Saturday the 28th of April, we went on another fieldtrip, this time around in Uppland, our home region in Sweden, this time with a different course – about destructive processes (weathering, erosion, etc.) and Quaternary geology (i.e. the geology of the time period we are currently in).
Most of the features we encountered here are tied to glacial processes, since essentially all of Sweden has been heavily influenced by the Ica Age glaciations. The surface is covered by a carpet of glacial deposits, and glacier-shaped landforms are scattered across the land. Therefore, I think a brief summary of how glaciers work is in place.
Glaciers are thick sheets of highly compact ice – so-called glacial ice, formed when summers are not warm enough to melt all snow that falls in winter. Snow accumulates and compacts under the weight of overlying snow layers, and, once it reaches a thickness exceeding 50 m, it is effectively a glacier.
Glaciers move downslope, both as ice layers sliding across one another (termed plastic flow) and as the entire ice mass slides over the ground (referred to as basal slip). The ice mass grinds the rocks in the ground and any ridge or hillside it touches, a bit like a sheet of sandpaper. Moreover, the ice breaks larger chunks of rock loose by frost wedging – basically freezing water in the rock pores, which, as it expands, explodes the rock pieces loose.
As the glaciers move and break the bedrock apart, it also picks up and transports large quantities of material, both large and small particles. Unlike when transported by, say, a stream, the material is not eroded during actual transport, and neither is it sorted according to grain size (fluid transport agents transport smaller fragments farther away, and so sorts them out from the larger particles, which are dropped closer to the origin; in ice, however, size does not matter much). Glacial material is therefore typically angular and unsorted.
The glaciers drop the material when the surrounding ice melts. Melting occurs throughout the entire glacier, mostly at the bottom (where friction and heat from the Earth warms up the ice), but as the glacier advances, the deposited material can be picked up again. Thus, final deposition occurs when the glacier is either in a standstill, or when its front is retreating.
Our teacher would kill me if I said that the glacier itself retreats. Actually, glaciers always move forward, but, when substantial melting occurs, its front end retreats, because melting makes ice at the front disappear faster than the ice moves downslope. So, the glacier is constantly moving forward; when it seems to be retreating, it is only the front that shrinks.
The glacier advancing strongly affects the land it crosses, but it also affects land further on when it is melting, since vast amounts of water are released, causing massive stream erosion downslope, and raising sea levels, making coastal erosion affect more inland areas.
In the first site we visited, a school playground, we witnessed a glacial erosion feature. I do not know the English word for it, only the Swedish name, namely ‘rundhäll’, a rounded bedrock exposure.
The ‘rundhäll’ has a very characteristic shape: the side facing the direction in which the glacier moved (the proximal side) is gently sloped, while the other side (the distal one) slants very sharply. Thus, you can tell by the shape in which direction the ice has moved across it. The form is due to the fact that the ice could not erode the entire hard bedrock slab, only scrape the proximal side; the distal part formed by frost wedging of material directly behind it.
In the second picture, you can clearly see stripes along the length of the proximal face. These are called glacial striations, and were created by large rocks carried at the base of the glacier, scraping and carving out furrows in the ‘rundhäll’. These also tell of the general movement direction of the glacier. (The marks that run across the glacial striations have probably been formed later, by some other process.)
Next, we went to see a typical depositional glacial landform: an end moraine. End moraines are ridges of material dropped by the glacier while its front has been stationary, and, thus, large amounts of material has accumulated, as particles are constantly transported forward within the ice, and now dropped all in the same place.
This end moraine is exceptionally coarse-grained, consisting to a large extent of big rock blocks. I have no idea why.
End moraines form with their long axis roughly at right angle to the glacier’s movement direction, so these features can also be used to work out how the ice has moved. Perhaps more importantly, however, end moraines show the position of the ice front when the glacier was stagnant, and the size of the end moraine gives an indication to for how long time the glacier was stable.
When we went back with the bus, we passed through a nearby area with a myriad of end moraines, most covered by vegetation, but still preserving their typical shape. We even passed through one of the end moraines! Literally, the road went right through it.
The coming stop was the highest ground in all of Uppland, informally called ‘Upplandsberget’, or the Uppland Mountain, an astonishing 118 metres above sea level. (Uppland is very flat, which is ironic, since its name literally means up land.) Despite that, the whole of Uppland was covered entirely by the sea during the sea level rising caused by the last glacial melting period. Consequently, this whole area, and, indeed all of Uppland, has been affected by coastal processes, since the coast line has both risen and sunk back across the landscape. The degree of influence depends on for how long the sea levels were stable at each topographic level – i.e. for how long the waves were splashing against the rocks at a certain altitude.
The place we were at now showed clear signs of wave erosion. Recall from my previous posts that waves create steep, near-vertical cliffs.
Waves can also break apart larger chunks of rock from the cliff, by sheer pressure against the rock, compressing air in the rock pores (the tiny holes within the rock mass); as the water retreats and relieves the pressure, the air expands explosively, blasting rock fragments apart.
Soils at higher elevations tend to be drier than the lower surroundings, since both rainwater and groundwater are pulled down by gravity. Also, rainwater washes down acids released though the decay of dead organic matter in the topsoil down through the soil. The acid helps the percolating rainwater dissolve a significant proportion of the nutrients in the soil, so the upper portion of the soil becomes both more acidic and nutrient-poor. Such conditions are not very favourable for plants, and, therefore, only the toughest vegetation will do well in such places.
