Thursday the 17th of May, we went on a two-day fieldtrip to Dalarna, a province in the western central Sweden. Dalarna is rather important regarding quaternary geology in Sweden, as the highest coastline – the highest sea level since the last Ice Age – ran through it. The highest coastline is an important boundary between wave-affected land below and the land above, which has no features related to coastal erosion and deposition. Therefore, the landforms can be rather different above and below this line.
I am rather ashamed, but I did not make note of every locality we visited. I was very tired, you see. However, that might just be for the better for you! Consider it that I have spared you from all sights that will not astound you.
The first – actually, the first – stop was a dried-out waterfall. It is thought to have dried out about 7000 years ago, so it would not have been active for more than a couple of thousand years, which could explain why the channel is so rough and unrounded (recall that water erodes rocks to rounder, smoother shapes).
The most striking feature is the tilt of the rock blocks that make up the channel. I do not know what could have caused it, but it feels like tectonic forces were involved.
The next site of note was a forest where the highest coastline went right through. Magnus, our brilliantly pedagogical teacher, took us from the highest point in the forest and downward, so that we could see how the landscape changed in the direction the sea had receded. Not only did he point out various landforms, but also showed us several dug-out holes in the ground, where we could see how the deposited sediments changed as we went down. He was very fond of digging holes in the ground, you could tell. (Geologists…) And it was an excellent way of showing these things that seemed so vague in a textbook. Unfortunately, I am hopelessly incompetent when it comes to quaternary deposits, mostly because I cannot find anything interesting in them. They don’t tell me anything. They are just in the way, covering the nice fossils and bedrock! I guess it is just one of those things I am not naturally curious about, and, since I don’t know much about them, I don’t see anything interesting about them, and, therefore, I am not keen on learning more, since they seem boring, and so the vicious circle goes on.
However, this site was noteworthy, because, at the end, we came to a hole where the quaternary deposits had been converted into soil. Soil is basically the portion of the ground that is affected by, or supports, plant life. And plants are very much alive, and crucial for animals. Plants per se are fascinating organisms, but understanding plants also helps you understand animals. Understanding soil helps you understand plants. Any plant cannot grow in any kind of soil, so soil properties are crucial for what type of plant life it can support.
Now, I am not a soil expert, just a beginner. What we saw here was basically the most advanced thing I know about soil.
This is a typical soil profile, a real textbook example, with clear sections, called horizons. Different types of soil have different horizons, but this one is a classical profile. The soil type is called podsol (may also be spelt podzol), and is the most common in Sweden. The top layer is the O-horizon, and is basically the layer of dead, decaying organic matter (hence the O) covering the ground. Below is the A-horizon (the darker brown and black layers), where organic matter has been mixed with quaternary deposits, and thus is a blend of organic and inorganic matter. The pale grey zone is the E-horizon, where two important processes occur: eluviation (hence the E) and leaching. When rainwater percolates down through the upper soil layers, it picks up acids released during the decay of the organic material. These acids help dissolve nutrients in the E-horizon, and so the nutrients are washed away from this portion of the soil. This is leaching; eluviation is basically the same process, but about smaller, insoluble rock particles. The removal of organic nutrients bleaches this horizon, giving it its characteristic paleness. The nutrients are deposited in the lower, more alkaline portions of the soil, the B-horizon, where the acid is neutralised, thus precipitating the solutes. (Whether and why the insoluble components are deposited here as well is unknown to me.) Below the B-horizon, the C-horizon, consisting of unaltered inorganic material, stretches all the way down to the bedrock.
Now, what could be so interesting about this? Well, it makes you think about where the nutrients are located in the soil: at the top layers and somewhere in the lower half of the middle section. And this makes you wonder about the plant roots: where should they strive to go? The upper portion, or the B-horizon? And if it goes down to the B-horizon, what happens in the section of the roots that touch the E-horizon? Note that roots do not absorb nutrients from their tips, which consist mostly of hard, dead cells, used to plough down through the earth.
