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Precambrian and Paleozoic rocks at the mouth of the Grand Canyon
Precambrian and Paleozoic rocks at the mouth of the Grand Canyon

Geologic Maps

Strike and dip

Many kinds of rocks form in broad, flat layers, called beds, that stack up like the layers of a cake. In areas like northern Arizona, thick stacks of rock beds that have built up over millions of years remain in their original flat orientation (where they can be viewed as multicolored horizontal layers of rock that make up the spectacular walls of the Grand Canyon). In places like California near active plate boundaries, however, the forces that make earthquakes don't leave the beds flat for long, but bend and tilt them.

tilted sedimentary rocks
Tilted sedimentary rocks
tilted sedimentary rocks
Geologic map represetation of tilted sedimentary rocks.

Tilted beds are shown on a geological map with a strike and dip symbol (location 10). The symbol consists of three parts: a long line, a short line, and a number. The long line is called the strike line, and shows the direction in the bed that is still horizontal. Any tilted surface has a direction that is horizontal (think about walking on the side of a hill, there is always a way to go that is neither up nor down, but is level).

The strike line shows that horizontal direction in the beds. The short line is called the dip line, and shows which way the bed is tilted. The number is called the dip, and shows how much the bed is tilted, in degrees, from flat. The higher the number, the steeper the tilting of the bed, all the way up to 90 degrees if the bed is tilted all the way onto its side. Strike and dip symbols can be modified to give more information about the tilted beds just like lines can be, and these modifications are also explained in the MAP KEY.

Map Key

All geologic maps come with a table called a map key. In the map key, all the colors and symbols are shown and explained. The map key usually starts with a list showing the color and letter symbol of every geologic unit, starting with the youngest or most recently formed units (in the example map those are the man-made deposits), along with the name of the unit (if it has one) and a short description of the kinds of rocks in that unit and their age (in the key, the age is described by Epochs, subdivisions of the Periods shown in the letter symbol). After the list of geologic units, all the different types of lines on the map are explained, and then all the different strike and dip symbols. The map key will also include explanations of any other kinds of geologic symbols used on a map (locations where fossils were found, locations of deposits of precious metals, location of faults known to be active, and any other geologic feature that might be important in the area shown by the geologic map). Because the geology in every area is different, the map key is vital to understanding the geologic map.

GEOLOGIC MAP OF PART OF THE SAN FRANCISCO BAY AREA, CALIFORNIA, USA
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SUMMARY

Although the geology of every area is different, all geologic maps have several features in common: colored areas and letter symbols to represent the kind of rock unit at the surface in any given area, lines to show the type and location of contacts and faults, and strike and dip symbols to show which way layers are tilted. The geology of an area has a profound effect on many things, from the likelihood of landslides, to the availability of groundwater in wells, from the amount of shaking suffered in an earthquake, to the presence of desirable minerals, from the way the landscape is shaped to the kinds of plants that grow best there. Understanding the earth underneath is the first step in understanding the world around us. So what's the geology like in your neighborhood? If you can read a geologic map you can probably find out!

To find out how to obtain geologic maps of your region, visit the Western Region Geologic Mapping Site

HOW TO READ A GEOLOGIC MAP, was originally developed by Russell W. Graymer, Western Region Mapping Team, U.S. Geological Survey, for the 1997 USGS Open house.

| What is a geologic map? | Colors | Letter symbols |
| Lines | Faults | Strike and dip | Map key |

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