This site was clearly dominated by pine conifers and some moss type, maybe peat moss (?). Pines prefer dry, acidic soils, perhaps because their physiology favours such conditions, but also because few other trees grow well in such environments, so there is little competition.
Our next stop was truly a wonder, almost magical. I had never seen or imagined anything like it in my entire life.
A spring – incredibly still, clear ground water naturally pumped up through the ground and into a small, tranquil pond. I love the sound of water, roaring waterfalls, purling rivers, rocks splashing through the surface, even waves washing across the beach. But the spring was completely mute, and I was surprised to discover that it liked this too. It was… so different.
And the view! Almost as finding yourself in a fairy tale. The surface was so still that the trees were reflected perfectly. However, if you looked carefully, you could see green spots on the bottom, making me think of lake spirits and tales of how they could lure you down to drown.
Piles of dead wood at the bottom did not make it less eerie.
But everything has a logical explanation. (Ahem, so we like to think, at least.) The “green” spots are the points where the groundwater is pumped up, thus washing away the dark, dead organic matter, leaving a patch of clean sand, which looks green through the water. (Such a boring explanation… I prefer the one with the evil spirits.) If you look even more carefully, you can actually see the sand bubbling as water pours out. I caught this on film later, but I guess I could show the clip now.
The water in the spring comes from a nearby esker, a glacial landform, quite like an end moraine in its gross shape, but consisting of material transported by meltwater through cracks in the glacier – i.e. finer, more sorted material – and forms long ridges with the length roughly in the direction of the ice movement, not across it as for the end moraines. Eskers work as natural pipelines for water, and, more importantly, have a built-in natural water cleaning ability. Water that comes out of eskers is therefore essentially drinkable. Later, we tasted water from the spring. (It was, however, rather slimy… nothing I would try again.)
I think you can see a piece of the base of the esker in this picture.
The spring flowed further on, calmly and quietly, through a small meandering channel, which we followed.
Shortly, we came to another, larger pond, where they had built a bridge, not because it was particularly deep (two or three metres at most), but because it was a nice place to stand and look at the marvels of the spring.
Once again, the green spots!!
Moving on along the bridge, the pond got shallower, until you could see the bottom as clearly as if there was no water.
And yes, in the shallower portions, there was even thriving plant life! And more than so. This pond was teeming with fishes.
(These are all the same fish, but there were many more, trust me. This was the only one I got good pictures of.)
We even discovered lumps of tadpoles, amphibian eggs. (Although, it could have been fish eggs, for all I know – eggs of lower vertebrates for sure, at least.)
Of course, people making jokes about caviar were unavoidable here.
Last, but not least, our teacher pointed on a patch of flowers and said we should learn these. I did not quite catch the name, but, with a faint memory that it began with ‘gull’, and a flora (plant species book) at home, I was able to find out that it is a ‘gullpudra’, Chrysosplenium alerifolium – alternate-leaved golden saxifrage in English. (I am a simple person, so I favour the Swedish name.)
The thing about the alternate-leaved golden saxifrage (do you see why I like it more in Swedish? hehe) is that it grows where there is near-surface groundwater, such as this spring. In other words, you can get a rough idea of the groundwater level if you find this plant, which should be quite easy to spot, now that we have paid attention to it.
This plant made me realise even more now that I need to learn more about plants. It has been too much rocks this year. On the bus, a friend tipped me of a summer course about plants and animals, and I have made a late application for it, hoping I can get a place anyway. It would be perfect! A month of mostly fieldtrips, this time looking at actual living things! Wow!
The spring was definitely my favourite of all sites I have visited and written about in this blog, except for those with fossils – actually, this place even beats some of the fossil sites, such as the coquina bed in Estonia.
The next stop illustrated a not as magnificent usage of eskers. We went to a gravel pit, where they take the esker’s material for industrial use.
Industry is really not an area of interest of mine, so I do not have much to say here. But, of course, there is a problem with taking material from the eskers, how convenient it may be, as the eskers are also important for our use of the groundwater, an important water resource, at least in Sweden. Luckily, the government has apparently ceased to give new permissions for extracting gravel from eskers. (Frankly, I deem water more important than gravel.)
The final stop was a rather steep, narrow valley, with a small stream flowing along its bottom.
The big tree to the left was close to where I stood when taking the photo, and it would be pretty much the same thickness as those trees down by the stream bank. That might give you a rough feeling for the scale.
This was probably the only locality of today that had not been directly influenced by a glacier, since they tend to make the valley much broader, with a flatter valley floor, commonly called U-valleys, because their walls have the shape of the letter U in transection. This valley, however, was more like a V – a V-valley – the typical shape of stream-eroded valleys. Moreover, the material on the sides and at the valley floor was not till, but some sort of sandy sediment.
However, the valley does have some connection to glaciers, partly because it is in Uppland, which, as already mentioned, at some point was entirely submerged under water, thanks to glacial melting, and partly since it probably was initially formed by a meltwater stream.
Recall the way the tree trunks were curved in the meteor impact site in Estonia. The soil and sediments were moving downhill very slowly, and the tilting trees bent their trunks in order to grow upright. This is characteristic of a process called creep, because the ground creeps downslope. We saw the same phenomenon here.
The stream was remarkably calm, almost matching the spring. This would be partly because the steep valley, with its fairly dense vegetation, kept much of the wind away. Also, the stream was deep enough for the surface water to flow without being disturbed by friction against the stream bed (the bottom) – with a few exceptional sections.
As we walked along the stream, we could see more evidence of creeping.
And, we stumbled upon another familiar face.
Lastly, there was a creepy rodent swimming across the stream.
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