Geologic Map Day

As part of Earth Science Week, today is Geologic Map Day!

Long before I became a geologist, I had already developed a love of maps, a love dating back to a cross-country trip I took with my family when I was five. Besides the road maps we traveled with, I had a large wall map of the U.S., on which I kept track of the states I had visited and the roads that had taken me there—these were the major highways of the day, like U.S. Highway 40, now replaced by I-80 in most places. As time went on, I learned more about other types of maps, in part thanks to National Geographic, and eventually, maybe in high school, learned how to read topographic maps and came to know the basics of reading a geologic map.

I got my first real taste of geologic mapping at field camp. I found it fairly challenging: I was still getting up to speed on the tools in use—simple tools like a quadrant-based Brunton compass—and I was also becoming gradually familiar with following contacts up and down and around hills, and across broad and hot expanses, mostly of desert.

Geologic mapping really became fun for me on our second mapping project during the summer of 1978 (several early stories are listed here), when I found that I could do it well and that a good geologic map tells a story about what has taken place in an area during past geologic times. I learned then that preconceived ideas about how the geology of an area might be such and such, accompanied by ideas that certain processes or events couldn't have taken place, or that others must have taken place, often didn't fit with the story the rocks were trying to tell me.

As geologic mappers, we must follow the contacts, over hill and dale as necessary; we must check out, ascertain, and describe the textures and rock types; we must follow basic geologic principles, while observing obvious signs and vague hints in the rocks that point to the order of events as shown by the principles of superposition and cross-cutting relationships. If the rocks are telling us something unexpected—or something thought to be unlikely or even impossible—we must listen to them. For myself, geologic mapping became much easier after I was willing to give up my limited preconceived ideas about what I should find out there in the field.

Geology of the Conterminous United States,
Digital version at 1:2,500,000 scale, courtesy USGS.

A geologic map is a thing of true beauty—although I don't really like the use of gray for Quaternary in this particular rendition of the geology of the 48 states.

Map courtesy USGS Geologic maps of US states.

Maps and Mapping Resources:

Geologic Map Day resources at AGI

FAQs about geologic maps at AGI

What is a geologic map? USGS at National Park Service

Introduction to geologic mapping, by the National Cooperative Geologic Mapping Program at the USGS

Geologic maps of US states at the USGS

Geology of the Conterminous United States, at the USGS

Nevada geologic map at the USGS

A puzzle of geologic regions at the USGS

The Basin and Range province at the USGS, Tapestry of Time and Terrain

Tuff All Over: ESW field trip by the NBMG on Saturday or Sunday, October 20 or 21, 2012