Today is Geologic Map Day, which gives me a great excuse to write about one of the most interesting, beautiful and informative information sources available, the geologic map.

A geologic map illustrates the rocks beneath our feet: their age, distribution, relationships and basic composition. Geologic maps illustrate faults and folds, and are used to help geologist find oil, water, minerals and precious metals. And because bright color is used to distinguish between different types of rock, they are very pretty.

The first modern geologic maps were produced over two hundred years ago in England by a canal surveyor named William Smith. Smith was a working man, and his humble origins prevented him from getting the recognition his work deserved until much later. He recognized that fossils can be used to make connections between similarly aged rocks, even over long distances and even when the composition of the rock changes. This understanding allowed him to create the first geologic map of England and Wales (and a bit of Scotland). Readers interested in 19th century science and the development of geologic maps should check out Simon Winchester's book on the topic, The Map That Changed the World.

Print geologic maps can be found as stand alone publications, often accompanied by a brief "explanation." Libraries and geosciences departments often keep them tucked away in special map drawers, or folded up in envelopes attached to the brief explanations that accompany them. The National Geologic Map Database can help users locate geologic maps of areas they are interested in.

This gorgeous map was produced in print in 2005 by the Geological Society of America (click to see a much larger image):

Try this at home:

Click on the map image to see the large version of the map. Locate your hometown on the map, and make note of the color, pattern (if applicable) and two or three letter code. Next, open up the PDF of the map explanation here . (It's a big map with a lot of detail, so the explanation is a large file.) Are the rocks beneath your hometown sedimentary, igneous (volcanic or plutonic), or metamorphic (see the labels across the top of the explanatory chart)? How old are they (see the geologic time labels along the left side of the chart)?

I live near Buffalo, NY, and the map illustrates this as a light blue color (teal perhaps?) with the two letter code "mD." Looking at the explanation, I can see that the rocks near my house are sedimentary rocks from the middle Devonian period, formed about 390 million years ago (see this Table of Geologic Time to turn your geologic time period into millions of years).

While print maps are still incredibly useful, scientists need to be able to manipulate the data they contain and combine that data with other information to make new discoveries. So more and more geospatial information is available as GIS datasets or Google Earth downloads. GIS data, metadata and the map explanation for the Geologic Map of North America (illustrated above) can be downloaded from the United States Geological Survey.

Other small maps are often published as figures in journal articles. These can be more difficult to find, as it isn't always clear from a journal article title whether the article will contain a usable map. Elsevier has a tool for searching for maps within publications (aimed at the petroleum industry) called GeoFacets. And I am excited by an upcoming announcement by GeoScienceWorld, about their new geologic maps finding tool, OpenGeoSci. GeoScienceWorld will debut the beta version of this new tool at the Geological Society of America meeting at the end of the month.

Geologic maps, an information format developed 200 years ago, continue to help scientists make discoveries and find solutions. Although much of this data has now gone digital, these are still some of the most beautiful information resources out there.

Additional geologic maps resources: