1.3: Detailed Figure Descriptions

Geologic map of San Joaquin County, California

The image is a geologic map showing a region divided into multiple irregularly shaped areas representing different rock units. This map demonstrates how geologists use spatial distribution, labeling, and structural features, such as faults, to interpret the geologic history of a region, including rock formation, deformation, and erosion.

Each unit is identified by a short label (such as “Qls,” “Kp,” “Mvq,” and “Qo”) placed within its boundary. These labels correspond to specific geologic formations or time periods. The map uses multiple colors to distinguish rock units, including yellow, green, red, tan, and brown. Because color alone can be difficult to interpret, each unit is also clearly separated by solid boundary lines and labeled directly within the region. The boundaries between units are uneven and follow natural geologic patterns rather than straight lines.

Several black lines cross the map diagonally and horizontally. These represent fault lines. Some faults are thicker and more prominent, indicating major structural features. In a few places, fault lines intersect or branch, suggesting a complex tectonic setting. Thin blue lines run across the map and represent rivers or streams. These lines follow winding paths and often cut across multiple rock units, indicating erosion and surface water flow. Contour lines appear in some areas as thin, closely spaced curved lines, indicating changes in elevation. These lines are most visible in the green and red regions, and help show the topography of the landscape. Place names and geographic references, such as “San Joaquin” and “San Joaquin Peak,” are printed in light text across the map, providing regional context. The left side of the map is dominated by a large yellow region labeled “Qo,” while the central and right portions contain a more complex mix of smaller, interlocking units in green, red, and tan. The density of labels and boundaries increases toward the center and right, indicating more geologic complexity in those areas.

Unit Abbreviations (decoded)

• Qls: Quaternary landslide deposits
• Qo: Quaternary older alluvium
• Kp: Cretaceous plutonic rocks
• Mvq (and variants): Miocene volcanic rocks

Geological Map of California

This image is a poster titled “Geology of California.” Across the top are institutional credits: California Geological Survey; John G. Parrish, Ph.D., State Geologist; State of California – Edmund G. Brown Jr., Governor; The Natural Resources Agency – John Laird, Secretary; Department of Conservation – David Bunn, Director; and “Map Sheet 57, Geology of California.”

To the right of the title are two explanatory paragraphs. The first explains that a geologic map shows the distribution, relationships, and composition of Earth materials, including rocks and surface deposits such as sediments and landslides. It states that each color represents a different rock type or age, and that thick black lines represent faults. The second paragraph explains that geologic maps are used to understand geologic history and to plan for the future. They help identify resources such as oil, natural gas, groundwater, and mineral deposits, as well as hazards including landslides, volcanoes, earthquake faults, liquefaction zones, and tsunami hazard zones. They are also used in environmental and engineering studies to identify areas suitable for agriculture, urban development, and construction, thereby supporting safer, more sustainable decisions.

The central portion of the image is a detailed geologic map of California. The state is divided into many irregularly shaped regions representing different geologic units. Each region is distinguished by both color and clearly defined boundary lines. Colors include greens, reds, yellows, browns, purples, and blues, but meaning is not dependent on color alone because units are separated by boundaries and referenced through the legend.

The map includes numerous labels for geographic locations, including “San Joaquin” and “San Joaquin Peak.” The Pacific Ocean is shown along the western edge in blue, and rivers are depicted as thin blue lines crossing multiple geologic units. A large, light-colored region extending north to south through the center represents the Central Valley, while surrounding areas show more complex patterns of smaller, interlocking units. Black lines across the map represent faults. These lines vary in thickness and style to indicate different levels of certainty. Solid lines represent well-located faults, while dashed or dotted lines indicate approximate or concealed faults. Some faults intersect or branch, indicating complex tectonic relationships. Thrust faults are indicated with symbols showing the direction of movement.

To the right of the map is a legend titled “Generalized Geologic Units.” This section organizes rock types into categories, including:

• Marine sedimentary rocks
• Nonmarine (continental) sedimentary rocks
• Volcanic rocks
• Marine sedimentary and metasedimentary rocks
• Mixed rocks
• Plutonic rocks

Each category includes colored boxes paired with text descriptions of rock types, such as sandstone, shale, volcanic flows and deposits, granitic rocks, and metamorphic rocks. These categories are arranged vertically to align with geologic time divisions labeled Cenozoic, Mesozoic, and Paleozoic, allowing connections between rock types and ages.

Below the legend is a key explaining map symbols. It identifies:

• Contacts between geologic units (boundaries)
• Faults, with line styles indicating whether their location is well known, approximate, or concealed
• Thrust faults, marked with directional indicators showing relative motion

In the lower left corner, an inset map titled “California Geomorphic Provinces” shows major physical regions of the state, providing additional context for landscape organization. At the bottom right, a compass rose indicates cardinal directions (north, south, east, west), and a scale bar provides distance in both miles and kilometers. The map scale is approximately 1:750,000. At the bottom of the poster are logos for the California Geological Survey and California Conservation, along with a copyright statement indicating that the map was compiled by the California Department of Conservation and that no warranty is made regarding its use.