How to Read a Geologic Map

Unlike your basic road map that focuses on the locations of roads and cities, geologic maps provide information about the rocks that are on or near the earth’s surface. To the trained eye, these maps help to explain the series of geologic events that led to the present-day distribution of rocks and other earth materials.

This page deconstructs a geologic map, identifying commonly found features. It also explains how to interpret a maphow maps are made, and shows some of the types of geologic maps available.

Who uses geologic maps?

Geologic maps provide a valuable source of information for geologists, engineers, planners, the mineral and construction industries, county and local units of government, and landowners who may be concerned with mineral resources, water supply, construction siting, utility routing, waste disposal, and other issues.

Geologic map of Marathon County with the parts of the map labeled 1-5

1. Map

Geologic maps are idealized pictures of the distribution of earth materials at or near the ground surface.

Close-up of the Marathon County map's Description of Map Units section
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2. Explanation

The description of map units defines each earth material (map unit) shown on the geologic map. Each map unit is defined by its own unique color, name-abbreviation, and text describing physical characteristics.

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3. Scale

The map scale relates the size of geologic features shown on the map to the actual size of those features. A scale of 1:100,000 means that one unit (an inch, for example) is equal to 100,000 units in real life.

Close-up of the Marathon County map's symbols section
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4. Symbols

This area shows diagrams, names, and descriptions of every symbol used on the map.

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5. Correlation of map units

The correlation of map units shows the relative age of each type of earth material (map unit) shown on the map.

This element may be absent from a geologic map when the units simply stack one on top of another, from oldest to youngest. In this case, the explanation does double duty, describing each unit and presenting it in the order of its relative age.

Close-up view of part of a geologic map
Detail from Bedrock Geology of Marathon County, Wisconsin

In addition to a geologic map’s features, there are several other elements that are commonly found on geologic maps. To know how to interpret a geologic map, it’s important to understand these elements.

Map units

Example of geologic map units keyMap units indicate different types of rocks or other earth materials by a distinct color and symbol.

In the map shown above, different colored map units show the distribution of various rock types at the earth’s surface.

Contacts and faults

Detail cropped from example map showing a fault mapped as a shear zoneContacts between different map units show where one kind of rock or other earth material passes into another. Sometimes the transition is clear (often depicted by a solid line) and sometimes it’s uncertain or approximate (a dashed line).

Faults are places where the earth’s crust has moved. On a geologic map, fault lines show where one unit has slid, been pushed up, or dropped down relative to the neighboring map unit.

In this example, a very deep part of a fault is exposed at the earth’s surface. In this case, there is a zone of deformed rocks rather than a single fault. This deformation zone is mapped as a shear zone, as indicated by the squiggly lines.

Strike and dip (orientation of rocks and faults)


Strike and dip example using a house roof for illustration
Another way of remembering strike and dip is to think of the roof of a house. “Strike” is the direction the ridge of the house points, while “dip” measures the slope of the roof.

Strike and dip symbolStrike and dip symbols show the orientation of rocks and fault planes. The strike, the long line, shows the direction the rocks point. The dip, designated by the short line and number, indicates the direction and angle that the rocks are tilted (90 degrees would mean that the rocks are completely vertical).

Cross section

A cross section represents a vertical slice of the earth’s crust showing the subsurface rock layers in much the same way that a slice of cake shows the layers of cake and frosting. Cross sections help the map user understand the arrangement of rocks beneath the earth’s surface.

Cross section from La Crosse County, Wisconsin, geologic map.
Detail from Geologic Map and Cross Sections of La Crosse County, Wisconsin (Bulletin 101, plate 1).

The information used to make a geologic map can come from a variety of sources.

Types of data used to make a geologic map

Well construction reports (WCRs)

Example well construction report
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Well construction reports (WCRs) are completed by drillers for wells drilled in Wisconsin. WCRs contain information about the location of the well, the geology encountered, and water-table elevation. The sheer number of WCRs makes them a useful tool for interpreting Wisconsin’s bedrock geology when it is buried under glacial sediments.

