the best way to print maps for a long time to come. (This is a separate question from map databases, which are essential for analysis and have advantages of their own.) The limitations of the computer screen are severe, in both size and resolution. With a map on the wall, or spread on the car hood out in the field, we can step close or stand back without a thought, whereas that still is clumsy to do on the screen.
Map images must be compressed for the Web, and geologic maps suffer under that compression. Part of it is color compression. The GIF standard uses too few colors for any but the simplest maps, though GIF maps can work well if carefully made for that standard, like the online geologic map of New Mexico or my own South American maps. The JPEG standard is better for scanned images, but it's weak at rendering dark, contrasting colors. With some image processing, I can produce versions with reasonable file sizes. Still, the colors that result are no match for the printed version. Compression also degrades maps when the images must be shrunk to manageable file sizes.
I continue with this work, though, because even though online versions of printed maps aren't as good as the originals, without them there isn't much on the Web. If you like what you see, think about buying the real thing. Geologic maps are even better in person.
PS: A feast of old geologic maps is available at the David Rumsey Collection, including the first geologic map of the United States, by William Maclure in 1809. Just search on the keyword "geology." The Rumsey site uses the MrSID compression scheme for a really nice experience.
