MUDSTONE
mudstone, shale and clay
Colour: Black, grey, white, brown, red, dark green or blue.
Texture: Grain size less than 1/256 mm; individual grains are
too small to be distinguished with the naked eye. Mudstone and
shale feel smooth, and a pure clay is not gritty when smeared
between the fingers. Clays are plastic and often sticky when wet.
Structure: When consolidated and relatively massive it is known
as mudstone (or claystone); if finely bedded so that it splits
readily into thin layers it is called shale. When soft and uncompacted
it is termed clay. Sun cracks, rain prints etc. sometimes occur
on bedding surfaces; and fossils and concretions are common.
Mineralogy Too fine-grained for minerals to be distinguished with
the naked eye, or usually even with the microscope. Clays consist
of a mixture of clay minerals together with detrial quartz, felspar
and mica. Iron oxides are usually abundant and contribute the
red and yellow colours. Black shales are rich in carbonaceous
matter, and pyrite and gypsum commonly occur in them, sometimes
as well shaped crystals.
Field relations Clays tend only to occur in the younger geological
formations, being consolidated into mudstone and shales with time.
Being very fine-grained, clay is easily transported water into
the sea and lakes, where it accumulates with silt, sand and calcareous
organisms to form typical sequences of shales, siltstones, sandstone
s and limestone s. Some clays are residual, having formed in situ
as soils; such are the bauxitic clays." (Hamilton
et al 1976, 196)
