Download - Relative geologic time
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RELATIVE GEOLOGIC
TIME
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Geologists use a
combination of
common sense and a
few simple principles
to determine the
order in which rocks
formed and changed
over time.
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Principle of Original
Horizontality
• The principle that most
sediment is deposited as
nearby horizontal beds,
and therefore most
sedimentary rocks
started out with nearly
horizontal layering.
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Principle of
Superposition
• The principle that in any
undisturbed layers of
sediment or sedimentary
rock, the age becomes
progressively younger from
bottom to top, younger layers
always accumulates on top
of older layers.
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Principle of Crosscutting
Relationships
• The principle states that a
rock or feature must first
exist before anything can
happen to it; thus, if an
intrusion of rock cuts across
an existing rock, the dike
(intrusion) is younger.
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Theory of Evolution
• It states that life-forms
have changed throughout
geologic time
• Species emerge, persist
for awhile; and then
become extinct
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Principle of Faunal
Succession
• The principle states that
species succeeded one
another through time in a
definite order, so that
sedimentary rocks of the
same age contain identical
fossils and rocks of different
ages contain different fossils.
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