How do you use cross-sections and profiles to complement a geological map?

1 What are cross-sections and profiles?

Cross-sections and profiles are graphical representations of the vertical or oblique slices through the earth, showing the rock layers and structures as they would appear if exposed by cutting or drilling. Cross-sections are usually drawn perpendicular to the strike of the rock units, while profiles are drawn parallel to the strike or along a specific line of interest, such as a road or a pipeline. Cross-sections and profiles are often drawn at a different scale than the map, to emphasize the vertical variations and relationships.

2 How are cross-sections and profiles constructed?

Constructing cross-sections and profiles requires the use of information from geological maps, such as locations, shapes, and symbols of rock units and structures, strike and dip measurements, and map scale and projection. Additional data from boreholes, geophysical surveys, or geological reports can provide more details and constraints on subsurface conditions. The basic steps to construct cross-sections and profiles involve choosing a line of section or profile on the map, transferring it to a blank sheet of paper, plotting the surface elevation along the line of section or profile, transferring rock units and structures from the map to the cross-section or profile using strike and dip symbols, borehole data, and geophysical data, filling in gaps between data points with reasonable assumptions and interpretations, drawing boundaries and labels of rock units and structures, and checking the consistency and accuracy of the cross-section or profile.

3 How do cross-sections and profiles help you interpret the rocks?

Cross-sections and profiles provide insight into the three-dimensional geometry and history of the rocks beneath the surface, such as their thickness, attitude, composition, deformation, and age. These tools can help answer questions about stratigraphic sequences and correlations of rock units, the types and orientations of folds, faults, fractures, and other structures, the directions and magnitudes of stresses and strains that affected the rocks, the depositional environments and tectonic settings that formed and modified them, as well as potential geotechnical properties, hazards, and resources. By combining a geological map with cross-sections and profiles, you can gain a more comprehensive understanding of subsurface conditions and geologic history to apply to your engineering projects.

4 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?