If we bulldoze a hill to make way for a road or a house, the scars are very apparent in the landscape. When a glacier does the same, we don’t notice it because it takes so much longer. Clemens explained that the glacier moves continuously and grinds away on the underlying surface even though it looks like it’s lying still. The glacier alters the landscape in several different ways. It can level mountains and carve out fjords. It moves massive boulders, rocks, sand and clay, and pushes debris as it advances.
The dirt that moves away from under the glacier mainly consists of clay and fine-grained sand. But in addition, the glacier also picks up and moves car-sized boulders. When the glacier shrinks it leaves lots of rubble in the areas earlier covered by ice. New plants and grass can now settle and grow on the newly revealed surface, or these areas can be washed out by rain and meltwater from the glacier.
Water erodes and transports sediments in rivers over large distances. If you’ve tried panning for gold in an amusement park or in the wild, you’ve seen how the lighter sand grains are washed out and the heavier gold is left behind in the tray. This also happens with the glacial debris when exposed to rain or meltwater from the glacier. Whilst the bigger boulders are left behind, the sand and clay is easily transported away with the water.
When the sediment rich rivers meet a lake, sediments falls out as the water slows down. On rare occasions we can be lucky and see a sharp contrast in lakes between gray sediment rich melt water and the clear blue water surrounding it as shown in Figure 1. The amount of clay and sand we can see in the picture reflects how efficient the water is in moving things along. The amount of meltwater varies a lot through time depending on the rate of melting and on where the meltwater goes. In some periods the lake can be almost clear due to little debris brought down from the glacier.
Since the river flow varies so does the amount of sediment reaching the lake. In this way the sediments act as an archive. They tell us how a glacier has evolved, when it’s been growing and when it’s been melting. Figure 2 shows how we can go about extracting the information. By taking sediment cores from the bottom of the lake, and in all surrounding lakes, we can use this great archive to figure out whether and when the lake received meltwater from the glacier, whether the glacier covered the lake or whether it was too small to provide the lake with any sediments at all. By comparing cores from the different lakes we can create a map of the region and the glacier back in time. Slowly but surely these lakes are filled in and the archive is hidden.
So the glacier changes the landscape in many different ways, both locally and far away. It grinds away on the mountain underneath. When it melts, the glacier disappears and boulders appear. Meltwater makes new rivers, and lakes are filled with sediments exposing new plains for grass and flowers to grow. We might not be able to move mountains, but the glacier, nature’s bulldozer, certainly can!
[This post was developed and written for Turspor in collaboration with ClimateSnack at the ResClim science communication course in Finse 16-20th September, 2013]