Ground-penetrating radar (GPR)

Ground-penetrating radar from a Wisconsin quarryGround-penetrating radar uses the reflection of high-frequency radio waves (radar) waves off of boundaries between different types of rock or sediment buried under the earth’s surface. Geologists use GPR to interpret the geometry and distribution of these rock or sediment boundaries.

Aeromagnetic and gravity surveys

Composite aeromagnetic map of Wisconsin. The kaleidoscope of colors shows changes in density across the state.
Courtesy of the USGS. Map by David L. Daniels.

Aeromagnetic and gravity surveys measure physical properties of bedrock. Aeromagnetic surveys (shown in this image) show the distribution of magnetic minerals in the rocks while gravity surveys indicate variations in rock density over an area. Geologists use knowledge of the distribution of minerals and rock density to interpret what rock types are present buried under the earth’s surface.

Well logs

Example geophysical log, shown as a line graph.
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The graph at left shows an example of a geophysical log. Geophysical logs are created by lowering a tool into a well hole to measure certain rock properties. The rock properties are displayed as a graph, with the depth of the well hole on the y-axis, and the rock property on the x-axis. This example shows a gamma ray log, which measures gamma radiation produced by certain minerals contained in the rock column.

Rock cores

A box of rock core. The box has five columns of core from the same drill site laying side by side in order of their depths. The rocks in each column are broken into multiple pieces.The cylindrical rock core in this box came from a hole drilled into the earth. In Wisconsin, much of the bedrock is hidden by a thick pile of sediments that were deposited during the last ice age. Cores allow geologists to study a continuous column of this buried rock.

Other sources of data

Geologists also look at the rocks exposed in road cuts, outcrops, quarries, and river cuts when mapping an area.

Putting the information together

Geologic information is drawn on top of a base map. The base map orients the user in space by showing the location of rivers, lakes, roads, hills, and valleys.

The following images show a base map (aerial photo), a geologic map, and the same map combined with a satellite image to highlight the topography. (Click the images to enlarge them.)

Aerial photo of Devils Lake and the surrounding area.
Aerial photo
Geologic map of Devils Lake and the surrounding area. This map is color-coded to show different rock units and has contour lines to show elevation changes.
Geologic map
Geologic map of Devils Lake and the surrounding area. In addition to the color-coding and contour lines of the previous map, this map uses hillshading to give it more of a 3D look.
Geologic map on satellite image

The kinds of maps made at the Wisconsin Geological and Natural History Survey reflect Wisconsin’s geography and geologic history. The following map types offer a sample of our maps.

To see all of our maps, visit our Publications Catalog.

Bedrock maps

Detail from a very colorful geologic map. Each color represents a specific type of rock or geologic deposit, and this map shows more than forty different types.
Click to enlarge.

Bedrock maps show solid rock (bedrock) buried beneath Wisconsin’s glacial material or exposed at the Earth’s surface in places like the Baraboo Hills (in Sauk and Columbia Counties) or Copper Falls (in Ashland County).

This map shows a detail from the Bedrock Geology of Marathon County, Wisconsin, by G.L. LaBerge and P.E. Myers.

Glacial maps

Detail from a glacial geologic map. This map shows the lakes of the Madison area. It also includes part of the boundary between the unglaciated Driftless Area and its neighboring glaciated lands, showing the distinct difference between the two geographies.
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Glacial maps show the sediments and landforms (such as drumlins and moraines) that were left by glaciers. These maps may be called Quaternary (referring to the period) or Pleistocene (referring to the epoch).

This map is a detail from Glacial Geology of Dane County, Wisconsin (part of A Guide to the Glacial Landscapes of Dane County, Wisconsin, by D.M. Mickelson and M.C. McCartney).

Water table maps

Detail of a geologic map with contour lines representing the heights of the water table. Here the water table tends to be lowest near the Wisconsin River and Lake Wisconsin.
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Water-table maps show the approximate elevation of the top of the saturated zone, where all underground cracks and pores are water-filled.

This map is a detail from the Preliminary Water-Table Elevation Map of Columbia County, Wisconsin, by S.M. Sellwood.