![]() 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 16.2). The geologic age (typically the geologic period) is listed for each unit in the key and uses standard symbols ( Table 16.1). This allows the map reader to quickly assess the relative age of each unit. The symbols typically follow superposition and are stacked in an age sequence from oldest at the bottom to youngest at the top. The symbol for each formation, or unit, is shown in a box next to its name with a brief description. Figure 16.3: General formatting for the legend of a geologic map. The map legend to a geologic map is usually printed on the same page as the map and follows a customary format ( Figure 16.3). Figure 16.1: Simplified geologic map illustrating Mesozoic terranes in and around the Bay Area. Geologic maps are also vital in finding and developing geological resources, such as sediments, groundwater, fossil fuels, and minerals. Second, geologic maps are essential tools for practical applications such as zoning, engineering, and hazard assessment. 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. Geologic maps are important for two reasons. Geologic maps take practice to understand, since they display three-dimensional features, such as folds, on a two-dimensional surface. At each outcrop, geologists record information such as rock type, strike and dip of the rock layers, and relative age data. Geologists make these maps by making careful field observations at numerous outcrops (exposed rocks at the Earth’s surface) throughout the mapping area. These mappable units are shown as different colors or patterns on a base map (usually a topographic map), over which information about geologic contacts and strikes and dips are included. Generally, a mappable unit is a geologic formation. 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. Typically, a geologic map will depict mappable rock or sediment units. What Are Geologic Maps and Why Are They Important?Ī geologic map uses lines, symbols, and colors to illustrate information about the nature and distribution of rock units within an area ( Figure 16.1).
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