Basic Geologic Principles
Principle of Superposition
Principle of Original Horizontality
Principle of Lateral Continuity
Principle of Crosscutting Relationships
Principle of Superposition
Younger sedimentary and volcanic rocks are deposited on top of
older rocks.
The principle of superposition states that in an undisturbed layer
of rock, the oldest rocks are on the bottom and the rocks become
progressively younger towards the top.
Exception: overturned rock layers due to folding.
Principle of Superposition
Relative Dating
Relative dating is used in geology to determine the order of
events and the relative age of rocks by examining their position
in a sequence.
Principle of Superposition
TIMING: RELATIVE vs. ABSOLUTE
270 Million Years Old
YOUNGER
OLDER
600+ Million Years Old
Geologic Time
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Basic Geologic Principles
1)  The geologic cross section below shows a series of layers of sedimentary units. As you have learned, sedimentary rocks are laid down in layers. a)  Which of the units in the cross section do you think is the oldest? b)  Which unit do you think is the youngest? c)  How do you know (which geologic principle)? Principle of Original Horizontality
Sedimentary and volcanic rocks are laid down in approximately
horizontal layers.
This principle is important in the analysis of folded and tilted strata.
Originally
Horizontal
Layers on top of layers
Folding & Tilting
The Flatirons Boulder, CO
Folding & Tilting
The Atlas Mountain system of northwest Africa
Folding & Tilting
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Basic Geologic Principles
2)  Sedimentary and igneous extrusive rocks are originally laid down in nearly horizontal layers. a)  Why do you think that layers a & c (shown on the right) are not horizontal? b)  Put the cross sections (shown on the right) into a sequence in the order in which they would occur, from oldest to youngest. Principle of Lateral Continuity
Sedimentary and volcanic rocks are laid down in layers that are
usually much greater in lateral extent than in thickness.
-Layers of sediment initially extend laterally in all directions (laterally
continuous).
-Rocks that are otherwise similar, but are now separated by a valley or
other erosion feature, can be assumed to be originally continuous.
Principle of Lateral Continuity
Principle of Lateral Continuity
Principle of Crosscutting Relationships
If a rock unit or geologic feature cuts across another rock unit or
geologic feature, it was formed later in geologic time.
The principle of cross-cutting relationships states that a rock or
fault is younger than any rock (or fault) through which it cuts.
Principle of Crosscutting Relationships
In a series of horizontal sedimentary beds, there is an igneous
dike which cuts vertically through them. The dike is younger
than the sediment beds though which it crosses, as the beds
would have had to be around before the dike could have
intruded.
Principle of Crosscutting Relationships
Cross Cutting
MOST RECENT
Cross Cutting
Grand Canyon, Arizona
Basic Geologic Principles
3)  Look at the two cross sections shown to the right. a)  What is the youngest feature in each of the two cross sections? b)  How do you know (which geologic principle)? Basic Geologic Principles
4)  The geologic cross section below shows a sedimentary rock unit A and an intrusive rock unit B. a)  From what you know about how intrusive igneous rock units form, which of these units do you think is older? b)  How do you know (which geologic principle)? Unconformity
A layer of rock is a record of past events. But most rock
records are incomplete- there are layers missing. These gaps
are called unconformities.
Unconformities develop when agents of erosion remove
existing rock layers.
They also form when a period of time passes without any new
deposition occurring to form new layers of rock.
Unconformity
Unconformities Animation
In Figure 2, there are 8 different rock layers seen in this exposure. Layers
1-3 are horizontal while layers 4-8 are tilted. The contact between the
horizontal layers and the lower tilted layers indicates an unconformity
where the lower, older layers got tilted, then eroded down to a horizontal
surface, followed by deposition of layers 1-3. Processes such a faulting,
perhaps associated with mountain building, can create this kind of tilting
and unconformity on Earth.
Unconformity
Unconformities Animation II
Unconformity
Unconformity
Unconformity
Olympic Coast, Washington: 4th Beach near Kalaloch
Basic Geologic Principles
5)  The rock units in the cross section below have been assigned approximate age ranges. a)  Are the ages continuous, or do you see any gaps? b)  Assume that these are sedimentary rocks that were formed as sediment slowly deposited, layer upon layer. Can you think of an explanation for why there is a time gap in the geologic record? 6)  The cross section below shows several rock units in an area that has had a long and varied geologic history. a)  Put the rock units and the other geologic features marked with letters in the cross section in order of occurrence from earliest (oldest) to latest (youngest). On the top left corner of the loose leaf paper you have been
given write your name, the date, Earth Science, and the
period.
Then write “Geologic Principles Analytical Writing” centered
and underlined on the first line of the paper.
Write a Seven to Ten Sentence Paragraph
Analyze Geologic Cross Section shown on the screen
(and on the last page of your skeleton notes).
Choose three of the following geologic principles and explain
an example of each as they are shown in the in the crosssection:
•  Principle of Superposition
•  Principle of Original Horizontality
•  Principle of Lateral Continuity
•  Principle of Crosscutting Relationships
•  Unconformity