Activity 3: Geologic Cross Sections
    
Geologic cross sections are useful for visualizing spatial and temporal relationships of geologic units that occur in an area. From geologic maps, we can construct geologic cross sections (vertical planes of the subsurface) that show geologic structures beneath the surface. The top of a cross section shows the topography of the land and below that the geometric relations of the different rock units that crop out at the surface. Geologic information such as strike and dip is used to project the units that crop out at the surface into the subsurface. Cross sections are extremely useful for visualizing the spatial and temporal relationships of the geologic units that occur in an area. With a little bit of practice, it is a fairly straightforward procedure to create a cross section from a map.
Steep river canyons, sea cliffs, and mountain sides are natural cross sections. Roadcuts are also cross sections. Most cross sections are made by projecting surface features into the Earth or by interpreting data from seismic waves. Cross sections have been used to identify potential petroleum drilling sites, understand the movement along faults, unravel geologic structure in complexly folded regions, determine the flow of water, and plan mining operations.
Steps for making a geologic cross section from a geologic map include (Fig. 1)

gmapwithsection.gif (6093 bytes)

Fig. 1: Construction of a geologic cross section ( bottom) from a geologic map (top).

  1. On the geologic map, select the cross-section traverse, a line of section that should cross as close to right angles as possible the geologic features you wish to show in the subsurface.
  2. Construct a topographic profile along the line of the cross section. Refer to Lab 1 for a refresher.
  3. Transfer the location of all contacts, faults, and/or folds that intersect the traverse line to the cross section.  (This can be done simply by laying a piece of paper on the map along the cross-section line and marking each intersection. Transfer the locations to the cross section by laying the piece of paper along the section and marking off all geologic intersections.)
  4. For each geologic feature marked on the profile, examine it on the map for evidence of dip. Use this angle to project the feature into the subsurface. Remember that dip-and-strike symbols are usually regional features so use the ones closest the line of section.
  5. Keep in mind that sedimentary units rarely change thickness across a section and usually do not match up across faults.
  6. Mark each unit in the cross section with its symbol from the map legend.
  7. Label each fault with its proper symbol.
   
To complete Activity 3, select one of the following files:
   

 

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