relative geologic time

RELATIVE GEOLOGIC

TIME

Geologists use a

combination of

common sense and a

few simple principles

to determine the

order in which rocks

formed and changed

over time.

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.

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.

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.

Theory of Evolution

• It states that life-forms

have changed throughout

geologic time

• Species emerge, persist

for awhile; and then

become extinct

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.