Stratigraphy 101
Sleuth- ____________________ Per. _______ Date _______
There is a book that is called Theory of Earth. Now that would be no pocket size edition! Not only was the topic rather
large, but try to imagine explaining the Earth several hundred years ago, when public opinion was a bit less friendly.
Well, that is what James Hutton did in 1795. In this book Hutton described the idea that the processes we see changing
the Earth’s surface today are the same as those that changed its surface in the past, even though we weren’t around to
see it happen. Hutton called this idea the Principle of Uniformitarianism, (another great Scrabble word!) and he set the
stage for modern geology.
1.
What is the Principle of Uniformitarianism?
Okay, so things have changed over time, and are still changing as we sit in this classroom. But how has it changed? Rock
strata are like pages in a book. They have to be read to understand the changes that have taken place. Let’s try it and
see how we do at “reading” those pages. Look at the rock layers in this picture. Make a prediction about the sequence
of when each layer formed. Using a colored pencil, put a number #1 on the layer that you think occurred first, and so
on. Number all the lettered areas with your best guess!
Prediction Color
Final Answer Color
We will return to this diagram later. But first let’s learn about some basic principles of stratigraphy in geology.
Stratigraphy is the result of what geologists and archaeologists refer to as the “process of stratification”, or
the process by which layers of soil and debris are laid down on top of one another over time. The easiest way
to think about stratigraphy is as if you were making lasagna: first you put down the first layer of pasta, then
cheese on top of that, then sauce, then another piece of pasta, and so on. There is no way for you to put the
bottom layer of pasta on before you put the sauce on, and still maintain the same sequence or location of
these different layers.
This works the same way for archaeology, and can be used to determine a sequence of events. This is what
archaeologists and geologists call the Law of Superposition. Simply put, where one layer overlies another, the
lower layer was deposited first.
2. What is stratigraphy?
3. What does the Law of Superposition state?
There are several other guiding principles in geology. For example, the Law of Lateral Continuity states that strata
originally extended in all directions until it thinned out to zero or ended against the edges of another structure or
deposit. This means that layers are formed over large areas, not in patches. It is also understood that deposition occurs
in horizontal layers, known as the Law of Original Horizontality, and that folding or intrusions of other rock are younger
than all the layers that they cut through, which is the Principle of Cross-Cutting Relationships.
4.
5.
What does the Law of Lateral continuity state?
What does the Law of Original Horizontality state?
6. What does the Principle of Cross-Cutting Relationships state?
But what if layers are eroded away? An unconformity is a place where rock layers are missing. In the Grand Canyon
there is a major erosion of several layers at the very bottom that is called “The Great Unconformity” due to its size. In
this part of the rock strata there has been a massive shearing off of diagonal rock layers.
7. What is an unconformity?
Draw an arrow in the diagram to the left showing
where it is.
Finally, there is a pattern, and, oh yes, yet another law, about how fossils are deposited! Can you think of what this
might be?
It is based on the observation that
sedimentary rock strata contain
fossilized flora (plants) and fauna,
(animals), and that these fossils
succeed each other vertically in a
specific, reliable order that can be
identified over wide horizontal
distances. This is called the Law of
Faunal Succession and it allows
scientists to date fossils and rocks
using relative dating.
8.
What is the Law of Faunal Succession?
Now go back to the original drawing at the beginning. Using a different colored pencil, go back and renumber the layers
with what you believe to be the sequence of rock formation. Check your work!