Basic Geologic Principles*
Principle of Superposition
Principle of Original Horizontality
Principle of Lateral Continuity
Principle of Crosscutting Relationships
(*Notes)
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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.
(*Notes)
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.
(*Notes)
Principle of Superposition
TIMING: RELATIVE vs. ABSOLUTE
270 Million Years Old
YOUNGER
OLDER
600+ Million Years Old
Geologic Time
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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.
(*Notes)
Originally
Horizontal
Layers on top of layers
Folding & Tilting
The Flatirons Boulder, CO
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Folding & Tilting
The Atlas Mountain system of northwest Africa
Folding & Tilting
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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.
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Principle of Lateral Continuity
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Principle of Lateral Continuity
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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.
(*Notes)
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.
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Principle of Crosscutting Relationships
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Cross Cutting
MOST RECENT
Cross Cutting
Grand Canyon, Arizona
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.
(*Notes)
Unconformity
Unconformities Animation
In Figure 2, there are 8 different rock layers seen in this exposure. Layers 13 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.
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Unconformity
Unconformities Animation II
Unconformity
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Unconformity
Unconformity
Olympic Coast, Washington: 4th Beach near Kalaloch
Basic Geologic Principles