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