Since I’m currently away teaching mapping, this seems an opportune moment to fill everyone in on exactly what that entails, with this repost from ye olde blog. This advice is chiefly based on my own experiences in the field and extensive supervision, marking and viva-ing of undergraduate mapping projects back in my Southampton days. Things are actually a bit different in South Africa – the four or five day exercise I’m currently helping to supervise is the most advanced mapping students here will ever do, rather than acting as preparation for four or five weeks of independent mapping – but most of this advice still applies. Since the geoblogosphere is much bigger and wiser nowadays, perhaps other geologists will be willing to chime in with their own insights in the comments.
The aim of mapping
The ultimate aim of the project is not the map. Your goal is to uncover the geological history of the mapping area, principally:
- The sequence of deposition of the different formations, and the environmental and tectonic changes which have produced these distinct rock types.
- The structures (faults and folds) which have led to the present distribution of the different formations, and the tectonic events which produced them.
A good map is essential for developing this understanding, but if you confine your fieldwork to simply colouring the distribution of outcrop, you’re making it difficult for yourself.
Thinking and hypothesising in the field
There are various scales of observation to consider when you’re recording the geology at a particular locality, from whole outcrop to hand lens. When mapping, you also have to think about the biggest scale of all: regional context. As well as the obvious (e.g., ‘this is 200m downstream from the last locality’), you should always be asking yourself, what’s going on at this locality compared to the last one you saw, and others nearby? Can you explain any changes in rock type, or structure? It is especially important to properly record such observations. If a formation you don’t expect suddenly appears, note down that it’s unexpected. Record and sketch any ideas you might have about what’s going on, and what you might find at the next outcrop if your idea is right. Then go and have a look. Even if you never do field geology again, this is great experience of doing real science.
Field slips as working documents
Field maps are not something to passively colour in outcrop on; you should be trying to infer boundaries, and drawing them on, as early as possible (the same is true of faults, folds etc.), preferably as you are mapping them. Doing this turns your field slips into a powerful tool to guide your mapping, allowing you to visually check, whilst you are still in the field, whether what you are observing makes geological sense, and highlighting areas that you will need to concentrate on to sort out what’s going on. It’s far easier to do something about problems and missing data out there, than months later back home.
Annotation of your field slips is also important – if you’re inferring a boundary due to a break in slope, or a change in vegetation, then record this on your field slip. If you have observed a good exposure of a fault plane, or an unconformity, highlight it. All of this tells a marker that you are doing geology, rather than colouring in.
Don’t be a slave to the literature
In your final project, you will be expected to justify all of your conclusions and interpretations in terms of what you have actually observed in the field. It’s fine to compare what you have seen to what is stated in the literature, but published research is not a substitute for your own observations – even (especially!) if they disagree. Like any geologist, the authors of papers about your mapping area have pieced together the history of the area from incomplete information, inferring boundaries and structures where there was no exposure. What is more, their research usually encompasses a much larger area in far less detail, meaning that the observant undergraduate might well see features, structures, and even outcrop that they did not. This new information might change your interpretation completely, indicating that in your area at least, things were different from the published interpretation (you should also bear in mind that general conclusions for a larger region may not be a good fit for your specific area).*
As an example, a statement such as
At locality 56, the boundary with the overlying formation was seen to be unconformable; a clear erosional surface with rip-up clasts was observed. A hiatus of at least 25 million years has been suggested by Bloggs et al. (1978), based on fossil data.
will get much higher marks than
The boundary with the overlying formation is unconformable (Bloggs et al., 1978).
Observation and interpretation in your notebooks
By this stage of their geological education, students shouldn’t need reminding about separating interpretation from observation, but they do need to be aware of the biggest enemy they will face in their 30 days in the field – boredom, the effect of which is to make people forget about the observing part and move straight on to the interpretation.
