Tuesday
We arrived at Tallinn’s harbour around 10 am. First, we were taken on a trip (literally) through Tallinn’s downtown area. It was… alright. For lunch, we went to the nearby Tilgu cliffs to look at a geological outcrop. An outcrop is a site where bedrock is exposed to the surface – usually, bedrock is covered by soil or sediment, but in some places, mostly steep cliffs, one can see the rock layers.
We stood on a windy shore looking at the cliff exposure. It was very cold and difficult to hear what the teachers said, so I only managed to pick up small fragments. (Once again, a silly joke that is irresistable after having studied erosion for a month.) The rest is just observations.
The setting was clearly divided horizontally into three main sections from the shoreline landward: first there were smaller rocks, rounded and smoothened by the waves, then there were larger, rougher and more angular blocks, which probably had broken loose from the cliff and fallen to the ground, and finally there was the cliff. This is quite a typical division for coastal areas: the parts closer to the shore are more affected by the waves, and therefore look different; further away, the rocks are affected by waves only during floods, so they are shaped little by the water.
Water smoothens the surfaces and edges of rocks in several ways: by dissolving soluble components (chemical weathering), by breaking loose rock particles through sheer water pressure (erosion), and by bombarding the rocks with material from other rocks that it carries with it (abrasion). Wind works pretty much in the same way as well, barring that it cannot dissolve anything. Wind is, however, much slower than water in shaping the rocks, so on a coast, where water is abundant, the water is responsible for the way these rocks look.
The cliff consisted of very soft sandstones. You could probably blow a hole in it by just sneezing. (I’m serious!) This indicates that the sediments the sandstone was made of were not buried very deeply, and therefore not compacted by tons of overlying layers – in other words, it was poorly consolidated. There were also some knobs of the mineral pyrite, which contains substantial amounts of sulphur. This was probably an oxygen-poor environment where anaerobic, sulphur-producing bacteria dominated among the microbes. We rarely get such anoxic conditions on land, so this was probably an aquatic environment, most likely marine, since we were on a coast, after all. If we combine this with the shallow burial of the sediments, it points toward a shallow marine setting. However, normally, such environments tend to teem with life, because if sunlight reaches the bottom, there is potential for photosynthetic microbes at any level, and because the nutrients, which are released by decomposer bacteria at the bottom, are easily stirred up to all levels within the water column – in other words, there is food everywhere.
Then, why was this environment so anoxic? Well, it may be coupled to the prominent sea level rises, or transgressions, that occurred throughout the Cambrian period. (These are Cambrian sediments, did I forget to mention that?) Could it be so that this shallow environment was so young that marine organisms had not even had time to establish properly? It is an idea, at least.
There were also some beautiful features in this site. Some rocks had very clear ripple marks, or currents. They can be formed both by water and by wind, and can indicate the direction of the currents (unless they have been moved after they were formed). There were also some really nice breccia conglomerates along the shore. Conglomerates are a type of sedimentary rock with large lumps of rock of different types locked in a much finer matrix. If the large rock fragments are angular, the conglomerate is called a breccia.
Next we went to and area called Türisalu, where we looked at two different outcrops. The latter was at Pakri Cape, but I did not quite catch precisely where the first exposure was. Anyway, we were still long the northern coastline.
The first outcrop was in tall cliffs along a sandy beach. Here, the horizontal division was slightly different, as we had sand closest to the shore, and, more landward, a mix of small, flat, rounded rocks and large, fairly smooth blocks. I interpret this as being due to the floods being much stronger here than in the previous area: the flood waves pushed the small rocks far up the shore, where they could not be reached by the regular waves and pulled back; the water has also affected the large blocks more here than at Tilgu.
On the way to the cliff we found some amazing cave-like structures, or maybe more like longitudinally tapering pillars, depending on what you look at. I have no idea how they might have formed (I thought quite a lot about it, more than about the cliff we were supposed to be examining… hum… not good), but being so round (both as caves and as pillars, hehe) and at the base of the cliff, I would guess that water erosion was involved. These structures are fairly recent, however, not millions of years old. Nonetheless, they were interesting, and quite pretty.
About the cliff, it was around ten metres high had very clear bedding, or layering. There were also some black shales interbedded in the limestone exposure, which indicate an oxygen-poor environment (the shales are black because of a high organic content, simply due to poor decomposition – efficient decay requires considerable amounts of oxygen, so when there is anoxia, the dead matter is not recycled properly, but instead become part of the rock). That was pretty much all I heard from what the teachers said – it was even windier here!
Our next stop, Pakri Cape, which showed a succession from the Cambrian period into the Ordovician, was more interesting. The cliff was roughly as tall as in the previous place, and the pattern formed by the very different layers at the exposure were quite remarkable. (I missed to take a clear picture of the section, because I did not understand that that was the important thing until the day after… No excuses.)
The picture below shows at least the basal part of the cliff, which consist of black shales, which are dark redish to brown due to recent oxidation as a result of exposure to the air. In Cambrian times, however, the environment was low in oxygen, indicated by the presence of the black shale. The mass of black shale looks like a myriad of thin plates lumped together randomly. They were incredibly soft, nearly as soft as the sandstones at Tilgu, indicating poor consolidation – but, in this case, it was probably because the sediments were very fine-grained, so the water carrying the dissolved compounds that normally form the cement of sedimentary rocks (i.e. the mass that keeps the sediment grains together) was unable to circulate through the small pores efficiently, and was thus unable to bind the sediment particles together efficiently. Compare this to building a house of bricks.
Further up was a layer of dark green-blue, wet, sticky mud. You can see some of it in the picture above. The green colour comes from a mineral called glauconite, which is typical of shallow marine environments. Glauconite also turns red in oxygen-rich environments, so its green colour also hints toward the environment being rather anoxic.
At the top, there was light grey limestone, which you can see on the first picture of Pakri Cape. Limestone is also indicative of a shallow marine environment, and this particular type of limestone (cannot remember which one it was) typically forms in clear waters – i.e. waters with little microbes floating around, meaning water with few photosynthetic oxygen-producing bacteria.
Now we started to get a pretty clear view of what the Cambrian and Ordovician periods were like in Estonia: shallow, oxygen-poor marine environments. This was probably the case for most of the inland areas as well, which were submerged under the rising sea levels. This can be examined through drill cores where the sequences are not exposed at a cliff. (Some geologists are fond of drilling holes in the ground. If you use a cylindrical drill, you can get a long tube of bedrock, which gives you the vertical rock profile of the ground.)
We were tasked to sketch the profile of the Pakri Cape exposure, roughly to scale. Later, this got me thinking that this is what is important about geology and paleontology – examining and understanding the sequences of sediment layers, which could tell a lot about the environment where they were formed, and so on.
However, this was completely forgotten the next day, when we began to find actual fossils.
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