Next, we visited an old ravine, but, lamentably, it was not a rocky one, as I had pictured in my imagination. It was full of quaternary deposits, grasses, shrubs and trees, and a tranquil stream flowing through. Then, it is not difficult for you to guess which part was interesting: quaternary deposits or water.
The stream bank consists of silt – a very fine-grained sediment type, second only to clay – and is therefore very easily shaped by the currents, which easily carry such small particles around. Recall ripple marks – fossilised waves, evidence of past currents, either of wind or water – well, here we could see fresh ripples ad the bottom of the stream!
Now that I had seen ‘recent’ ripple marks, it was clearer that fossilised forms truly do represent ancient currents. As the famous phrase goes: the present is the key to the past.
However, the greatest aha-feeling came when our teacher illustrated a concept that really was hard to grasp in theory: the process of soil liquefaction – moist sediment made to flow like a liquid by strong vibrations, such as an earthquake. Our teacher took us to this exposed river bank platform.
He showed how stable this moist silt was by stepping out on it: it bore him without effort. Then he began kicking his boot fiercely on a spot, simulating an earthquake, and we could see how the silt turned more and more gooey, until it was more like a viscous fluid.
There I had seen it with my own eyes, and was completely mesmerised. This was the apex of the day.
Well, the apex of the geological stuff. I spend the night in a nice cottage with lovely people. We fixed a mouthwatering pasta salad with delicious salmon – a feast for kings when you are out in the field! As they retired to watch ice hockey – an important match for the Swedish team – I sat beside, half reading a book, half watching the game with them (I’m not much for such sports). Knowing that nothing usually happens in the games I watch, not until I get bored and leave, when there was a minute left and the score was 3-3, I said: “I’ll just go on reading now, so there will be a goal soon.” Seconds later, they cried out, and one of them said to me, half accusingly: “Wrong team!”
True sportsmen as the Swedes are, their faces were not sullen the next morning when we boarded the bus. Not particularly cheerful, but neither indignant. Neutral.
Spirits lifted by the pleasant night and the sun shining in the morning, I did not expect what a spectacular day this was to become. I had heard before that we would visit a site where there were fossils – a wall full of lots and lots of crinoids (sea lillies), not really my favourite, and I had seen enough of them in Estonia, I though – but little did I anticipate what it would feel like to see fossils again.
Knowing how fossil-crazy I am, our teacher said I could go ahead while the others were finishing their lunch. So I marched together with a couple of blokes (would never have found my way there alone).
(We weren’t the first to arrive, I noticed.)
To routine stratigraphers, this exposure must look odd, and you will soon see why.
Already on the rocks that had slid down to the base of the slope, we could spot a myriad of crinoids. Some embedded in larger rocks.
Others lying loosely among the gravel.
To the inattentive eye, they could surely slip by unnoticed.
Further away, they were coloured reddish brown, and looked much like cigarette butts.
But the true wonder of this place was the exposure. While climbing the steep slope up to it, I focused so much on my steps and balance, that, when I reached the wall and gazed upon it, it came as a shock.
And there was more!
A lot more! The whole length of it was packed full of crinoid stalks!
And when you look at it along its length, you notice what is weird about this place.
The layers are vertical! Not horizontal, as usually. Something had flipped the strata up and pasted them on the cliff behind.
The culprit is a large meteor that struck Dalarna in the late Devonian period, around 370-360 million years ago. Remember the impact crater in Kaali, Estonia, where huge rock blocks had been upthrust by the shockwaves of the impact? This was the same case, only that these blocks happened to be packed with fossils. (It might not be obvious, but the fossil strata were formed long before the meteor struck.) Now, what are the chances of that happening?!
Finding fossils here was no sport – the challenge was to find something else than crinoids. Our teacher’s PhD-student found a large trilobite head almost immediately, and others found small brachiopod impressions, so I knew there had to be other creatures here. I just wanted to find one myself.
And find one I did. A piece of a brachiopod shell, or possibly just a thin impression.
What was curious about it was that it had a crinoid stalk right under it. Was the brachiopod eating the crinoid?
