The geology under the city of Tallinn is made up of rocks and sediments of different composition and age. Youngest are the Quaternary deposits. The material of these deposits are till, varved clay, sand, gravel and pebbles that are of glacial, marine and lacustrine origin. Some of the Quaternary deposits are valuable as they constitute aquifers or, as in the case of gravels and sands, are used as construction materials. The Quaternary deposits are the fill of valleys that are now buried. The buried valleys of Tallinn are carved into older rock likely by ancient rivers to be later modified by glaciers. While the valley fill is made up of Quaternary sediments the valleys themselves originated from erosion that took place before the Quaternary. The substrate into which the buried valleys were carved is made up of hard sedimentary rock of Ediacaran, Cambrian and Ordovician age. Only the upper layer of Ordovician rocks protrudes from the cover of younger deposits cropping out in the Baltic Klint at the coast and at a few places inland. The Ordovician rocks are made up from top to bottom of a thick layer of limestone and marlstone, then a first layer of argillite followed by first layer of sandstone and siltstone and then another layer of argillite also followed by sandstone and siltstone. In other places of the city hard sedimentary rock is only to be found beneath Quaternary sediments at depths reaching as much as 120 meters below sea level. Underlying the sedimentary rock are the rocks of the Fennoscandian Craton including gneisses and other metamorphic rocks with volcanic rock protoliths and rapakivi granites. The mentioned rocks are much older than the rest (Paleoproterozoic age) and do not crop out anywhere in Estonia.