Day 28, and you come across an outcropping of limestone. It looks like a formation you’ve seen a hundred times by now, it’s about where you expect it to be. You write in your notebook, “Locality 278. Exposure of Intractable Limestone Formation,” record any structural information, and move on. The only problem is, assigning a formation is an interpretation, and it has been written down without recording the lithology, fossil content, and other observations which explain why you think it is that formation. Months later, you start to suspect that at that outcrop it was the Inscrutable Limestone instead of the Intractable Limestone. If you have recorded your primary observations, you can check this – you may see that at this particular outcrop you didn’t see the fossils characteristic of the Intractable, making it possible you misidentified it. If you just have a formation name, you’re stuck.
So work hard to maintain the quality of your notebook. After 30 days, you can’t possibly remember everything you’ve seen in your area; you will need to have it all written down so you can refer back to it.
What you should (and shouldn’t) be doing in your evenings
There is a growing trend for students to put very little on their maps, and sometimes even in their notebooks, in the field, only to waste hours reproducing immaculate versions of what they have seen in the evenings. What you will get for all this effort is suspicious markers, who want to see evidence that the field slips are being used, as I have already discussed. Filling in as much information as possible as you go means that in the evening all you should have to do is make sure that everything is legible and inked in, freeing up time for what you should be doing, namely thinking. Did you understand what you have seen today, or are there still areas you don’t grasp? Does it fit in with your current understanding of the whole area? It goes without saying that you should write down these thoughts in your notebook.
Ah, the term which strikes fear into the heart of every undergraduate – which is all the the more reason not to leave it until you’re writing up before thinking about them. From the very beginning you should be devoting some of that evening thinking time to sketching cross-sections. Amongst other things, this is another check on whether you’ve mapped things correctly – projecting things into the third dimension can reveal problems which aren’t immediately obvious in plan view.
Although within a week you should have enough information to produce a reasonable first stab at a section through your area, a fully accurate one will require a lot of structural data along the line of section. It is a really great idea to select a sensible cross-section line (or lines) before you finish in the field, then take a day to traverse it (or them), checking that what you see agrees with your map, and that you have enough structural data to properly constrain things. Again, this can reveal problems with your map which are much easier to fix out there than back here.
Sketches or photographs?
Both. If you have a digital camera, it’s very tempting to rely on cold hard optics rather than your own sketches, and it’s certainly true that a photo can record more detail, more accurately, than all but the best of us can manage with a pencil. But there is a lot of interpretation involved in the process of sketching – you are picking out and highlighting the most important features of the outcrop. You can’t do this when taking a photograph, which means that when you look at again it’s very easy to forget what it is supposed to show. A photo cannot replace a sketch, but it does complement it, and vice versa.
Small-scale tells us about large-scale
Because rocks tend to deform at all scales, clues to the overall structural style of an area can often be found by looking at features of smaller scale folds (e.g., shape, vergence) and faults (e.g., orientation, sense of displacement from slickensides). Detailed measurement of these features can be very useful.
Should you be lucky enough to get some field supervision when mapping, make sure you exploit this to the full. If they’ve visited the area before, then obviously you can benefit from their insider knowledge; but even if they are as new to an area as you, their greater field experience means that they will understand how to extract relevent geological information, and spot key bits of evidence at an outcrop much faster than you can. However, they’re not just going to tell you this stuff – ‘independent’, remember? So, should you be out with your supervisor, and they start to do any of the following:
(1) Sit pointedly on or near a particular outcrop as you wander past;
(2) Deliberately wander off in a different direction to the one you’re taking;
(3) Spend several minutes closely examining a particular part of the outcrop you’re currently working on;
(4) Say, “aren’t you going to look at that outcrop up there?” or, “maybe you should check what’s over the over side of that ridge…”;
(5) Jump up and down and shouts, “Look at this!”;
it might be worth your while to take the (increasingly unsubtle) hint (and bear in mind that they may well stop at number 3).
So endeth the lesson.
*I didn’t get anywhere with my own mapping in North Wales until I folded up the BGS sheet for the area and put it at the bottom of my bag for the rest of my trip. It just wasn’t at a small enough scale to properly represent all the details I was finding – for example, it had a lot of random faults running through my area, when I thought there was a lot more regularity there. As it turned out, there was.