Colors of Sedimentary Rocks
Red or reddish sandstones, mudstones, or shales may contain iron that has been oxidized, often indicating
an environment in which the sediments were exposed to the atmosphere before or during burial. An example
might be stream deposits or paleosols (fossil soils).
Greenish rocks may contain iron that has been reduced. A reducing environment may be one in which the sediments
are buried in an oxygen-depleted environment (with inadequate oxygen) under water, such as a sea environment
with limited circulation of nutrients or the quick burial of stream deposits before the iron has been
oxidized. Good examples of reddish and greenish rocks in West Virginia formed in these manners include those of
the Conemaugh Group exposed along I-79 north of Charleston.
Brown, black, or very dark gray rocks may contain large concentrations of organic matter (decayed plant
and/or animal matter) and were generally deposited in an environment lacking oxygen. A prime example is coal,
but other examples include black and very dark gray shales.
A distinction should be made between rocks colored all the way through like those above and rocks coated with
a surface coloration. Reddish/yellowish coatings can often arise from acid mine drainage. Additionally,
some iron and manganese minerals can also give a dark color to rocks, but most typically this coloration is
in the form of a surface coating. Breaking open rocks with a surface crust of iron/manganese oxides and
hydroxides often reveals that the coating is only a few millimeters thick and the "fresh" color of the rock is
Page last revised: November 11, 2005