SYAS RIVER, LENINGRAD REGION, RUSSIA–“Do you have a light? We are going into caves tomorrow.” I must admit to a bit of trepidation, hearing that question. I indeed have a small flashlight, but not one I’d like to have with me in a cave. Turns out the caves today were actually abandoned mine tunnels in a Cambrian quartzose sandstone on the right bank of the Sass River (N60.05429°, E32.59517°). The mines were designed to extract the quartz sand to make glass — some kind of Stalinist enterprise in the 1930s to turn the local peasants into proletarians. These mines were abandoned once the Germans invaded in 1941.
“We will have to crawl a bit.” As you can see from the photograph of the tunnel entrance, “crawling” is not quite the term. “Squirming on your belly” would be closer. I didn’t want to play the soft American, so I just made sure someone was behind me and in front of me, and in I went. The squirming through the mud with my back scrapping against the ceiling seemed interminable, especially since I couldn’t hold the light and pull myself through at the same time. This had better be a good blog entry in the end, I thought. The substrate dropped below me and I slid down into a comfortable cavity (relatively speaking) about five feet high and lit fleetingly by the flashlights of the others ahead. I hunched over and crab-walked to the source of the lights, finally emerging in a mine tunnel fortunately higher than my head. Half success! (Full success is getting out, of course.)
Now it is more interesting than terrifying. The mine cuts horizontally along bedding, so you can follow a single unit of rock throughout the tunnels, watching it change character with distance. The main rock is very white, relatively soft, and marked by the pickaxes of the miners. At about eye-level are darker beds with contortions which contrast dramatically with the flat-bedded sandstone below. Cracks descend down into the sandstone and are filled with iron pisolites (spherical structures made of hematite and from bb- to marble-size). In places the darker beds drop downward into complex breccias (conglomerates in which the clasts are very angular).
The interpretation of these rocks by Andrey Dronov is that they represent a Cambrian permafrost buried by Ordovician sediments. The reason the Ordovician clays, pisolites and siliciclastic sediments are contorted is that they had collapsed to varying degrees as the ice below them was melted after burial. We are looking, then, at a wonderful example of “cryokarst”, or the deformation of sediments associated with ice formation and melting. I had known of only such features in theory, never in practice. It is even more remarkable that these structures are almost half a billion years old. Well worth a bit of slithering through dark Russian mud!