Geologic maps rely on color to symbolize the different ages of rocks. But what colors to use? When that question first arose in the late 1800s, the geologists of the world couldn't agree.
A Dispute over Colors
After several decades of intensive mapping in the 1800s, the different periods of the geologic time scale were established and 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 at the Second International Geological Congress in Bologna, Italy.
American geologists had their preferences, too, imposed by their particular geology—North America has large areas of Paleozoic rocks, whereas Europe is largely of Mesozoic and Tertiary age. The European scheme assigned lots of colors to these 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 been elaborated into the American standard, and the European colors chosen in 1881 evolved into the International standard. The colors used on most of the U.S. state geologic maps on this site follow the American standard.
A Contrast in Continents?
People have strong color preferences for reasons that we can only speculate about. I speculate that the differing geologic layouts of the United States and Europe may have influenced their Earth scientists in at least two respects. I will speak here in exaggerated, highly schematic terms for the sake of argument.
1. Tectonics: Europe is a compact continent of irregular shape, corrugated by mountain belts running in many different directions. Its geologic map is a collection of small basins lying between the cryptic granitoids of Scandinavia and the bewildering metamorphosed Alpine chain. The United States presents a relative simplicity with two long cordilleras—the Appalachians and the Rockies—flanking a broad hinterland of flat-lying sedimentary rocks rich in fossils. Its geologic map shows a clean progression of rock ages with a sweep of continental scale. Where Europe is a puzzle for structural geologists and petrologists, America is a stratigrapher's dream. America lays out geological history in an integrated vertical stack, spread across the midcontinent like a swept-out deck of cards.
2. Politics: Geology arose in Europe during the 17th and 18th centuries among several different national scientific communities who worked in partial isolation due to geography and language. There was a gulf in social class between practical and academic thinkers. Worthy science was carried out by English, French, German and Italian speakers who communicated in Latin. Political turmoil and difficulty in travel hampered collaboration. Science relied on royal patronage and independent wealth. America was mapped in the mid-19th century by a wave of English-speaking geologists, funded by the state, who traveled widely on railroads and horseback and shared a common focus on geologic resources.
To summarize these in a label, I would suggest that Europeans saw their geologic map as a patchwork quilt and Americans saw theirs as a unified tapestry. Thus the American color standard is more muted and harmonious than the European.
Color Questions and a Modern Resolution
I can't tell whether my argument is valid or just a fun parlor game. Does the American scheme hide meaningful differences? Does the European standard ignore larger-scale connections? Is it meaningful to have a jarring contrast between the adjoining Permian and Triassic in European maps, or an equable gradation across the Mesozoic in American ones? Is the Devonian Period slate-blue, or is it tan? Are map colors meaningful, or are they an exercise in synesthesia?
I do know that in 2009, the Committee for the Geologic Map of the World introduced a new color scheme that lies between the two traditions, and that's the one I use for the geologic time scale. Whatever the standard is, it's only the starting point for putting the variety of the Earth's rocks on a piece of paper.
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.
