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Maps Collections in the Research Center: Geological Maps Basics

What are Geological Maps?

Most types of maps show man-made or natural features found on the earth's surface. Geologic maps are different from other maps since they typically show what is beneath the surface of the earth. Geology itself is the study of the earth and its history. Thus, geologic maps are used to present, among other things, formations of rocks and minerals, underground liquids and gases, and the movements and changes of the earth that occur over time.

Geologic maps serve an important purpose since, as stated by the United States Geological Survey (USGS), "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."

Map Reading Tips

The USGS has detailed information on reading geologic maps. Some of the basic things to remember are:

  • A base map, which is a regular map, is printed under the geologic map in light colors to help orient the user.
  • Look for a map key to identify symbols and colors used in the map. Sometimes the key is on a separate map sheet if it is extensive.
  • Letters on the map refer to the age and name of the particular geologic unit.
  • Lines on the map represent contact between geologic units. The shorter the dashes in the line, the more uncertain the location of the units.
  • Short lines with numbers are used to show the angle and direction of tilted rock beds that have been thrust up unevenly.