15.7: Geologic Maps

Geologic Maps

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What Are Geologic Maps and Why Are They Important?

A geologic map Links to an external site. uses lines, symbols, and colors to illustrate information about the nature and distribution of rock units within an area (Figure 15.20). Typically, a geologic map will depict mappable rock or sediment units. A mappable unit of rock or sediment is one that a geologist can consistently recognize, trace across a landscape, and describe so that other geologists may recognize and verify its presence and identity. Generally, a mappable unit is a geologic formation Links to an external site.. These mappable units are shown as different colors or patterns on a base map (usually a topographic map Links to an external site.), over which information about geologic contacts and strikes and dips are included. Geologists make these maps by making careful field observations at numerous outcrops Links to an external site. (exposed rocks at the Earth’s surface) throughout the mapping area. At each outcrop, geologists record information such as rock type, strike and dip of the rock layers, and relative age data. Geologic maps take practice to understand, since they display three-dimensional features, such as folds, on a two-dimensional surface. 

Geologic maps are important for two reasons. First, as geologists make geologic maps and related explanations and cross-sections, they develop a theoretical understanding of the geology and geologic history of a given area. Second, geologic maps are essential tools for practical applications such as zoning, engineering, and hazard assessment. Geologic maps are also vital in finding and developing geological resources, such as sediments, groundwater, fossil fuels, and minerals.

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What Are the Essential Components of Geologic Maps?

Most geologic maps have the following features (Figure 15.21):

1. The map itself 
2. The map legend or key that explains all the symbols on the map
3. Geologic cross-section(s) of the map area. These will be explored further in the next chapter.

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What Is a Geologic Map Legend?

The map legend to a geologic map is usually printed on the same page as the map and follows a customary format (Figure 15.22). 

The symbol for each formation, or unit, is shown in a box next to its name with a brief description. The symbols typically follow superposition and are stacked in an age sequence from oldest at the bottom to youngest at the top. This allows the map reader to quickly assess the relative age of each unit. The geologic age (typically the geologic period) is listed for each unit in the key and uses standard symbols (Table 15.6). The map legend also contains an explanation of the symbols shown on the map, such as the symbols for different types of faults and folds (Table 15.5). The explanations of rock units often follow the map symbol on the map, but for very large maps are often given in a separate pamphlet that accompanies the map. The explanations include descriptions with enough detail for any geologist to be able to recognize the units and learn how their ages were determined.

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General Rules for Interpreting Geologic Maps

• Folds
  • Anticlines have their oldest beds in the center, and their limbs (sides) dip away from the fold axis or hinge. Plunging anticlines plunge towards the closed end (nose) of the V-shaped outcrop belt.
  • Synclines have their youngest beds in the center, and their limbs (sides) dip toward the fold axis or hinge. Plunging synclines plunge toward the open end of the V-shaped outcrop belt.
• Streams
  • They intersect dipping beds of rock, they will cut V shapes in the direction of dip.
  • They cut “V” shapes into horizontal beds and formation contacts that point upstream.  The formation contacts are parallel to topographic contour lines, and the stream drainage system developed on horizontal and/or unstratified formations has a dendritic pattern that resembles the branching of a tree.
  • They cut across vertical beds, there will not be a V-shape
• Faults
  • Upthrown blocks of faults tend to be eroded more (down to older beds) than downthrown blocks.
• General
  • Geologic contacts migrate downdip upon erosion.
  • True dip angles can only be seen in cross-section if the cross-section is perpendicular to the fault or to the strike of the beds.

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