Based in Mahone Bay, nova scotia, agile libre is an independent publisher of technical books about the subsurface.  

Maps are interpretations

Learning to make geological maps is one of the most fundamental elements of any geology training course. But when you make a map what are you actually doing — creating a document of truth, or a fiction of your imagination? The reality is perhaps a combination of the two. To new geological trainees this may be a shocking admission, but geology as a science is all about interpretation and reasoning (Frodeman 1995). The fact that there isn’t a right answer introduces challenges for both the trainee and the tutor. Geology undergraduates are taught and assessed in a world where there are right and wrong answers; changing this perception of education and learning is a process that takes time and experience (Bond at al. 2011).

Think about the process you go through when making a geological map in the field. From the moment you walk, map in hand, into your field area you are collecting information from the landscape. All this conscious and sub-conscious information is combined to build your geological map. There are specific data points based on outcrop which may include quantitative measurements of strike and dip, and qualitative descriptions of the rock structure and properties. Before marking the information gathered in your field notes the interpretation process begins. The colour you choose to mark the outcrop on the map categorizes it into a rock type, an interpretation based on your observations. If well trained you will start making predictions, based on a virtual geological model, from the first outcrop — or from looking at how the rock units intersect the landscape. This initial model is continually tested and refined as the geological map is created.

Critically, although the geological map created is 2D, the virtual geological model in the mapper’s head is 3D. This three-dimensionality is what makes the map make sense. Think of the classic children’s join-the-dot puzzles. The dots could be joined in any order, but the numbering of the dots allows the child to create a picture. Imagine now your 2D geological field map with no 3D conceptualization or visualization of the geology it represents; you could join the outcrops in any way. But the map contains lots of 3D information, topographic contours, strike and dip measurements of planes, and orientations and plunges of lines. When joining the outcrop ‘dots’ the geologist uses this information to create a 3D solution represented by the 2D geological boundaries drawn on the map.

It is much easier to create and visualize this 3D solution in the field, standing in the three-dimensionality of the landscape, than trying to recreate it at your desk. It also allows you to test and build a model in the field. This is why tutors will say, ‘Draw your boundaries in the field’ — listen to them, they are trying to make it easier for you. Of course the 3D solution created may not be the only possibility, but if it honours the outcrop data and the 3D geometries created make sense, then the solution is a viable geological model and so is the geological map.


References
Bond, C, C Philo, and Z Shipton (2011). When there isn’t a right answer: interpretation and reasoning, key skills for 21st century geoscience. International Journal of Science Education, 33 (5), 629–652, DOI 10.1080/09500691003660364.
Frodeman, R (1995). Geological reasoning: Geology as an interpretive and historical science. Geological Society of America Bulletin, 107 (8), 960–968, DOI 10.1130/0016-7606(1995)107<0960:GRGAAI>2.3.CO;2.

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