Siltstone

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Siltstone is a geological term for a sedimentary rock whose composition is intermediate in grain size between the coarser sandstone and the finer mudstone.

[edit] Description

As its name implies, it is primarily composed of silt, defined as grains smaller than 62 micrometres. Siltstones were at one time often referred to as sandstones, but differ significantly due to their smaller pores and higher propensity for containing a significant shale fraction. Although often mistaken as a shale, it lacks the fissility and laminations which are typical of shale. Siltstones are important geological elements in certain stratigraphic series, such as in Muldraugh Hill in Kentucky. They 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.