1. Principles of Relative Dating:
Laws of Stratigraphy [struh-tig-ruh-fee]
Learning Target: I can describe the principles of relative dating and Laws of Stratigraphy.
I know I am successful when I …
Define and explain Laws of Stratigraphy: Law of Original Horizontality, Law of Superposition, Law of Lateral Continuity, Law of Cross-Cutting Relationships, and Principles of Faunal Succession
Distinguish between relative and absolute dating
It’s all about LAYERS!

2. Stratigraphy
The study of how sedimentary rock layers form and their relationship to one another.
Nicolas Steno
17th century Danish Catholic Bishop and Scientist
Surmised that sedimentary rock formed from sediment that settled out of water over time in horizontal layers.
Laid the foundation (lol) for geologists to determine which geological events preceded other events and allowed for relative age relationships.

3. Sediments – fragments of organic or inorganic material that have been broken down
Strata – layers of sediment
Stratigraphic Succession – the way rock layers are built up and changed by geologic processes

4. Click here for a video on: Law of Original Horizontality
Laws of Stratigraphy
Click here for a video on:
Law of Original Horizontality
Layers of sediment are deposited by gravity into flat layers
If the layers are not horizontal, an event occurred after the layers were deposited

5. Click here for a video on:
Laws of Stratigraphy
Click here for a video on:
Law of Superposition
The order in which layers of sediment are deposited indicate their relative age
Oldest layers are beneath younger layers
Which layer is oldest?

6. Click here for a video on: Law of Lateral Continuity
Laws of Stratigraphy
Click here for a video on:
Law of Lateral Continuity
Rock layers will extend outwards until the environment that produced them changes
Similar rock layers that are separated by a valley or some other erosion feature were once continuous and will match up

7. Law of Cross-Cutting Relationships
Laws of Stratigraphy
Law of Cross-Cutting Relationships
If a rock layer is cut by a fault or igneous intrusion, then the rock that is cut must be older than the layer that cuts it

8. Click here for video on: Principle of Faunal Succession (Stop at 1:30)
William Smith
1815 England
Realized that certain rock layers contain certain fossils
Older rock layers held more primitive fossils
Georges Cuvier
Noticed the same fossils in the same layers of rock in France as in England
Determined that relative ages could be determined over great geographic areas
Helped support that the Earth was older than believed and supported the time needed for evolution

9. Relative Dating – assigning an age order based upon position of sedimentary layers in relationship to each other
Which layer is oldest? C
If we know layer
A was deposited 50 mya and
layer C 75 mya
What can we determine about layer B?
Deposited between 50 and 75 mya
50 mya
75 mya

10. Absolute Dating Actual Numeric Age Also known as radiometric dating
Method used to date rocks based on the known decay rate of radioactive isotopes
Radioactive decay is the process by which an unstable atomic nucleus loses energy by releasing radiation
Half-life () is the time required for a quantity to fall to half of its starting value

11. Absolute Dating / Radiometric Dating
Uranium 238  Lead 206
 = 4.46 billion years
Uranium 235  Lead 207
 = 704 million years
Potassium 40  Argon 40
 = 1.3 billion years
Volcanic ash often contains minerals like zircon, feldspar, and biotite which contain natural amounts of these radioactive isotopes.

12. Unconformity - contact between sedimentary rock layers that are significantly different in age
Means a gap in the geologic record not represented by any rock present
Two potential reasons
Sediment deposition stopped for a considerable length of time
Existing rock layers eroded prior to being covered by younger sediment
Erosion surface is buried under younger horizontal layers

13. Hutton’s Unconformity –tilted beds or eroded sandstone are covered by horizontal beds of younger sandstone

14. Disconformity - older rocks were not tilted before being eroded
Erosion is parallel to both the older and younger rock layers
Erosion is often not uniform and undulating (wavy)
Very difficult to determine
Requires radiometric dating of rocks to determine if there is a significant difference in ages

15. Grand Canyon Unconformity
So, why is this important?
Because it allows geologists to create a timeline of events that have shaped the Earth we live on
It’s your turn
to try
Observe the picture of rock layers
Create a relative timeline that explains what took place to make the rock layers look like this
Talk out your ideas at your table
Grand Canyon Unconformity

16. Shale deposited in horizontal layers
Top of shale was eroded
No deposition
Today
Relative Timeline
Earthquake folded layers up
Sandstone layers deposited on top of eroded shale

17. References
Koch, A. (2018). What is Relative Dating. Retrieved March 9, 2018, from
Spencer, C. (2018). Unconformities in Geology. Retrieved March 9, 2018, from
[Steno's Laws]. (n.d.). Retrieved March 11, 2018, from
Tvelia, S. (2018). Principles of Relative. Retrieved March 9, 2018, from
[Unconformity]. (n.d.). Retrieved March 11, 2018, from
Vermont, U. O. (2008, October 02). The Principles of Faunal Succession. Retrieved March 9, 2018, from
Vermont, U. O. (2008, October 02). Relative Dating (Steno's Laws). Retrieved March 9, 2018, from