Haakon Fossen is a structural geology professor at the University of Bergen, Norway. His website is http://folk.uib.no/nglhe/
I grew up among (proto)mylonitic gneisses, LS-tectonites and other strongly deformed metamorphic rocks in the Caledonides near Bergen, Norway. This was all normal to me, and the first rock that caught my attention, as a kid, was an ultracataclasite. Flinty and pointed as the rock was, I wondered if it could be a tool or arrowhead, but having given up that theory I somehow figured out that it could be a mylonite, and later that it was more appropriately to be named ultracataclasite. I soon came to envy others who had the opportunity to see less deformed rocks, particularly sedimentary rocks. And the rock that, at the time, stood for me as the most exciting one of all was – conglomerate.
I was 16 or 17 years old before I had the pleasure of getting close to a conglomerate. I remember it as a great moment – Silurian conglomerates in what I later learned to be part of the cover sequence to an ophiolite complex. But an even more awesome moment was to come a few years later. A beautifully deformed conglomerate (Fig. 1) turned out to be located in the mountains near Bergen, and I was totally blown away by the beauty of these strained rocks. I think the concept of deforming anything as hard as quartz like it was soft modeling clay was what really turned me on. There was little doubt in my mind; structural geology was the way to go, and the observation of naturally deformed rocks in the field was what triggered it.
And by this I have reached the main point of this blog article; field observations. Field observations and fieldwork have been an important aspect of structural geology for me ever since. It is a necessity, a condition for life as a structural geologist the way I experience it. Physical and numerical models are indeed both interesting and useful, but they cannot replace the joy and importance that comes with real rocks and making your own field observations.
At the same time, the large number of models and interpretations that can be made from field observations, the large degree of freedom if you like, sometimes makes it frustrating to be a structural geologist. It is a well-known fact that a model that explains a given set of data does not have to be correct, simply because there may be other, usually many other reasonable models that fit the data equally well. Just think about infinite number of strain paths that conglomerate pebbles can follow to reach a given state of strain. From my own experience it seems that we tend to become more aware of this fact as we gain experience as geologists. And as this happens, it becomes even more important to go back to the rocks and outcrops to look for more clues through new observations and descriptions. This pushes me back to my first thrilling encounter of beautifully deformed conglomerates. Back to the roots, back to enjoying the beauty of deformed rocks, to observing and describing. And it feels right.
Don’t forget to bring your sketchpad
Let me immediately admit that I spend too little time drawing and sketching while doing fieldwork. It is just too easy to hit that camera button and leave it with that. Don’t get me wrong: the camera is my best friend in the field. But sketching, and the visual aspect of structural geology in general, add to the quality of fieldwork for several reasons. Not only does sketching allow us to better capture the aesthetics and beauty of structures and deformed rocks (which is important in its own right), but it also enables us to capture more of the details, details that may be extremely important in how we understand or interpret their formation and evolution. A field locality should always start with a sketch. Sketching and drawing takes our observation ability to a higher level. The information that can be extracted from outcrops through sketching is amazing, almost regardless of the observer’s drawing skills.
My students, like most other students, are exposed to “the sketching experience” during field courses (Fig. 2). They are “forced” to sketch up the outcrop that they are about to examine. I hear the phrases such as “but I’m a really lousy drawer” or “my drawing skills suck”. But my students (and I too for that matter) often times get really surprised by what they get out of 10-15 minutes of sketching in the field. The difference in the level of observation between students who do and do not spend time sketching is remarkable.
Then, once they discover the usefulness of sketching, the next thing that comes up is a request for a course in drawing techniques. Why do hardly any geology departments offer such courses? I guess some departments have skilled field instructors who teach some of the basics of field sketching, but few do this in a systematic way. If a field-oriented drawing course for geology students exists, then I would really like to know more about it. Interestingly enough, a student recently asked me if I would be kind enough to run a little workshop on field sketching, and lacking any good alternative teacher for such a class in our department, I reluctantly agreed. Well, the student posted it on Facebook, and after a day or two more than 100 students had signed up for it! It is happening this week, and students voluntarily come in to the U at night to spend time on this no-credit mini-course.