Probably not. Brachiopods are filter-feeders. Besides, there appears to be a hole on the side rim of the brachiopod shell that fits the crinoid stalk well. Probably, both animals were already dead, and the crinoid was shot through the brachiopod and settled inside it.
The most amazing find, however, was a weird-looking specimen, not like the typical crinoids I have seen so far – in fact, nothing like anything I have seen. Lying there among the gravel, I could not resist picking it up and bringing it back home to try to figure out what it was.
A part is bulbous and ends sharply in a narrow, stubby, circular projection, which may have been longer in life. On the other end, there is a more gently tapering extension that seems knobby at the very end, maybe reflecting its life shape, or just poor preservation. Which side is forward/upward and backward/downward I cannot say. On one longitudinal side, at about midlength, just the base of the lengthy section, there is a circular hole or depression that has been filled with darker material, which I am tempted to liken to the holes through which ‘roots’ stuck out on crinoids like those in Kaugatuma cliff, Estonia.
Of the limited set of fossil marine invertebrate groups I know of, I would only guess that this would be either a crinoid or a cystoid – a close relative, with a shorter stalk and a sac-like main body – but both types have a so-called pentagonal symmetry, or five-sided symmetry. This is most characteristic in them having five tentacles, or groups of tentacles (i.e. the number of tentacles is dividable by five), but this specimen doesn’t have any tentacles, just two tapering ends.
They way I decided to research might seem primitive, but most fossil identification is based on comparing what you find with what you have already seen, so I began to look at crinoids and cystoids on Google Images.
I though this fossil could be the calyx of a crinoid, but it is more cup-shaped, and, on all pictures, it ends already branched in five tentacles or tentacle groups. However, looking at cystoids (most pictures are of an eye condition called cystoid macular edema, but they are not relevant here; search for “cystoid fossil” if you want to get rid of them), you can see that many are very similar to my bulb and do not seem to end directly in branches. A picture of a cystoid on Wikipedia removed all doubt.
It even had the circular hole, which might be one of the pores characteristic of cystoids.
Ahh, it feels so great to have that solved out! Or, at least think that it is solved – who knows, I might be completely off – but, only time will tell… or maybe an expert. Regardless, for now, I feel like an incipient fossil detective! Hihi!
Next, we went to a large hill wherefrom we could see the actual impact site of the Devonian meteor. At the centre, a large dome had formed, much in the same way as when you drop a rock into still water: the centre sinks first, but then rises once the rock is gone. This meteor probably vaporised on impact, relieving pressure in the centre, making it rebound into a dome. Between this dome and the border of the crater, a lake formed from meltwater during the last glacial melting period. Here is a panorama of what we could see from where we stood (right to left).
Here, you can really see how enormous the meteor must have been. Yikes!
Our final stop was a large, famous mine: the Great Copper Mountain. Apparently, it had been used for maybe more than a thousand years. (Luckily) we never went into the actual mine (which would have freaked me out), but went to see a part that had collapsed long ago. Miraculously, the collapse occurred the day after a Swedish holiday, so everyone was at home with a hungover, and nobody got hurt! Since the collapse, the hole has been further excavated and expanded into what we have today.
As this is industrial stuff, I do not have much to say about it, but it looks pretty impressive, as you can see in another panorama (right to left, but there is a section missing in the middle).
To get an idea of how huge that pit really is, look closely at the second picture: those orange machines at the bottom are actually excavating machines. Just look! They are small as toys!
The most important event in this trip was definitely finding fossils again. Just the feeling, that trance I go into when I’m in a place like that, like in a different world – and not the usual ‘my own world’ people say I’m stuck in when I do stupid things like taking the wrong buss or go in the wrong direction, but something entirely different – it is impossible to explain in words, you just have to experience it yourself. Therefore, I urge you to go fossil hunting sometime this summer! I know for sure I intend to! It should not be difficult to find a good fossil site somewhere within a days reach, no matter where you live. Just look it up on the internet, or ask people around, and I’m sure someone will know a good place. Imagine what a fantastic family vacation!
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