How to Read a Geologic Map

A geological map is a map that shows the different types of rocks and their distribution in an area. It is a fundamental tool for geologists and other earth scientists, as it can be used to understand the history of a region, identify potential resources, and assess hazards such as landslides and earthquakes.

To Read a Geological Map, You Will Need to

Identify the Map's Title, Scale, and Legend

The title of the map will tell you what area it is depicting, and the scale will tell you how much distance is represented by each unit of measurement on the map. The legend will explain the different symbols and colors used on the map to represent different rock units, structures, and other features.

Examine the Map's Orientation

The map should be oriented so that north is at the top and south is at the bottom. If it is not, you can usually orient it by using the compass rose, which is a symbol that shows the four cardinal directions.

How to Read a Geological Map Wildly Colorful Geologic Maps of National Parks

Interpret the Map's Symbols

Use the legend to identify the different types of rocks and features that are represented on the map.

Identify the Different Rock Units

The different rock units on the map will be shown in different colors or patterns. The legend will tell you what each color or pattern represents.

Determine the Age of the Rock Units

The age of the rock units is usually shown by the order in which they are stacked on top of each other. The oldest rock units will be at the bottom of the map, and the youngest rock units will be at the top.

Identify the Contacts Between Rock Units

The contacts between rock units are the lines that show where one rock unit ends and another begins. The contacts can be either conformable (smooth and continuous) or discordant (abrupt and irregular).

Identify the Structures on the Map

Structures are features that have been created by the movement of the Earth's crust. Some common structures include faults, folds, and dikes. The symbols and colors used to represent structures on the map will be explained in the legend.

Find the Strike and Dip of Rock Layers

The strike of a rock layer is the direction in which it is flattest. The dip is the angle at which it slopes downward. These two measurements can be used to reconstruct the three-dimensional structure of the rocks in an area.

Draw a Cross-section

A cross-section is a two-dimensional representation of a three-dimensional feature. You can draw a cross-section of a geological map by first selecting a line on the map that you want to represent the cross-section. Then, use the information on the map to determine the rock units and structures that would be encountered along the line of the cross-section.

Interpret the Geological History of the Area

Once you have identified the different rock units, their ages, and the contacts between them, you can begin to interpret the geological history of the area. You can use your knowledge of geological processes to reconstruct how the rocks were formed and how they have been modified over time.

With a little practice, you will be able to use geological maps to learn more about the Earth's history and understand the natural hazards that may affect your area.

Geologic Map Symbols

Geological mapping symbols are used to represent the different types of rocks and rock features on a geological map. They are standardized so that geologists can easily interpret geological maps from different sources.