Definition: Rock composed of grains < 0.06 mm (Blatt & Tracy, 1996). For an ideal size definition, siltstone has greater than 2/3 silt, mudstone has between 1/3 and 2/3 silt, and claystone has less than 1/3 silt where silt is small quartz grains that may be large enough to be seen with a hand lens. Grains with diameters smaller than 60 micrometers usually travel suspended in water or air currents (Blatt & Tracy, 1996).

The composition of sediment in intermontane areas or alluvial plains may be classified as a siltstone/mudstone/claystone. This is not to imply the landform is rock but that it is primarily composed of sediments of that composition and may have rock fragments of other compositions and/or have windblown sediments from undetermined origins as well. Sediments and older sediments that are or were lacustrine in origin come under this classification though they are not rock and may not be derived from previous rock of this classification.

outcrop photograph of shale

(Calico Mud Hills, )

Oblique Air photo

of shale and sandstone

Practical Implications

Building Material: Siltstone can be crushed and used as road metal. Claystones can be used as pigments, fillers and as the raw material in the manufacture of ceramic products, brick and tile. Some shales can be heated for use as aggregate in concrete (Dietrich & Skinner, 1979).

Economic: Some shales contain oil, phosphates or uranium in sufficient amounts to make them economical (Dietrich & Skinner, 1979).

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(last modified 10/16/98)