Geologic Maps

The  National Geographic Society  defines a map as a symbolic representation of selected characteristics of a place, usually drawn on a flat surface. Maps can convey detailed information in a simplistic and visual way. The two broad categories of maps are thematic and reference. Find out a little more about each below.

Thematic maps convey relationships or patterns between themes (topics) and geographic areas. Information conveyed on a thematic map will likely change over time. An example of a thematic map is a weather map such as the one shown here. This map is conveying the amount of precipiation over a geographic area. Other types of thematic maps may show population density for a given country, rainfall amounts in the Amazon basin, or even a map that displays political affiliation for counties in North Carolina.

General reference maps show physical and/or cultural features in an area and their main purpose is to answer questions about where something is located in a space. A road map is a general reference map that you can use to get from one location to another and the map might include locations of cultural interest along the route. A topographic map that shows elevation is also a great example of a general reference map, as it tells you the elevations of certain features in relation to sea level.

We're going to discuss one type of map in particular - the geologic map. Which category of map does a geologic map fit in to? Since geologic maps show physical features that aren't likely to change much over time, it falls into the general reference map category.

Most folks aren't familiar with geologic maps but if you ask a geologist they're likely to tell you that they are critically important maps, which we'll discuss in more detail below. Not only are geologic maps important, they're also works of art! Just check out the geologic map below of the United States and Canada. If that's not an intersection of art and science then I don't know what is.

If you're not a geologist, you might be thinking there's no way you could read a map like this. I mean, just look at all those colors! Even without knowing what the colors mean, I bet you've already noticed patterns in the map. Did you notice the circular pattern of colors over Michigan? What about the northeast/southwest trending color pattern along the east coast? Where do we even start with everything going on in the Rocky Mountains area? You might not realize it, but by recognizing patterns in the map, you're already thinking like a geologist.

Let's start our journey into all things geologic maps - what they are, the vital information they can convey, why they're important resources for society, and what it takes to make one.

Generally, geologic maps show rock units, sediment that covers the rocks, and structures that are present above and below ground surface.

All the colors, patterns, and symbols on geologic maps provide loads of information to help us understand the materials that make up the Earth and the way those materials are positioned on and within the Earth. Rock types are represented by various colors and geologic strata and layers are shown with symbols. Structural features such as folds, faults, dips, and bedding planes can be shown using symbols that give us three-dimensional information that help us understand what's going on below Earth's surface.

Geologic maps provide important information for resource location and identification (gravel for roads, groundwater for drinking), for land-use planning (zoning), for hazard awarenenss and mitigation (landslides and coastal erosion), as well as important data for our growing world economy (minerals that are critical to society).

What does a geologic map show us? So much information! Don't get too overwhelmed - we'll break down the components of a geologic map so the next time you see one, you'll be better prepared to read it and understand the infomation it conveys. Check out some cool features below.

We've learned so far that geologic maps show us rock units and geologic structures. They can provide cross sections of features below ground and data about samples that have been collected. Now let's explore why this information is important and how we can use the data provided on the maps.

Geologic maps can provide information to help you prepare for, and minimize risks from, hazards like landslides, rock falls, flooding, earthquakes, and even volcanoes. If you think you are located in a hazard zone, reach out to a geologist with the NC Geological Survey to learn about measures you can take to minimize your risks.

Landslides and Debris Flows: There are two ways that geologic maps show us where landslides, rock falls, and debris flows might happen - by pinpointing the rocks and sediments that are susceptible to these hazards and by locating the places where they have previously occurred. Check out this interactive and informative Landslides in Western North Carolina  website  and its associated geologic maps from the NC Geological Survey.

Earthquakes: Geologic maps identify fault zones, places where there has been or might be movement between rock units. Earthquakes occur where fault zones are active.

Flooding: Land-use planners and government officials use geologic maps to identify locations of flood basins and flood plains as well as other low-lying or swampy areas to help predict areas that might be prone to flooding.

