by CKR
The Cambrian is the geological age in which the earth becomes home to many kinds of creatures. That was five hundred million years ago. Even before the Cambrian, there were bacterial colonies called stromatolites, whose fossils look very much like rocks. Then, suddenly during the Cambrian, there came some lifeforms that aren’t much like anything we know now, along with many that do. Good fossils of the strange ones, including soft tissues (fossils usually are just bony parts), were first found in the Burgess shale in Canada. There were trilobites and brachiopods too, very popular fossils that many of you may know. Their descendents are pillbugs and, well, brachiopods, which are still around today. Stromatolites are still around, too.
Those strange Burgess organisms died out, and organisms that begin to look familiar to us became more prevalent. We don’t know why larger organisms showed up all at once, we don’t know why there was such a riot of forms, and we don’t know why some of them died out.
There’s a strange reminder of that strange place on the north coast of Estonia. Estonia is separated from Finland by the Gulf of Finland, from Tallinn and Helsinki east to St. Petersburg. I haven’t found the geology of it yet, but there has to be a fault under the Gulf. To the north, Finland is all shield geology: granites and pegmatites, old, old, Precambrian rocks, Precambrian, no life. To the south, Estonia is all sedimentary rocks: oil shale, limestone, dolomite. The Cambrian blue clay outcrops below all of them on the glint (or klint; g and k are very close in Estonian), the northern cliff on the sea.
Finland has contributed rocks to Estonia. The northern Estonian coastline is distinguished by giant boulders of that shield rock pushed there from Finland by the last glaciers. The Baltic Sea, along with surrounding lakes like Peipsijärv, is similar to the Great Lakes of the United States. It was carved by glaciers, and the land is still rebounding.
But that blue clay. Blue it is, bluer than any rock I’ve ever seen. And clay indeed. After a half-million years, under the pressure of all the rocks that were laid over it by the seas, heated inside the earth, it has never solidified into rock. The sea hits the glint during storms and pulls material away. The clay flows out. Trees tilt, struggle, fall, die and are washed away. Blocks of limestone fall. Fossils are etched out by the sea. I think these are Ordovician, slightly more recent than Cambrian, if I recall some of my museum visits correctly. The snails (gastropods) are beginning to look like snails. That segmented pole seems to be a member of the Hyalithes.
One of the young women at the Saka Cliff Hotel said that in grade school, her modeling clay was the blue clay.
Its properties clearly depend on its moisture content. Where springs emerge, it is sticky and flows easily. It dries into hard, brittle chunks.
And it’s blue. Really blue. My camera seemed not to want to record such a color in a soil. Here’s a photo of paiseleht (Tussilago farfara) flowers coming up from the blue clay. I found these plants fascinating; apparently broad leaves will emerge later. My guidebook says that they are found in disturbed areas, and that was where I saw them. I think this is one of the best photos I managed to get for reproducing the color of that blue clay.
Springs flowing out of the blue clay or snow melting through it lay down clay liners in their paths in the sand. It can look like some weird pollutant, but everything in this photo is natural so far as I could tell.
BTW, one of the problems in stabilizing the Sillamäe tailings pond was that layer of blue clay beneath it. Clay is notorious for failing catastrophically; you don’t know it’s going to move until it does, and then it goes all at once. So pilings had to be sunk well below the blue clay around the dam to hold it in place.