Erosion caused by various meteorological factors is constantly wearing down the bedrock, this effect being particularly pronounced in areas with large height differences such as mountain ranges. These erosional forces are able to reduce even the hardest of rocks to small grains. It is in this way that the soils and other loose deposits that cover the bedrock are created. The water that falls on the earth’s surface as rain and the resulting streams and rivers transport this material to lower-lying areas in the course of time, and eventually into the sea, at the same time sorting it, so that erosion products of different grain sizes are separated out and areas or deposits are formed beside the watercourses which are almost entirely composed of material of the same grain size, such as clay, silt or gravel. The till that is typical of the loose deposits in Finland is a mixture of erosion products of varying grain sizes and is a consequence of the glaciers that once covered the whole country, as these similarlyransported erosional material but did not sort it as flowing water does.
Sedimentary rocks arise when the erosion products eventually accumulate in thick layers, the lowermost layers being forced so deep that the increase in both heat and pressure causes them to fuse into rock.
The main sedimentary rock types used as building stone are the sandstones, limestones and travertine.