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Lesson2 - Earth
Steno’s Laws of Stratigraphy
Historical Perspective of Stratigraphy
• First advances were made in the mid to late 1700s. This was due to advances in mining and canal building.
Historical Perspective of Stratigraphy
• Abraham Werner, mining geologist • In later 1700s he noted that the same strata
could be found in the same order at widely separated locations.
• Implied that local strata could hold clues as to how the global Earth had changed with time.
• Coined the term “Neptunism” which refers to a now obsolete theory of geo-stratification.
Historical Perspective of Stratigraphy
• William “Strata” Smith, English canal surveyor and consulting engineer (until 1799) and Father of Geology.
• Carefully examined strata along canals, roads, railway cuttings and quarries while crisscrossing the English countryside.
• Found that “the same strata were found always in the same order and contained the same fossils.”
William Smith (1815)
First Geologic Map of Great
Britain
Historical Perspective of Stratigraphy
• James Hutton (Founder of Modern Geology; 1726 -1797)
• Examining the sea coast in England he realized that strata are laid down by deposition of sediment in water.
• The sediment came from erosion of the continent.• Internal forces on the Earth later raised the strata
above sea level.• The cycle can repeat over and over.• Time to form a single strata layer from deposition is
many thousands of years.
Stratigraphy (Steno 1669)
• Law of Original Horizontality-infers that sedimentary rock layers were originally deposited as flat-lying (horizontal) layers.
Stratigraphy (Steno 1669)
• Law of Original Horizontality-infers that sedimentary rock layers were originally deposited as flat-lying (horizontal) layers.
• Law of Lateral Continuity-states that sedimentary rock layers are deposited over large areas
Stratigraphy (Steno 1669)
• Law of Original Horizontality-infers that sedimentary rock layers were originally deposited as flat-lying (horizontal) layers.
• Law of Lateral Continuity-states that sedimentary rock layers are deposited over large areas
• Law of Superposition-states that, in a cross-section view, rock layers are oldest at the bottom and become progressively younger upwards.
Stratigraphy (Steno 1669)• Law of Original Horizontality-infers that sedimentary rock
layers were originally deposited as flat-lying (horizontal) layers.• Law of Lateral Continuity-states that sedimentary rock layers
are deposited over large areas• Law of Superposition-states that, in a cross-section view, rock
layers are oldest at the bottom and become progressively younger upwards.
• Law of Cross-Cutting Relations-infers that a rock body (e.g. igneous dike) cutting through another rock body (sandstone beds) is younger than the layers it intrudes; that is, the igneous dike would be younger than the sandstone beds.
Original Horizontality
Lateral Continuity
Superposition
Younger Strata
Older Strata
Even Older Strata
Cross-cutting relations
Fracture is younger than strata because it cuts through
the strata.
Let’s recap
• The story in the rocks.
Something easy. Rank the layers from oldest to youngest
D
A
B
C
Something easy. Rank the layers from oldest to youngest
D, C, B, A from superposition
D
A
B
C
Little harder. Rank the layers from oldest to youngest
D
A
C
E
B
Igneous intrusion
Little harder. Rank the layers from oldest to youngestD, C, E, B, A from cross-cut relation and
superposition
Igneous intrusion
D
A
C
E
B
Little harder. Rank the layers from oldest to youngest
Igneous intrusion
D
A
C
E
B F
Fault
Rank the layers from oldest to youngest
D, C, E, B, A, F
Igneous intrusion
D
A
C
E
B F
Fault
What is the youngest feature?
D
A
B
C
ERiver Valley
What is the youngest feature?E from cross-cutting relation
D
A
B
C
ERiver Valley
Which feature is the youngest?
Which is the youngest?
0 of 150
1 2 3 4
0% 0%0%0%
1. E2. D3. B4. F
Which is older, D or A
Relative Age Date the Features from Oldest to Youngest
A
B
D
E
C
Relative Age Date the Features from Oldest to Youngest
C, E, B, D, A
A
B
D
E
C