Limestone is formed as a product of erstwhile life activity. With the help of bacteria dead organisms are usually going to recycling. Small fraction of the remains of calcareous shells are sedimenting to the sea bottom and departing from the circulation of substance. Greater part of the sunken substance is degrading into uniform paste, lesser part will be preserved as small pieces or even whole intact shells. Occasionally traces of soft interior tissues have been discovered. Remains of onetime organisms, preserved to this day are called petrifactions or fossils.
Estonia is a land with remarkable geological history. The oldest history of the Earth 542-359 million years ago is exposed here. (skeem) During all this time the territory of Estonia was submerged by the sea and sedimentation was permanently going on. By the end of the Devonian Period Estonia’s territory protruded over the sea level and have endured until today (except some sea level fluctuations, caused by glaciation). During different epochs Estonia has probably encountered everything, including dinosaurs, but the glacial ice has destroyed all traces younger than 359 million years.
In Estonia one can find fossils from all geological periods, while upper Ordovician and Silurian limestone layers are especially rich in petrifactions. Beautiful intact fossils can be found from Estonian mineral resource deposits – oil-shale and phosphorite (both originate from the Ordovician period). There are no large and conspicuous fossils in Cambrian formation – caused by the position of Estonia in the Cambrian period. These days Estonia was located in chilly temperate zone close to the South Pole and there were few creatures with calcareous skeletons. The situation changed when the continent of Baltica including Estonia started to thrift towards the Equator and stayed there for the next 130 milloion years. The shallow tropical sea was swarming with vivacious life and there are many traces of life activity (fossils) in the sediments of that period.
In the Devonian Period life conditions changed, as Estonia was situated in the vicinity of the giant Scandinavian mountain range. Sediments of sand, formed as the result of mountain erosion, were holding limestone formation back. Nevertheless one can find in Devonian sandstone fossils of ancient Placoderms.