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Suspended matter, reflections on geomorphology fieldwork
Which geomorphologic processes shaped the surface of our 6 km2 fieldwork area? The professor pointed us on the gullies forming a drainage pattern in an old meander bow. After this, I would see gullies everywhere, even when just biking to the Lidl for groceries.
Beforehand, I actually knew few very plant names. The first day, looking at some green bushes, I thought: how will I ever be able to distinguish one species from one another? Especially when there turned out to be a million types of grasses and ferns. However, after carefully observing a plant and its characteristics once or twice, and attaching a name to it, species started to crystallize out of the bushes, with a remarkable sharpness. I would, after five weeks, even when just peeing in the bushes, not see bushes anymore but only see a variety of species.
Suspended opacities. Naturally, as an Earth scientist you carry around a magnifier at all times, for example to study the size of sand particles. We were sitting at a restaurant at night and I used my magnifier to study the tablecloth and other objects, when accidentally I looked into a street light. My gaze got stuck because the magnifier was filled with something I knew as Muscae Volitantes (latin for ‘flying flies’) but then extremely focused and in this case, not flying. Muscae Volitantes can be seen, especially when you just wake up and look into bright light. The ‘flies’ are small particles suspended in the vitreous humour, a jelly liquid keeping the eyeball in shape. Floating around, they slowly follow the movement of the eye and cast a conic shadow on the retina, analogue to how the Earth casts a shadow on the moon during a lunar eclipse. Where the blood vessels can be filtered out of our field of vision by our brain, because of their fixed position, the 'flying flies' cannot since they’re floating around in suspension.
Suspended matter. When swimming against the current, I would have exactly enough power to stay on the same position relative to the shore. Sediments in suspension can’t resist the erosive force of the Sûre river and quickly passed by my body. How did these particles end up in the river, I wondered, while trying to resist the current. Gully erosion, rock fall, creep, solifluction, processes recently engraved in the lens through which I now looked at the landscape, hollowed out the hills next to the Sûre. Along the course of the river particles had been collected in drainage systems, while chemical and physical weathering broke up stones into ever smaller and smaller particles. In the water, gravity pulls the sediments towards the riverbed, since the density of a sand particle is higher than the density of the water. How are these sediments kept in suspension? The water shows heavy turbulent behaviour behind each particle, exercising pressure into all directions, while the water in front of the particle is only moving in one direction, creating a drag force stream downwards.
However, when the Sûre is flooding the valley, the water staying behind on the land becomes quiet. The force to keep the grain into a downstream journey is lost, and for decennia or maybe even millennia the sediment particle will be locked in a floodplain. Then it may be eroded into suspension again when the river is changing its' path. It follows the Sûre, draining into the Moessel, ending up in the Rhine to be finally deposited in a sediment bank in the north sea. And then, in some far away future the North Sea might be lifted up by tectonics to form new hills to be eroded by gullies, solifluction, slumps, and rock toppling.
Excerpts of lecture 'suspended matter' at the finnisage of Kati Kärki's and Hanne Lippard's exhibition Aberrations