This blog is written by Charlie Bristow. Charlie is a professor of sedimentology at Birkbeck, University of London. Being a sedimentologist involves wading through mud, scrambling up sand dunes, and measuring rocks often in remote locations. It is a great job if you enjoy that sort of thing. Get in touch with Charlie at c.bristow@ucl.ac.uk
Geological fieldwork is based upon good observational skills. Becoming a trained observer takes time and experience; learning what to look for and what matters most. This is the first part of the field class mantra ‘Observe, Record, Interpret’.
In sedimentology, the type of geology that looks at rocks which were once sediments like sands and muds, we use a graphic log to record our observations in the field (“graphic” because it’s a type of graph, and “log” as in Captain’s Log!) We stand in front of the rock face, perhaps a cliff on a beach, and measure, note down and record the rock in a way that will allow us to go back to our desks and think about what it all means. But what if you can’t get to the rock face? The aim of the virtual graphic log is to bring the outcrop into the class room, onto your desk-top or mobile phone, using digital photographs and a suitable scale to show sediment grain size, and the other features and structures needed to make your graphic log (Bristow 2020a).
What does a graphic log show?
The geological graphic log starts life as a graph with an x-axis and y-axis. The vertical y-axis shows the thickness of the beds (layers of rock) from bottom to top working up, and the horizontal x-axis shows the particle size in those layers, getting coarser (bigger) from left to right. Whether a rock contains tiny grains of mud or coarse pebbles can reveal important clues about how that rock formed, and the past environments of that location. Creating the graphic log allows us to spot trends and pattern in the rocks- perhaps the particle size gets finer or coarser as you go up; or perhaps the different beds of the rock get thicker or thinner. But the graphic log is more than just a graph of particle size and bed thickness. It is a picture, a representation, of the rock. The contacts between the beds of rock can tell us a lot about what happened when the sediments were deposited. They may be sharp or gradual, flat or irregular; perhaps they are erosional, cutting into the layers beneath. Each of these types of bedding contact can be shown on our graphic log. Being good at drawing is not essential but it does help, and drawing is something that does improve with practice.
The graphic log is also used to record rock type (there are standard patterns we use for each type of rock), and fossil content. Additional columns can be added to show structures in the rock- shapes made by ripples or dunes, for example- as well as “trace” fossils, the footprints made by ancient animals like trilobites or dinosaurs.
It has been suggested that “The log should be as detailed and realistic as the artistic ability of the drawer will allow” (Anderton 1985, p.37). Some people use special symbols to record the structures and fossils in a rock, but sketches tend to be better than symbols. If you’re not sure about what something is, you can make sketches and figure it out later with the help of books, the internet, or an expert. The main thing is to record as much information as possible, in a way that is easy to understand and interpret. The ultimate goal is to unlock the past and figure out what environments formed these rocks (Middleton 1978), completing the field class mantra “observe, record, interpret”.
Virtual graphic logs at Birkbeck
Birkbeck, University of London, has a long history of teaching part-time students through evening classes, and about twenty years ago we started to offer a degree in Geology by distance learning. Originally, all lectures were written as PDFs and onto CDs that were sent out in the post. Nowadays, modern technology allows students to stream lectures and watch them at home. At first, the distance learning resources helped with course accessibility for students who could not attend class, perhaps due to work, child-care, or illness. It meant that they did not have to borrow notes in order to catch up, but would have a complete copy for themselves. However this year, distance learning has become a necessity for us, and universities all over the world, due to the COVID-19 pandemic.
Figure 1. A) Outcrop of Thames terrace gravels with survey staff for scale and B) the size of pebbles can be estimated using the cm scale on the survey staff (from Bristow 2020a).
As part of Birkbeck’s distance learning programme, we’ve created a series of virtual graphic logs. Examples include Thames terrace gravels that show changes in grain size and sedimentary structures (Figure 1). This one is a great example to start with and can be downloaded from the Seds Online website. The beds were deposited on sand and gravel bars in a river. A second example from the Woburn Sands, near Leighton Buzzard, has a limited range of particle sizes, fine to medium sand, and silty clay, but the contrast in tone picks out sedimentary structures formed by tidal currents that moved sand and mud on the sea bed and trace fossils from shrimps that lived there during the Cretaceous, just over 100 million years ago (Figure 2). A third example from the Bama Ridge in NE Nigeria (Figure 3) includes changes in grainsize as well as a range of sedimentary structures which makes a more detailed and interesting graphic log (Bristow 2020b). Other examples include the base of the Ainsa 1 turbidite channel and Triassic fluvial sandstones in south Devon.
Figure 2. Outcrop photographs and graphic log from the Woburn Sands, near Leighton Buzzard in southern England (Bristow 2020a). The sedimentary structures and burrows show that this area was covered by the sea just over 100 million years ago.
Top tips for educators creating a virtual graphic log
It is really important to choose a suitable outcrop of rock that has a range of particle sizes and sedimentary structures that will show up on digital photographs. Try to avoid outcrops with faults, fractures or plants because these can make it more difficult to see the beds and sedimentary structures. It is essential to include a scale in all of the photographs and I have used a survey staff that is marked with cm, 10 cm and meters so that you can see exactly where you are on the section. A near vertical face and vertical scale make it easier to measure bed thickness but this can limit safe access. Strong sunlight can create shadows on the rocks and a flat light from a bright but overcast (cloudy) day works better for photography. Take the photographs from a constant distance and avoid using a wide angle lens.
Figure 3. Example of a virtual graphic log dataset from Bristow 2020b. The outcrop from a sand and gravel pit in NE Nigeria includes a range of sedimentary structures as well as particle size that makes an interesting graphic log from a relatively short section.
Pros and cons of University distance learning
Studying for a degree by distance learning takes a lot of dedication as well as organisation and self-discipline making time to study. At Birkbeck, all of our lectures are video recorded and new technology allows lectures and seminars to be streamed and watched live with interactions online. Use of virtual microscopes, high resolution digital images and resources such as Google Earth™ have improved the learning experience. However, students can miss out on the collegial experience of classrooms, coffee bars and social events that are part of the standard university experience. On the other hand, social media helps students keep in contact and share problems. In addition, students can live at home which is cheaper, and many work during the day, studying in the evenings and at night, adding the ability to earn while you learn, and greater flexibility. Part-time and distance learning also opens up higher education, widening participation and inclusion within the geosciences.
Resources:
Lecture notes can be downloaded from the Seds Online website https://sedsonline.com/sedimentology-teaching-library/
Graphic logs lecture_1-2_Bristow_2020,
Virtual graphic log 1-2_Bristow_2020,
Virtual graphic log 1_answer_Bristow_2020.
Anderton, R., 1985. Clastic facies models and facies analysis. In: Brenchley, P.J., Williams, B.P.J. (Eds.), Sedimentology: Recent Developments and Applied Aspects. Geological Society, London Special Publications 18, pp. 31-47.
Bristow, C.S., 2020a. Observe, record, interpret: some examples of teaching sedimentology by distance learning including virtual graphic logs. The Sedimentary Record 18, No. 2, doi: 10.2110/sedred.2020.2 (out soon!)
Bristow, C.S., 2020b. A virtual graphic log for clastic sediments. doi: 10.1016/j.sedgeo.2020.105703.
Middleton, G.V., 1978. Facies. In: Fairbridge, R.W., Bourgeois, J., (Eds,), Encyclopedia of Sedimentology. Hutchinson and Ross, Stroudsburg, Pennsylvania, pp. 323-325.