The concept of interpreting rocks in terms of modern
processes dates back to the 18th and 19th centuries
(‘the present is the key to the past’). ‘Sedimentology’
has existed as a distinct branch of the geological
sciences for only a few decades. It developed as the
observational elements of physical stratigraphy
became more quantitative and the layers of strata
were considered in terms of the physical, chemical
and biological processes that formed them.
The nature of sedimentary material is very varied in
origin, size, shape and composition. Particles such as
grains and pebbles may be derived from the erosion of
older rocks or directly ejected from volcanoes. Organisms
form a very important source of material, ranging
from microbial filaments encrusted with calcium carbonate
to whole or broken shells, coral reefs, bones and
plant debris. Direct precipitation of minerals from solution
in water also contributes to sediments in some
situations.