2.7: Where the Rocks Are- California's Geologic Map
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- 21444
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Geologic Maps
A geologic map shows the geological features of a region (Figure 2.7.1). A rock unit is color-coded and identified in the map legend. Faults and folds are also shown on geologic maps (Appendix). The geology is superimposed on a topographic map to give a more complete view of the geology of the region.
The geologic data, in layer fashion, are typically printed upon a base map. The base map may show the topography, roads, rivers, or other cultural or natural features in the map area. Modern maps are in digital form, extending their usefulness in decision-making by allowing geologic information to be combined with other data, such as demographics or infrastructure. Explanatory text on the maps provides information on the geologic age of the materials as well as their physical properties.
A segment of a typical geologic map is shown in Figure 2.7.1. On the map the geologist has recorded the locations, types, and ages of the rock units and surficial materials by using various standard colors, symbols, and patterns overlain on a topographic base map. The basic rock units shown on the map are called formations. For example, on the map the green area labeled "Kp" shows the aerial extent of the Panoche Formation which is of Cretaceous age (symbolized by K). Similarly, the red portion of the map labeled "Mvqa" represents the aerial extent of an unnamed formation composed of a volcanic rock of Miocene age (symbolized by M). A formation is usually named after a geographic feature (mountain, canyon, or town) near the area where the rock unit was first identified. The geologically most recent deposits on the map are the landslides ("Qls") depicted in yellow with arrows showing the direction of movement or "flow". Using special symbols, the geologist has recorded other significant observations on the map such as faults (bold black lines), folds (pink arrows), contacts between rock units (thin black lines), and the strike and dip of formations (respectively, the direction of a horizontal line within a unit and the angle that the unit slopes in outcrop). The explanatory text for the geologic map below is not shown.
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, civil engineering, and hazard assessment. Geologic maps are also vital in finding and developing geological resources, such as gravel to help build the road you drive on, oil to power the car you travel in, or aluminum to build the more fuel-efficient engine in your next vehicle. Another resource that is developed on the basis of geologic maps is groundwater, which many cities, farms, and factories rely on for the water they use.
Essential Components of Geologic Maps
A complete geologic map has at least two features:
- the map itself
- the map legend that explains all the symbols on the map.
Professional geologic maps usually have two other components as well:
- an accompanying explanation of the rock or sediment units
- geologic cross-sections of the map area.
The legend to a geologic map is usually printed on the same page as the map and follows a customary format. The symbol for each rock or sediment unit is shown in a box next to its name and brief description. These symbols are stacked in age sequence from oldest at the bottom to youngest at the top. The geologic era, or period, or epoch–the geologic age–is listed for each rock unit in the key (see Geologic Time). By stacking the rock units in age sequence from youngest at the top to oldest at the bottom, and identifying which interval of geologic time each unit belongs to, the map reader can quickly see the age of each rock or sediment unit. The map key also contains a listing and explanation of the symbols shown on the map, such as the symbols for different types of faults and folds. See the Tables of Geologic Map Symbols for pictures and an overview of the map symbols, including strikes and dips, faults, folds, and an overview.
Generalized Geologic Map of California
Let’s now take a look at California’s generalized geologic map (Figure \(\PageIndex{2}\)). At first glance, you will likely notice some interesting patterns: Perhaps the light tan swath in the center of the state stands out, or the pink and red units along the eastern border catch your eye. These patterns are not random: they are the result of tectonic plates converging and diverging over many millennia (see Plate Tectonics and A Brief Geologic History).
California’s geology is particularly complex compared to some of the flatter states of the Midwest where the geology can be described as "layer cake" geology–simple undisturbed layers of strata stacked on top of one another. By contrast, California is the only state in the U.S. with all three types of plate boundaries and it is the only state that straddles two tectonic plates. We will revisit the generalized geologic map of California throughout this text.
The California Geological Survey has also made available an interactive geologic map of California which can be navigated by panning and zooming in an out. As you zoom in, more detailed geology appears. The data of this interactive map can also be downloaded for use with geographical information systems (GIS) software such as ESRI ArcGIS or open-source program QGIS.