Following on from our 2015 field school feedback session, it was evident that many participants struggled with constructing a geological cross section. I thus decided to make a very brief and simple guide. This cross section is located in Area 5 of the 2015 field school mapping region, west of Eksteenfontein, Northern Cape, South Africa:
Step 1. Choose a section line that runs perpendicular to the general bedding/foliation fabric:
Always choose a line perpendicular to the general orientation of the bedding or foliation fabric because it will allow you to cross more of the geology and get a true representation of the geological structure.
Step 2. Generate contour profile:
In the "old days" you would do this with a topographic map and mark elevation points along your cross section line and then connect the dots. These days, I suggest doing it automatically using a digital elevation model (DEM). This can be done using the Profile Tool plugin for QGIS or 3D Analysis in ArcGIS. This can also be done in Google Earth by creating a line and selecting "show elevation profile".
Step 3. Extrapolate geology along section line onto contour profile:
Extrapolate the outline of the geological polygons and structural lines, such as, faults and fold axes, onto the contour profile. Ensure that you note the dipping angles for these features.
Step 4: Complete the drawing:
Now complete the geological polygons and structural lines. Remember to ensure that your geological layers dip at the same angle as measured in the field (assuming no vertical exaggeration). Insert structures (e.g. folding) as defined by surface data (e.g. fold axes). Apply colour. Do a final cross check to make sure that the stratigraphy is in the correct order.
