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The rock consists of very fine-grained quartz and feldspar interlayered with clay-rich sediments. The quartz and feldspar grains can still be felt between the fingers or teeth.As its name implies, it is primarily composed (greater than 2/3) of silt sized particles, defined as grains between 3.9 and 62.5 micrometres. Siltstones differ significantly from sandstones due to their smaller pores and higher propensity for containing a significant clay fraction.


Although often mistaken as a shale, siltstone lacks the fissility and laminations which are typical of shale. Siltstones may contain concretions. Unless the siltstone is fairly shaly, stratification is likely to be obscure and it tends to weather at oblique angles unrelated to bedding.