Four principles of geologic stratigraphy

Introducing the science of stratigraphy.

READ THIS - This "active page" is an attempt to enhance plain old readings by having you engage with ideas and concepts. If your feedback is positive we hope eventually to enliven much of the content this way.
Even though there are "questions" and "answers", this is NOT a quiz. "Answers" and feedback are provided but do not just skip to the answers. Use your brain to figure stuff out. (The "Show all questions" button displays all questions for this page.)

For a 4:13 minutes introduction to these concepts see https://www.youtube.com/watch?v=rc3da3-znK4.

According to the Principle of Original Horizontality, sediments are deposited horizontally. After they have been transformed into rock, strata may become tilted or folded by various tectonic movements. However, this principle basically states that "tilted or folded layers used to be flat”.

The Principle of Superposition states that in layered strata (sedimentary rocks or lava flows), the oldest layer will be at the bottom of the exposed strata and the youngest at the top. Effectively "what's on top is youngest”. Although this might appear to be somewhat obvious it is important to remember that this principle was being introduced to a world that believed that everything around us had been created over a period of days. To suggest that a vast pile of strata had not simply been called into existence but had developed over a very long period of time was a new paradigm.

So ... which rocks were deposited first?
1. Rocks in layer A
2. Rocks in layer B
3. Rocks in layer C
4. Rocks in layer D
5. Rocks in making up those at E
But ... which layer was deposited first in this formation (displayed at http://faculty.kutztown.edu/friehauf/classphotos/structural_photos.html)?
1. Rocks at the top of the bluff
2. Rocks at the bottom of the bluff near the man
3. Left most layers
4. Right most layers
5. Needs more information
6. It won't be possible to know.
To explore original horizontality and other principles, we will work with an interactive geology modelling tool called Visible Geology. This is an interactive geologic model construction and analysis tool that runs in your web browser. YOU MUST USE EITHER FIREFOX OR CHROME BROWSER FOR RUNNING VISIBLE GEOLOGY. OTHER BROWSERS DO NOT UNDERSTAND HOW TO RUN THIS SIMULATION.
Before playing with Visible Geology, be sure to view these 3 short "how to" video clips:
1. Introduction: What you can do in VG: (2:04 mins)
2. First things you should do: (1:17 mins)
3. Geology tutorial; (1:12 mins)
4. Starting, using, and correcting beds and tilt: (1:20 mins))
First Question ... Which of the four labelled video clips is primarily an introduction to "geologic beds".
1. "Introduction ..."
2. "First Things ..."
3. "Geology ..."
4. "Starting, using ..."
Once you have watched these introductory clips (should take less than 5 minutes total), open Visible Geology here and start playing with the simulation. Answer remaining questions below as you go along. RECALL you MUST use either Firefox or Chrome web browser to run Visible Geology. Chrome is most reliable and fastest. We recommend copying this link
http://app.visiblegeology.com/profile.html
into a separate browser while you keep this course website open.

In Visible Geology Click “Geologic Beds” and place 5 beds. Edit any or all if you like (thickness and colour).

QUESTION: What is the default "width or length" of your volume in the tool's arbitray "units" (just enter the number - it could be kilometers, miles ... whatever).
Next (still In Visible Geology) - tilt your model. Explore use of the "tilt" tool, then make the tilt between 45 and 90 degrees.

QUESTION: What default angle does the “dip” tool start with?
While adjusting "tilt": What does a 90 degree dip angle cause beds to do? If you are puzzled by the "tilt" (or dip) tool, see the fourth Youtube video above. Recall that in the 'tilt' tool, the red "sheet" represents the angle that will be applied when you click the "Add new tilting event" button.
1. Tilt to a vertical orientation
2. Remain horizontal
3. Rotate 90 degrees about a vertical axis
In Visible Geology, click the "topography" tab and select "valley", "mountain", "cliff" etc. to see how the ground's topography affects what patterns you would see if you could stand on the geological scenario you just made.

The third principle, the Principle of Lateral Continuity, states that a stratum (layer) of sedimentary rock will continue in all directions until it thins, grades into another type of sediment, or comes against the edge of the depression into which it is being deposited.

Which two steps would you use to generate the simplest possible simulation of sedimentary layers continuing across an eroded valley?
1. geologic beds
2. tilting
3. folding
4. simple unconformity
5. Dikes
6. Faults
7. Topgraphy (hill or mountain)
8. Topgraphy (valley)
9. Topography (cliff)
The fourth principle, the Principle of Cross-cutting Relationships, was first articulated by James Hutton, a Scottish geologist, physician, chemist, and experimental farmer. Examples of crosscutting relationships in geology are shown in the upcoming section called “Unconformities”.

Hutton believed that the relationships between stratigraphic units indicated that Earth was very old. The famous geological section that Hutton used to demonstrate this is Siccar Point in Scotland. We will discuss this next.

If you have finished the activities above you can close Visible Geology, answer this last question and then move to the next section of Lesson 1, "Siccar Point".

QUESTION: Which three of the following tools in Visible Geology will allow you to add a "cross cutting relationship" to an already existing model?
1. beds
2. tilting
3. folding
4. simple unconformity
5. Dikes
6. Faults