A Little Bit of History…
• The history of geologic time begins with the human interest
in mining.
• Interests in rock units began to flourish with commercial
mining in the 1500s and 1600s
• People began to identify strata (rock layers) by the types of
fossils that were found within them.
• Two scientists were credited with contributing the most
research to this area -- Nicolaus Steno and James Hutton
.
Nicolaus Steno

Adapted two geologic principles
1.
Law of Superposition : sedimentary rocks are
found with the oldest on the bottom and
youngest on top.
E.x. Paint layers
2.
Principle of Original Horizontality: If left
undisturbed rock layers will stay in horizontal
order
James Hutton
18th-century Scottish
physician and farmer
 Studied his farm land for geologic changes
*The Present is the Key to the Past
 Principle of Uniformitarianism: geologic
processes from the past are the same as the
current geologic processes.
E.x. What happened then happens now
Relative Age: gives the age of a rock layer based on
the ages of layers around it (younger or older)
What sort of events can change the order of rock layers?
Law of Crosscutting Relationships
 Fault or body of rock is younger than any other body of rock
that it cuts through.
 Igneous intrusions that may be uplifted to the surface would
also be younger than the rock they push through
Unconformities- breaks
in geologic time where
rock layers erode away
or when sediment isn’t
deposited