In the beginning of the Silurian Period favourable conditions for seabed biota, especially Brachiopods arose in the Estonian territory. Therefore a huge deposit of the shells of the Braciopod Borealis borealis was formed, stretching from Hiiumaa Island to Lake Peipsi. During hundreds of millions of years these calcareous shells were turned into shell limestone (Borealis-limestone). By the way most of the Pandivere uplands consists of Borealis-limestone. The depth of the deposit is up to 10 metres.
Borealis-limestone is extremely decorative construction material and therefore extensively used. Due to high content of calcium carbonate Borealis-limestone is also valuable as industrial raw material.
Brachiopods (Brachyopoda) are ancient beings, resembling oysters or cockles (Bivalvia). Valves of Brachiopods are with different size, covering the dorsal and the ventral side of the animal. The creature anchors with a pedicle stalk (a stem) to the seabed. Most of the organism is made up by lophophores, which is used by the creature for foraging.
In the Silurian period the Brachiopods were having very important role in the ecosystem, but these days they are almost extinct. According to the fossils at least 10 000 species are known, while nowadays there are only 200 species. During the evolution Bivalves took over the role of Brachiopods.