Siltstone's a sedimentary rock which has a composition intermediate in grain size between the coarser sandstones and the finer mudstones and shales. Description As its name implies, it's primarily composed (greater than 2/3) of silt sized particles, defined as grains between 3.9 and 62.5 micrometres or 4 to 8 on the Krumbein phi (φ) scale. Siltstones differ significantly from sandstones due to their smaller pores and higher propensity for containing a significant clay fraction. Although o… (More on Siltstone)