What I am trying to communicate is that there is obviously a need and desire of both undergraduate and graduate students for this kind of teaching. Perhaps North American universities with their extensive field camps (see previous blog essay by Peter DeCelles) manage to put this into their curriculum in a better way than we do – it would be interesting to know how North American students think about this (please post comments!).
Comparing sketches and techniques is also interesting. Some prefer 3D sketches, while others are more comfortable working in 2D, producing cross-sections and map-view illustrations (Fig. 3). Some try to capture (sometimes too many) details, while others make very general or idealized sketches reminiscent of textbook or article illustrations typically found in early- to mid-20th century publications.
I believe that students (and their professors too) learn more efficiently if they engage “both sides of their brain”. Just taking a picture does not match the on-site sketching experience, but I have found that spending some time on location and then additional time on my computer drawing sketches based on photos has some of the same effect; tracing or sketching from a high-resolution picture forces you to explore structural complexities and relative age relations, and that process in itself may be quite rewarding, in spite of the many limitations of a photo.
We see what we are trained to see
What we are talking about here is all about learning from rocks, about extracting information from outcrops. One of the dangers involved in this game is the fact that we see what we are trained to see. We use textbook and lecture examples as references or “standards”, and we automatically try to force what we see in the field into those models. I guess you could say that we are all brainwashed to some extent. This does not mean that you should throw away your textbook – it’s just something to be aware of. Ideally, observations and descriptions should be our starting point, while presumptions and existing models should be secondary.
Let me share an example. During my PhD I did structural mapping in a part of the Caledonides where well-foliated protomylonitic rocks had been pervasively folded into asymmetric hinterland-verging folds and shear fabrics. These folds are extremely prominent and really eye-catching (even in the winter, see Fig. 4), occurring from microscale structures to kilometer-scale asymmetric folds. Still, the structural geologist who mapped the area and wrote two healthy books about the structural geology of this area hardly mentioned their existence. I suspect at least part of the reason was that the folds did not fit the general concept of foreland-directed Caledonian thrusting, and therefore were largely overlooked. The message here is that if you force yourself to sketch out the details of key exposures, you also force yourself to deal with the structures that are there.
The discovery of S-C and related kinematic structures found in shear zones and mylonites (Fig. 5) may serve as a more general, albeit related example. Detailed field-based descriptions were what made us realize that these structures relate to strain partitioning during shearing rather than representing classical crenulation cleavage. Shortly after the publication of some key papers on this issue, S-C structures were discovered in shear zones all over the world. The implications of the kinematic information that this provided were enormous, and helped among other things to distinguish between extensional and thrust-related detachments. It was also a valuable tool for me as a student, as it helped to link the aforementioned hinterland-verging folds and related shear fabrics to post-collisional extensional reversal of the basal décollement of the Caledonian orogenic wedge, and to relate this extensional deformation to plate-scale divergent motions.
The interesting question is whether there are other secrets hidden in rocks, secrets that we are missing out on because we are too caught up in our preferred models and entrenched ways of thinking. Are there other simple and, once understood and pointed out, intuitive, easy-to-use structures that we need to discover? Or will future advance in our field solely rely on more indirect methods, such as sophisticated isotope or geophysical data?
Regardless, the importance of objective observations through field sketching and descriptions will always be important, making the foundation for more advanced secondary work. Hence we should keep training our students (and develop ourselves as teachers) in this important part of geological research.
But it’s not only about research. Spending time in the field, observing, drawing, coloring (Fig. 6), geo-contemplating and perhaps enjoying a bit of a workout as we move around in the field while being away (?) from our computers and email provides rest and nourishment for our geologic souls. It brings us back to the joy and excitement that we felt when we first discovered an awesome fold, some beautifully deformed fossils or, as in my case, an intensely strained conglomerate.