Engineering Geology

Chapter 1
Figure 1.22
Salt teepees on the Devil’s Golf Course, a salina, Death Valley, California.
rocks, for example, carbonate rocks may be replaced by anhydrite and sulphate rocks by
halite (NaCl).
Organic residues that accumulate as sediments are of two major types, namely, peaty material
that when buried gives rise to coal, and sapropelic residues. Sapropel is silt rich in, or
composed wholly of, organic compounds that collect at the bottom of still bodies of water.
Such deposits may give rise to cannel or boghead coals. Sapropelic coals usually contain a
significant amount of inorganic matter as opposed to humic coals in which the inorganic content
is low. The former are generally not extensive and are not underlain by seat earths
(i.e. fossil soils). Peat deposits accumulate in poorly drained environments in which the
formation of humic acid gives rise to deoxygenated conditions. These inhibit the bacterial
decay of organic matter. Peat accumulates wherever the deposition of plant debris exceeds
the rate of its decomposition. A massive deposit of peat is required to produce a thick seam
of coal; for example, a seam 1 m thick probably represents 15 m of peat.
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