How to Read a Geologic Map 2

Page 2, Symbolizing Geology on Paper   page 1   page 3

Here's a small sample of a real geologic map. You can see the basic things discussed on page 1—the shorelines, roads, towns, buildings and borders—in gray. The contours are there too, in brown, plus the symbols for various water features in blue. All that is on the map's base. The geologic part consists of the black lines, symbols and labels, plus the areas of color. And just as ordinary maps have an explanation to show what the symbols stand for, a geologic map has an explanation that can be quite elaborate. It has to be.

The lines and symbols condense a great deal of information that geologists have gathered through years of fieldwork. A crucial thing to show is the contacts between different rock units. On the map, contacts are shown by a fine line, unless the contact is a fault.

A fault is a discontinuity so sharp that it's clear something has moved there. There's one in the lower right corner of the image. In this particular place, it's not clear what kind of fault it is, so it's shown with a simple heavy line. Most faults on a geologic map are inactive, rather than what people think of as earthquake faults. They were active at some time deep in the past, but a fault in today's rocks is evidence only that motion happened once upon a time, not that it's occurring right now.

The strike-and-dip symbols give us the third dimension of the rock layers—the direction they extend into the ground. Geologists measure the orientation of rocks wherever they can find a suitable outcrop, using a compass and transit. In sedimentary rocks they look for the bedding planes, the layers of sediment. In other rocks the signs of bedding may be wiped out, so the direction of foliation, or layers of minerals, is measured instead.

In either case the orientation is recorded as a strike and a dip. The strike of the rock's bedding or foliation is the direction of a level line across its surface—the direction you would walk without going uphill or downhill. Dip is how steeply the bed or foliation slopes downhill. If you picture a street running straight down a hillside, the painted center line on the road is the dip direction and a painted crosswalk is the strike. Those two numbers are all you need to characterize the rock's orientation. On the map, each symbol usually represents the average of many measurements.

The symbol near the bottom shows the direction of lineation with an extra arrow. Lineation might be a set of folds, or a slickenside, or stretched-out mineral grains or similar feature. If you imagine a random sheet of newspaper lying on that street, lineation is the printing on it, and the arrow shows the direction it reads. The number represents the plunge, or the dip angle in that direction.

Then there are the letter symbols. They signify the name of the rock unit in an area. The first letter refers to the geologic age. On the sample, J is Jurassic, P is Permian, and Z is the late Proterozoic. The other letters refer to the formation name or the rock type. (To see what these units are, take a look at the geologic map of Rhode Island, where this comes from.)

All of this information, strike and dip and trend and plunge and age and rock unit, is won from the countryside by the hard work and trained eyes of geologists.

But the real beauty of geologic maps—not just the information they represent—is in their colors. Let's have a look at them.

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