Wind and rain erode rock to begin the formation of siltstone. The rock particles break down further as they travel with the water and then settle to the bottom of the water when it slows. Layers of silt accumulate, creating heat and pressure that cement the silt together into rock.
Siltstone is a type of sedimentary rock formed from fine rock particles. Coarser rock particles are classified as sand or gravel and form sandstone, breccia or conglomerate rock as they are cemented together by heat and pressure. Very fine rock particles form a type of sedimentary rock known as shale or mudstone.
As the particles of eroded rock travel with water, the edges of the rock are worn by the water into a rounded shape. Once these rounded particles are cemented together, other particles often occupy the abundance of space between the particles of silt. Calcite and quartz are two minerals that often fill the space between particles of silt. Oil and gas sometimes occupy this space, creating a reservoir that the petroleum industry can mine. Many oil and gas reservoirs are found near the mouths of large rivers, though some reservoirs are located inland where ancient rivers once existed.