Geologic maps are powerful tools for understanding Earth's structure and history. They use symbols, colors, and patterns to represent rock types, formations, and features on the surface. By interpreting these maps, geologists can visualize the 3D arrangement of rocks beneath our feet.

Cross-sections provide a vertical slice through Earth's crust, revealing subsurface relationships. By combining map data with structural geology principles, geologists can reconstruct the sequence of events that shaped an area over millions of years, from rock formation to deformation and erosion.

Geologic Maps

Geologic map interpretation techniques

• Map symbols represent different geologic features
  • Points indicate locations of specific features (outcrops, springs, mines)
  • Lines depict contacts between rock units, faults, and fold axes
  • Polygons show areas of specific rock units or formations
• Colors and patterns distinguish rock types and geologic units
  • Different colors represent various rock types or geologic units
  • Patterns (dots, stripes, cross-hatching) differentiate rock units or indicate specific features
• Map scale and orientation provide spatial context
  • Scale relates map distance to actual distance on Earth's surface
  • Orientation is shown by a north arrow or compass rose
• Topographic contours illustrate elevation and land surface shape
  • Contour lines represent elevation
  • Closely spaced contours signify steep slopes; widely spaced contours indicate gentle slopes

Construction of geologic cross-sections

• Geologic cross-sections are vertical slices through Earth's crust
  • Show subsurface arrangement of rock units and structures
  • Constructed perpendicular to the strike of rock units or structures
• Steps in constructing cross-sections
  1. Determine the line of section on the geologic map
  2. Project surface geology onto the cross-section plane
  3. Use topographic contours to draw land surface profile
  4. Interpret and draw subsurface geometry of rock units and structures based on map data and field observations
• Interpreting cross-sections reveals geologic relationships
  • Identify rock units, thicknesses, and relationships
  • Recognize geologic structures (faults, folds, unconformities)
  • Infer geologic history and sequence of events

Structural Geology and Geologic History

3D geometry from 2D maps

• Folds are characterized by their shape and orientation
  • Anticlines are upward-arching folds with oldest rocks in the core
  • Synclines are downward-arching folds with youngest rocks in the core
  • Plunge is the angle between fold axis and horizontal plane
  • Limbs are sides of a fold that dip away from hinge line
• Faults are planar fractures with displacement
  • Normal faults have hanging wall moving down relative to footwall (extensional stress)
  • Reverse faults have hanging wall moving up relative to footwall (compressional stress)
  • Strike-slip faults exhibit horizontal movement of fault blocks (shear stress)
  • Fault dip is angle between fault plane and horizontal plane
• Unconformities represent gaps in the geologic record
  • Caused by non-deposition or erosion
  • Types include angular unconformity, disconformity, nonconformity

Synthesis of geologic information

• Relative ages of rock units determined by stratigraphic principles
  • Superposition: in undisturbed sequence, younger rocks overlie older rocks
  • Cross-cutting relationships: younger features (faults, intrusions) cut across older features
• Reconstruct sequence of geologic events
  1. Deposition of sedimentary rocks
  2. Deformation events (folding, faulting)
  3. Igneous intrusions or extrusions
  4. Periods of erosion or non-deposition
• Interpret tectonic setting and geologic environment
  • Recognize patterns and associations of rock types and structures
  • Consider regional geologic context and correlations with nearby areas
• Integrate data from multiple sources
  • Geologic maps, cross-sections, field observations, well logs
  • Geophysical data (seismic, gravity, magnetic)
  • Geochemical and geochronological data