Sinkholes: Sinkholes occur in areas that are underlain by sedimentary rocks that contain calcite, such as limestone, dolomite, and gypsum. Calcite is easily dissolved by acidic water and this dissolution can cause these circular, funnel-shaped depressions or holes. Geologic maps identify where these types of rocks are found.

According to the  US Geological Survey , about 15% of the the U.S. population relies on domestic (private) drinking water wells to extract groundwater for use. A geologic map such as the one shown here provides the locations of potential water-bearing fractures or features like diabase dikes, which can help you to determine where to drill a water well on your property.

Geologic maps can also help land use planners decide where to build infrastructure like roads and buildings based on areas of well-drained sediment or areas that aren't underlain by clay sediments that might shrink or swell. In order to build infrastructure for cities and towns, developers need raw materials such as gravel, aggregate, and other earth materials. Studying geologic maps can help determine where the best locations are for mining of those materials.

Geology can affect your health! Part of the job of mapping the geology of an area is finding rocks and sediments that contain hazardous elements or minerals. Geologic maps can showcase this important information.

Radon is a naturally-occurring gas formed from the natural, radioactive decay of uranium in rocks and soil. It's the leading cause of lung cancer in non-smokers, according to estimates by the  US EPA . Radon can enter your home and get in to the air you breathe. It can even be found in your drinking water. Geologists can map the locations of radon-bearing rocks which helps communities and health officials determine those most at risk of being exposed to radon. You can find more information on radon from the  NC Radon Program , a division of NC Department of Health and Human Services.

Arsenic is also a naturally-occuring element in rocks and is not inherently dangerous. When arsenic is released into the environment by the weathering of rocks, however, it can seep into groundwater that you might drink. Arsenic is found in groundwater throughout the cental part of the North Carolina Piedmont region. Long-term exposure to low levels of arsenic may pose health risks, such as cancer, to humans. The NC Geological Survey in collaboration with staff of the North Carolina Division of Water Resources developed an  Areas of Relative Susceptibility to Detectible Arsenic in Groundwater in Orange County, NC  information poster and associated geologic map.

These are just a few examples of the importance of geologic maps and the information that they convey. If you'd like more information on the importance of geologic maps, please visit these resources:

•  North Carolina Geological Survey 
• U.S. Geological Survey -  The National Geologic Map Database 
•  National Park Service 
•  American Geosciences Institute 

Geologic maps display information that was manually collected by geologists in the field, was analyzed by geologists in the lab, and was brought to life by a map maker called a cartographer. By walking and traversing the landscape, geologists are able to collect rock and sediment data, to make interpretations about rocks beneath the surface, and to interpret and make assumptions about the processes that occurred in the geologic past. Geologic mapping is very labor intensive and requires a great deal of walking, hiking, measuring, inferring, and analyzing to collect the data necessary to produce a map. Maps for the NC Geological Survey are produced and finalized by our staff cartographer.

Next year, 2023, the North Carolina Geological Survey will be celebrating its 200th anniversary! On December 31, 1823 the state legislature approved an act that lead to creation of the state's first survey of North Carolina's geological resources. Denison Olmstead (Yale, UNC Chapel Hill) traveled the state on horseback, from the mountains to the coast, in order to produce the first geologic map of North Carolina in November 1825. The map was hand-drafted with color ink and displays eight geologic divisions. The map is currently housed in the North Carolina Division of Archives and History in Raleigh. Pretty cool!

It has taken 200 years, many staff members, hundreds of miles walked, and immeasurable hours worked to produce the current version of our geologic map. You can order or download a copy of the map by visiting the  Map and Publications  page of our website.

If interactive is more your style, you can get an in-depth look at our geologic map and the geology below your feet with this online version:

We appreciate you taking the time for this geologic map journey. We hope you learned a bit about geologic maps, their importance, and why we produce them. If you have questions, comments, or suggestions, please contact  Amy Pitts , Education and Outreach Coordinator for the NC Geological Survey. Have a gneiss day!