How do you construct geological cross sections

Cross sections are a valuable method of displaying geological structural information. They are relatively easy to construct from a geological map. The first requirement is a topographical cross section. This means creating a profile of the topography along the line of your desired geological cross section. The topographic section must be at the same scale as your geological map. The topographical cross section is constructed in the same way as the geological cross section. Use a strip of paper to transfer contour heights onto a section, marking the heights along the section line.

Once the topographical profile is complete then you can start the geological cross section. The easiest way to do this is to lay a thin strip of paper along the line of your section. Then mark on geological boundaries (faults, dykes, sills, rock-type boundaries, etc) and other important geological information (dips, facies changes, etc). The strip of paper now contains an accurate representation of the geology along the section line. This paper is then used to transfer the information to the cross section. The upper half of figure 1 shows a geological map with the strip of paper overlaid. The paper is marked with the necessary information. The lower half of figure 1 shows the paper being used to transfer the information to the topographical section. Notice how the marks are transferred from the surface, which undulates, not a straight line. Also note how the dip is measured from horizontal, not the surface.

	Figure 1: A simple example of making a geological cross section. The figure shows how a strip of paper can be used to transfer the information from the map to the cross section.

The following three figures show map construction of three typical geological features. The last two also include the temporary sketch lines. The topography is flat for all three examples to simplify things. Figure 2 shows a simple cross section of some dipping beds.

	Figure 2: A simple cross section of some dipping beds.

Figure 3 shows the cross section of a synform feature. The best way to construct these is to draw the temporary lines the same as the dipping beds, but these will now cross. Once these have been drawn, simply round off the apex of the fold. Figure 3 includes the sketch lines in the centre diagram.

	Figure 3: A cross section of folded beds.

Figure 4 shows the cross section of a synform feature with a fault cutting it at an angle. The line of the cross section is also shown. The fault is marked on the paper as with all geological boundaries. The fault has displaced beds A and B, which looks like a lateral movement on the map, but it is clear on the cross section that it is a vertical movement.

	Figure 4: A cross section of folded beds with a fault. The dotted line on the sketch example is where the A-B boundary would be and is used to construct the boundary to the left of the fault.

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