Geological Time
Geology 12
Uniformitarianism
“The present is the key to the past”- the
fundamental principle that underlies
most of geology. Put simply, it
explains that the present processes
occurring on earth have occurred
throughout geological time.
 Relative time ("chronostratic") -- subdivisions
of the Earth's geology in a specific order
based upon relative age relationships (most
commonly, vertical/stratigraphic position).
These subdivisions are given names, most of
which can be recognized globally, usually on
the basis of fossils.

Relative Dating: Determining the
chronological sequence of rock units by using
six fundamental principles of geology. It
estimates the order of prehistoric and
geological events determined by using basic
stratigraphic rules, and by observing where
fossil organisms lay in the geological record,
often in horizontal, stratified bands of rocks
present throughout the world. Relative dating
can determine the sequential order in which a
series of events occurred, but not precisely
when they occur.
6 laws/ principles used to
determine the relative age of
rocks
1. Principle of Superposition -In an undeformed
sequence of sedimentary rocks, the youngest
beds are at the top and the oldest beds are at the
bottom (also applies to volcanic rocks).
2. Principle of Original Horizontality - The observation
that sediment particles deposited from water
under the influence of gravity form essentially
horizontal layers. Non-horizontal rocks have been
disturbed after deposition and lithification.
3. Principle of Lateral Continuity - Sediment
extends laterally in all directions until it
thins, pinches out, or terminates against
the edge of the depositional basin.
4. Law of Cross-Cutting Relationships An intrusion or fault that cuts
through another rock is younger
than the rock it cuts.
5. Principle of Inclusion - Inclusions
are older than the rock that
contains them.
6. Principle of Faunal Succession - Fossil
organisms succeed one another in a
definite and determinable order, so
any time period can be recognized by
its fossil content. General evolution
pattern is from simple to complex
organisms.
4 Types of FOSSILS:
 Actual
unchanged remains
 Remains replaced by minerals
 Molds and casts
 Impressions (i.e tracks,
footprints)
In order to be considered an “Index
Fossil” the organism must:

A) be found over a wide geographic area

B) have only existed for a relatively short
time span (and therefore found in only a
few distinct rock layers)

C) be easy to distinguish from other
similar fossils
 Absolute time ("chronometric") -numerical ages often in "millions of
years”; most commonly obtained via
radiometric or isotope dating methods
performed on appropriate rock types.

Absolute Dating: A general term
applied to a range of techniques that
provide estimates of the age of
objects or materials in real calendar
years either directly or through a
process of calibration with material of
known age. Such techniques include
radiocarbon and radiometric dating.