How to Read a Geologic Map 1

Page 1, Starting on the Ground   page 2   page 3

_{A portion of the online Geologic Map of Utah}

Geologic maps may be the most concentrated form of knowledge ever put on paper, and one of the hardest to put in a computer. So why do I put so many geologic maps online? Because I believe, like Keats, that truth—in this case, the ground truth of the Earth—is beauty.

The map in your car's glove compartment doesn't have much on it beyond highways, towns, shorelines, and borders. And yet if you look at it closely, you can see how hard it is to put all that detail on paper so it's useful. Now imagine that you want to also include useful information about the geology of that same area.

What's important to geologists? For one thing, geology is about the shape of the land—where the hills and valleys lie, the pattern of streams and angle of slopes, and so on. For that kind of detail about the land itself, you want a contour map, like those published by the government. Here's an example from the U.S. Department of Commerce.

You can see the roads, streams, railroads, place names and other elements of any proper map. The shape of San Bruno Mountain is depicted by fine lines that are contours—lines of equal elevation. If you imagine the sea rising, those lines show where the shoreline would be after every 200 feet of depth. (The thicker contour marks the 1000-foot level.) With some practice, you can get a good mental picture of what's going on. Here's the classic illustration from the U.S. Geological Survey of how a real landscape on the top translates to the contour map beneath it.

_{U.S. Geological Survey images}

Notice that even though the map is a flat sheet, you can still figure out accurate numbers for hill slopes and gradients from the data encoded in the image: you can measure horizontal distance right off the paper, and the vertical distance is in the contours. That's simple arithmetic, suitable for computers. And indeed the USGS has taken all its maps and created a "3D" digital map for the whole 48 states that reconstitutes the shape of the land that way. The map is shaded through another calculation to model how the sun would illuminate it. Here's a piece of it showing New York and southern New England (see the whole thing here.)

That's just the first part of a geologic map. The map also gets into the rocks that underlie the land surface. Turn the page to see how geologic maps can pack all of that—rock types, geologic structures and more—onto a printed page through colors, patterns and symbols.

Next page > The Full Geologic Monty > Page 2, 3

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