Stones of Estonia (5)

Friday

I might as well say it from the start: this day became unforgettable.

We were back in mainland Estonia, and had now reached the Devonian period, which commenced 416 million years ago. The Devonian is usually referred to the “time of fishes”, because they diversified like crazy during this period. Our first stop was a river bank from this time period, located in Tori. If there was any chance of finding a vertebrate fossil, it would be here.

People had had fun here before us.

The geological significance of the marks stop at indicating that the rock is soft (unless we suspect cavemen, or drunk geologists equipped with sharp tools?). So, a true geologist would naturally only put attention to the cross-bedding (layers lying at angles to one another). I cannot read Estonian, but most of the words seem to be names.

Sadly, there was not a fossil in sight. This could only mean one thing: we had to do geology work. This, in turn, meant to examine the exposure.

The sandstone cliff was about six metres tall, and quite clearly dividable into three sections. The top one is marked by its darker red colour and the abundance of fairly large holes. The two segments below were sticking out slightly, further emphasising the distinction of the top part. The middle section was quite “normal”, with even bedding and few holes, while the base portion, ranging from the ground to about one metre up, had crazy cross-bedding. In the picture below is a clearer part of this feature. 

Cross-beddings mark gross changes in the main current direction (I think, but I am not entirely sure). Although it is not seen in the pictures, there was an overall general tilt down to the right (when facing the cliff), which might show the stream’s main flow direction throughout the long time span during which this part of the cliff was formed.

We also looked at the sandstone grains in more detail, using hand lenses. They were fairly rounded, mostly spherical, sand-sized and well-sorted (sorting is the degree of difference between the sizes of the particles in a rock: the more different particles sizes there are, the less sorted the rock is). This all screams long-distance transport by water.

So, the sediments had been transported a long way, meaning that the environment where they were formed was far away from here. How, then, could we be certain that these are river sediments? We cannot, really, since such features can be found in marine environments as well. However, the absence of marine fossils gives a good hint. It is likely that this was a river deposit.

My friend Sara was excited about our second stop: someplace in the middle of nowhere where they had a storage room for drill cores, which are basically long drilled-out tubes of bedrock, showing the layer sequence in inland areas, where the beddings are not exposed. I had a feeling this would mean a lot of really dirty work.

It started out great with a long walk through the muddiest terrain so far (only matched in this trip by the next stop to come). I cannot complain, though: I had rubber boots; others were not so lucky.

Even more luckily, there was no exercise about the drill cores. The idea was more to give us a more compact view of the stratigraphic change over time, which we had seen only patchily so far.

The most distinctive piece was the green glauconite bed from the Ordovician.

There was not really much to say about this. There was nothing more than rock.

Having that in mind, the following picture is just sad.

And that is only the left half of the room…

On the way back to the buss, I got to know Heda Agic, a PhD-student, who would later become a good friend. The walk didn’t feel muddy this time.

The final stop for the day was the Silurian Võhmuta limestone quarry, which looked like this when we arrived.

As mentioned before, this place was even muddier than before. Now you know why.

There were some fossils here, which can be seen in the picture below, although not very clearly.

This quarry was known as a massive Borealis brachiopod bed, but our brachiopod expert, Lars, had never seen this type before. They were indeed very weird brachiopods, looking more like smooth, elongated balls.

Because the place was flooded, and mucky, we decided to leave. There was not much we could do here, unfortunately. Our brilliant guide, Heiki, however, managed to grant us permission to enter a nearby quarry, currently in operation, where the same beds were exposed.

This was clearly the most tricky bed to examine throughout the entire trip. There were no clear sections; everything looked the same. We had to look closer, then.

We found an incredible mass of fossils, all apparently Borealis, except for a few large stromatoporoids (third picture below).

It was as if the rocks consisted more of fossils than of other minerals. Such limestones are called coquina.

The exposure was criss-crossed with bands of extremely concentrated fossil shells.

As remarkable as this was, all features were present everywhere on the outcrop, so they provided no tools for distinguishing layers. It was the same along the entire exposure. We just could not figure out what had happened here. 

Naturally, our teachers were able to clarify: this bed, being almost excessively dominated by a single type of organisms, was an example of the strange ecological forces that were operating back in the Silurian. Exactly what had happened is not understood, but it is different from what we see today, perhaps because we have different environments and animals now, etc.

This is actually really interesting, thinking that the laws and processes we see today might not have been the same in the past. It took a whole revolution within geology to recognise this, to accept that what geology was based on back then, the idea that “the present is the key to the past”, was not true. (Curiously, the debate about whether the extinction that exterminated the dinosaurs was caused by a meteorite from outer space or not, had a part in this revolution!) Nowadays, the saying has cleverly been rephrased into “the key to the past is in the present”, implying that we can learn about past times from what we can observe today, but that we must be open for the possibility that events unknown to us, events so rare that they have not reoccurred within our time of recorded history, also may have occurred.

About this particular example, however, we can also speculate about the idea that ecology evolves together with the organisms – that there is a reciprocal, two-way relationship. It is well-known that ecology causes organisms to evolve, but what if the organisms in turn can change the laws of ecology? It is not entirely daft, because ecology basically depends on the organisms. The food chain depends on the presence of producers and consumers; population ecology depends on the types of populations, and so on. Ahh, so many questions, so many fascinating questions!

Remember that I opened this post saying that this day would be unforgettable? Well, so far, it has not been much spectacular. It had been mostly muddy and weird.  The truly wonderful thing came at the hotel. A nice evening playing table tennis with the guys I shared the ferry cabin with, followed by a glorious plate of schnitzel (as we all noticed, when being in the field for a week, any food is a blessing!), and finally a crazy, late night with Heda, Iliam, Stephen, Aodhán, Sara, and others. This night, I made truly great friends, at least from my part.