GEOLOGIC MAPS and REPORTS

James S. Aber


Table of Contents
Geologic maps Map details
Geologic reports References

Surficial geologic maps

Many kinds of geologic maps exist. For purposes of this course, we will focus on maps of surficial geology. A map of surficial geology is a graphic diagram that depicts surficial rocks and sediments as a scale model of reality. In most environments vegetation, soils, and human structures cover the surface, so that underlying rocks and sediments are not directly visible or exposed. For mapping purposes, the materials beneath the soil are depicted. This means the rocks or sediments that exist at shallow depth, usually 1 m (Europe) or 5 feet (North America).

The following map of Butler County, Kansas, demonstrates several basic attributes of typical geologic maps. Butler County is located in the Flint Hills of east-central Kansas. Bedrock consists mainly of lower Permian limestone and shale of the Council Grove and Chase Groups. Unconsolidated sediments include Tertiary and Quaternary alluvium plus Quaternary loess.

Map of surficial geology of Butler County, Kansas. Based on Aber (1994a, b); taken from Kansas Geological Survey--see geologic maps of Kansas.

The "map" consists of two main components--graphic model of the geologic units within the county and a legend of those units on the map. Notice the arrangement of the legend. Geologic units are placed in stratigraphic (age) order--youngest at top (left) and oldest at bottom (right). The composition of each unit is summarized in the legend captions.

Next look at the color coding. Closely related geologic units have similar colors--yellow and gold for unconsolidated Quaternary and Tertiary sediments, reddish purple for Chase Group (middle Permian), light blue for Council Grove Group (lower Permian), and purple-blue for Pennsylvanian strata.


Geologic map details

Geologic maps often include many more features in addition to the basic map units. The level of geologic detail is limited by map scale, which is often determined by the "base map" utilized for a geologic mapping project. The conventional base map for field geology in the United States is the 7½-minute topographic quadrangle (or equivalent airphoto). This map series has a scale of 1:24,000 (1 inch = 2000 feet). Map scale is closely related to another concept--map resolution. Resolution refers to the smallest identifiable object that can be depicted in true size and shape on a map. According to U.S. Geological Survey guidelines, the resolution is 40 feet (12 m) for map scale 1:24,000. This, in turn, leads to the concept of positional accuracy--the proper location of objects on the map. In practical usage, resolution and positional accuracy are the same.

More on map accuracy from the Geographer's Craft.

Another important feature of topographic maps are elevation contour lines. The elevation of a geologic feature is often just as important as its horizontal position. Elevation resolution is limited by the contour interval depicted on the map. Depending on terrain characteristics, the contour interval may be 10, 20 or 40 feet. Elevation data are particularly important for working out structural conditions--strata dip, folds and faults, as well as geomorphic relationships. The following maps are large-scale examples drawn from the general Butler County map.

Detailed map of the Rosalia vicinity, Butler County Kansas. Notice that each map unit is identified by a letter code--see legend below map. This helps to distinguish units that have similar color.

An anticline (<|>) crosses the middle of this area. Brown lines show topographic contours (10-meter interval); red numbers indicate sections in the township-and-range grid.

Structural symbols for folds and faults are especially important elements of surficial geologic maps, as these structures may influence the movement of subsurface fluids--oil, gas and ground water. Symbols are often placed to show features related to special ground-water conditions, such as springs, artesian wells, sinkholes, caves, etc.

Detailed map of the Augusta vicinity, Butler County, Kansas. Fold structures are indicated by anticline (<|>) and syncline(>|<) symbols. The linear symbols mark the surface trace of the crest of each fold. Paired anticline-syncline structures are common in the Flint Hills region. Black diagonal lines indicate the urban area.
Zone with numerous sinkholes is marked by the blue diagonal lining within the Barneston Limestone (section 34). A syncline parallels the Walnut River valley to the northwest of the sinkholes. This area is located south of El Dorado, Kansas.

Across eastern Kansas, bedrock strata are nearly, but not exactly, horizontal. Bedrock normally dips downward 20 to 40 feet per mile (less than 1°) to the west or northwest. This is indicated by the relationship between bedrock map units and topographic contour lines. Unconsolidated sediments, on the other hand, show more variable relationships with contour lines and underlying bedrock, as the following examples demonstrate.

Detailed map of the Winfield Limestone (Pwi) east of Augusta, Kansas. Note the change in elevation of the Winfield--higher to east (410 m) and lower to west (390 m). This slight, westward dip is typical throughout most of eastern Kansas.
Detailed map of the Grouse Creek syncline (>|<) in southeastern Butler County, Kansas. Notice the eastward dip of bedrock units (map center) toward the syncline that parallels Grouse Creek (sections 16 & 21).

Such "reverse" dip is unusual and is always associated with a nearby syncline. A parallel anticline crosses the northwestern corner of the map area (secion 17).

Detailed map of Tertiary terrace gravel (Tg) in vicinity of Leon, Kansas. The gravel deposits "drape" over the landscape and rest on different bedrock units. Two terrace levels are preserved here--a hill-top terrace at map center (410 m) and a lower terrace (400 m) along side the river valley.


Geologic reports

A geologic map is usually accompanied be a written report that provides further explanation of the surficial geology depicted on the map. A complete geologic report normally contains the following components arranged in this order.


References

Return to field geology syllabus.
GO 547/ES 747 © J.S. Aber (2006).