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as they grapple with describing sedimentary rocks). It has also been impli-
cated in the gteatest mass murders to have occurred on this planet. Sea level
change is a prime suspect in the periods of mass death, called mass extinc-
tions, that shaped out planet's hiota. The way geologists have deciphered
sea level change is thus a potent type of time machine for planetary detec-
tives on the global homicide detail.
Trekking through time
The study of sea level change is part of the discipline of sedimentology, the
study of the way sedimentary or layered rocks form. This branch of geology
gives us the tools necessary to reconstruct the geography of ancient lands.
Sedimentary rocks can be seen forming today in a variety of environments
on land and in water, such as in deserts, lakes, rivers, and oceans; and they
surely did so in similar fashion in the past. This belief forms the core of the
Principle of Uniformitarianism, which holds that the present is the key to
the past. For instance, the way in which sand grains accumulate into sedi-
mentaty beds on a modern-day beach or riverbed is probably the way such
processes occurred on ancient beaches or riverbeds. Time may change, but
the laws of physics, which control the accumulation of sedimentary rocks,
do not. The principle of uniformitarianism is most powerfully used in the
field of sedimentology, where it enables us to reconstruct past environments.
In like manner, we can often reconsttuct and deduce the "life habits" of
ancient organisms by studying how modern forms of similar species aggre-
gate and live.
Each sedimentary environment, be it a riverbed or desert floor, pro-
duces a specific type of sedimentary rock, for the physical forces operating in
each particular environment frequently dictate how the resulting assemblage
of sedimentary rocks will look. Deserts, for instance, accumulate sandstone
amid wind-blown sand dunes, which more often than not leave a tecord of
their existence in the rock itself, in the fotm of switls and irregularities in
bedding known as cross beds. Any such marks in sedimentaty rocks are
known as sedimentary structures, and these often provide the most specific
