Frequently Asked Questions

Geology of the San Francisco North quadrangle

What is a geologic map?

Like all maps, a geologic map shows where things are. But while other maps highlight where you can find things like streets and streams, a geologic map shows the distribution, nature, and age relationships of rocks, faults, strata, and other geologic features.

How do you read a geologic map?
Each geologic map has a map key, which is a table explaining the meanings of all colors and symbols used to represent geologic features in the map. For example, geologic units usually are listed in order from the youngest (most recently formed) rock types to the oldest (formed earliest in time). The key often will give the name of the each unit, as well as the age and a brief description of that unit’s rocks.

In addition to units, the key usually explains the map’s use of lines and symbols. Lines might show where two units meet and perhaps bend, fold, and warp up against one another. Symbols might indicate where you can find things like fossils, precious metals, or active faults.

A strike and dip symbol, for instance, is a T-shaped symbol that shows where layers of rock stack up in tilted beds. The number accompanying a strike and dip symbol indicates the angle, or tilt, of the rock bed.

How do you use a geologic map?
Like people all over the world, your life is shaped by the geology of your area. Is the ground good for building or farming? Are you likely to find available groundwater? What natural resources are there underground? What is the likelihood of a natural disaster such as an active volcano or earthquake?

As we can see on the map on the other side of this poster, geologic maps are used to identify many features and phenomena, from coal resources and potential landslides to vital ecosystems and animal habitats. Geologic maps are necessary to help us navigate among the many challenges and opportunities offered by the dynamic Earth systems that surround us.

(Adapted from Geologic Maps, USGS, and Meeting Challenges With Geologic Maps, AGI.)