Environmental Engineering Reference
In-Depth Information
FIGURE 5.12
Several interconnecting cross sections. (From United States Geological Survey, National cooperative geologic
mapping program, http://ncgmp.usgs.gov, 2009.)
(Great Lakes Mapping Coalition 2009). Cross sections of this type aid in interpreting the
depositional history of the area being mapped.
5.2.7 Creating the Base Map
After the review of existing data, conducting field work, facies analysis, and construct-
ing cross sections, a base map can be prepared. The steps required to prepare a base map
include (1) using appropriate nomenclature, (2) selecting appropriate symbols, (3) selecting
the scale for the map, and (4) determining what type of map is most appropriate.
Using recognized and acceptable nomenclature and symbols ensures the geologic map
being prepared maintains consistency and minimizes ambiguity. The standard nomen-
clature for geologic maps in North America is The North American Stratigraphic Code
(American Association of Petroleum Geologists 2005). This code is used for naming,
describing, establishing, redefining, and abandoning geologic units. For symbols, the
standard is the Federal Geographic Data Committee Digital Cartographic Standard for
Geologic Map Symbolization (USGS 2006). Items such as line symbols, point symbols, and
colors and patterns used to portray the different features on a geologic map are covered
by this standard.
The first step in preparing a geologic map is to start small and then add adjacent loca-
tions to build the entire map. One way to accomplish this process is to begin with a single
geologic unit that is relatively simple or easy to identify in the field and then add adja-
cent geologic units both horizontally and vertically through the entire unconsolidated
strata until bedrock is encountered. For reference, the applicable 7.5 min series USGS
Quadrangle map (“quad”) is recommended to construct the field base map. The 7.5 min
series quad is a 1:24,000 scale map—a scale suitable for many environmental applications.
These maps also have valuable information to greatly assist in mapping any given area,
including
• Each map is very accurate and reliable.
• Most urban areas in the United States have readily available 7.5 min series
Quadrangle maps.
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