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the close of the 18th century, the haze of fantasy and mysticism that
tended to obscure the true nature of the Earth was being swept away.
Careful studies by scientists showed that rocks had diverse origins.
Some rock layers, containing clearly identifiable fossil remains of
fish and other forms of aquatic animal and plant life, originally
formed in the ocean. Other layers, consisting of sand grains winnowed
clean by the pounding surf, obviously formed as beach deposits that
marked the shorelines of ancient seas.
layers of rock are in the form of sand bars and gravel banks - rock
debris spread over the land by streams. Some rocks were once lava flows
or beds of cinders and ash thrown out of ancient volcanoes; others are
portions of large masses of once-molten rock that cooled very slowly
far beneath the Earth's surface. Other rocks were so transformed by
heat and pressure during the heaving and buckling of the Earth's crust
in periods of mountain building that their original features were
the results of studies on the origins of the various kinds of rocks
(petrology), coupled with studies of rock layering (stratigraphy) and
the evolution of life (paleontology), today geologists reconstruct the
sequence of events that has shaped the Earth's surface. Their studies
show, for example, that during a particular episode the land surface
was raised in one part of the world to form high plateaus and mountain
ranges. After the uplift of the land, the forces of erosion attacked
the highlands and the eroded rock debris was transported and
redeposited in the lowlands.
the same interval of time in another part of the world, the land
surface subsided and was covered by the seas. With the sinking of the
land surface, sediments were deposited on the ocean floor. The evidence
of the pre-existence of ancient mountain ranges lies in the nature of
the eroded rock debris, and the evidence of the seas' former presence
is, in part, the fossil forms of marine life that accumulated with the
recurring events as mountain building and sea encroachment, of which
the rocks themselves are records, comprise units of geologic time even
though the actual dates of the events are unknown. By comparison, the
history of mankind is similarly organized into relative units of time.
We speak of human events as occurring either B.C. or A.D. -broad
divisions of time. Shorter spans are measured by the dynasties of
ancient Egypt or by the reigns of kings and queens in Europe.
Geologists have done the same thing to geologic time by dividing the
Earth's history into Eras-broad spans based on the general character of
life that existed during these times, and Periods-shorter spans based
partly on evidence of major disturbances of the Earth's crust.
names used to designate the divisions of geologic time are a
fascinating mixture of works that mark highlights in the historical
development of geologic science over the past 200 years. Nearly every
name signifies the acceptance of a new scientific concept-a new rung in
the ladder of geologic knowledge.
major divisions, with brief explanations of each, are shown in the
following scale of relative geologic time, which is arranged in
chronological order with the oldest division at the bottom, the
youngest at the top. Keyed to the relative time scale are examples of
index fossils, the forms of life which existed during limited periods
of geologic time and thus are used as guides to the age of the rocks in
which they are preserved.
View MAJOR DIVISIONS OF GEOLOGICAL TIME.