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 crosssection 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
A
B
C
D
Something easy.
Rank the layers from oldest to youngest
D, C, B, A from superposition
A
B
C
D
Little harder.
Rank the layers from oldest to youngest
A
B
C
D
E
Igneous intrusion
Little harder.
Rank the layers from oldest to youngest
D, C, E, B, A from cross-cut relation and
superposition
A
B
C
D
E
Igneous intrusion
Little harder.
Rank the layers from oldest to youngest
A
F
B
C
D
E
Igneous intrusion
Fault
Rank the layers from oldest to youngest
D, C, E, B, A, F
A
F
B
C
D
E
Igneous intrusion
Fault
What is the youngest feature?
A
B
E
River Valley
C
D
What is the youngest feature?
E from cross-cutting relation
A
B
E
River Valley
C
D
Which feature is the youngest?
Which is the youngest?
1.
2.
3.
4.
E
D
B
F
0 of
150
0%
1
0%
2
0%
3
0%
4
Which is older, D or A
Relative Age Date the Features from
Oldest to Youngest
A
D
C
B
E
Relative Age Date the Features from
Oldest to Youngest
C, E, B, D, A
A
D
C
B
E