Issues in Earth Science

“Eww, There’s Some Geology in my Fiction!”

Issue 10, Nov 2018

Teacher Resources

Suggestions for Activities and Discussions to accompany Readings of

Dinosaur Dig by Jacqueline Howland

Long before geologists could measure the age of a rock by radiometric dating, geologists figured out the stories in rocks (that is, the sequence of events recorded in the rocks) by determining the relative ages of rocks and features through consideration of key principles such as the following:

1) The principle of original superposition

2) The principle of original horizontality

3) Various cross-cutting relationships

These are properly thought of as principles rather than laws because, rather than being the conclusion of scientific investigations in the way that laws are, they are logical reasoning steps that guide us in interpreting observational evidence.

What do we mean by logical? Well, it makes sense that a sediment deposited first will be (originally) on the bottom because gravity will pull it down and later sediments will then be deposited on top of it. It makes sense that a crosscutting event (that is, an event that affects a rock) had to happen after the affected rock already existed. I (Russ) often ask my students, “Here’s broken pencil; which had to happen first, someone broke the pencil or someone made the pencil?” Clearly, the pencil has to exist before it can be broken. Rocks have to exist before they can be affected by later events such as erosion, magma intrusion, or faulting.

These principles seem very obvious and logical on the surface, but application of them is often more nuanced and complex than one might expect! These principles are at the very heart of geological investigation, and comprise a set of investigative methods that are somewhat unique to the geosciences. They provide a great opportunity to help students practice logical, scientific thinking.

Here are a few simple cross-cutting relationships to get started thinking about how they tell us about sequences of events in rock. The pictures below represent cross-sectional views of rock, that is, what we might see in the rock if we had a cut-away view down into the earth.

In the cross-sectional view seen below, which came first, the rocks or the fault that cuts across them? Like the pencil, the rocks have to be there before they can be broken!

In the cross-section below, which had to come first, the igneous dike or the other rocks? We can see that the igneous dike cuts across the other layers, therefore the other layers had to be there first (it doesn’t make sense that a big sheet of molten rock (later to become a dike) poked up into the air before the other rocks around it had formed).

What about the situation below? Can you figure out which had to come first, layer A or layer B?

We can see that chunks of layer A are found inside layer B. That means that layer A had to already exist when layer B formed, otherwise, we couldn’t get layer A inside layer B (We can’t put a pencil in our pencil bag until after the pencil already exists). Students might first think that layer B must be older because it is on the bottom, but yes, sometimes layers can get tipped upside down, like these two (although not commonly)! The inclusion of one rock inside another is not unlike the scenario in the story in which pre-existing fossils of Stegosaurus get included into a later rock layer (although according to the story, the rocks in the story are not tipped upside down).

Consider the two different scenarios below. In the first, which is older, A or B? How do you know (that is, what is the cross-cutting relationships)? Can you tell if these rocks have been tipped upside down or not? How did B have to form?

What about the second case? What is the age-order of A, B, and C? How can you tell? What must be different about how layer B formed in the second case versus the first?

In the first case, the rocks on both sides of B had to already be there when B formed, otherwise they would not have been affected by the high temperature of the magma. This tells us that the magma intruded in between the pre-existing rocks. This feature is called an igneous sill.

In the second case, only rock A was there at the time that B formed, telling us that B must have been a lava flow and layer C formed after the lava cooled. We can infer that these rocks are tipped upside down!

Consider the cross-sectional view below that includes an unconformity—a boundary in the rock that tells the story of an ancient erosional surface that later got covered by new rock. Unconformities are often illustrated in cross-sections with a waving line, symbolizing the unevenness of the erosional surface. Can you figure out which has to be older, A or B?

Because the erosion surface cuts across layer A, but not layer B, we can infer that layer A had to be present before the erosion (you can't erode a rock layer that doesn't exist yet). Layer B was then deposited (horizontally, originally) on top of the old erosional land surface. Thus, A is older than B (once again, the rocks are tipped upside down—rare in real life, but common in our puzzles!)

Now, Let’s turn our attention to the geological setting of the story, Dinosaur Dig.

Which of the following cross-sections below best represents the setting of the dinosaur dig in the story, where both Stegosaurus and Sauropelta fossils were found in a single layer of rock?

This stratigraphic puzzle can be solved by recognizing that the bones were all found in the later (Cretaceous) rock, so the dig must be located in that layer, and by realizing that the Jurassic rocks had to form before the Cretaceous ones, thus the dig site in the Cretaceous rocks must be above and after the unconformity represented by the wavy line. That leaves us with option B.

You might encourage students to imagine or sketch a cross-sectional view of what the topographic landscape might have looked like at the time that the Stegosaurus bones were washing out of cliffs and mixing with Sauropelta. This would represent a time before the unconformity between the Jurassic and Cretaceous had finished forming, before the deposition of most of the Cretaceous rocks or any of the later rocks, and before the modern erosion that has exposed the dig site. One solution might look like that below. This picture shows the insight into past landscapes that Jace got in the story when he started thinking about rocks washing down from the top of a nearby hill.

If you would like to try out a couple of additional stratigraphy challenges, check out the stratigraphy puzzles Lost Civilization! and Mysterious Planet in our Reading the Earth and Sky Challenges (or try the student version which includes hints).

You can find a variety of similar stratigraphy and cross-cutting challenges by searching online using the words:

“order of geological events image”

You can learn more about cross-cutting relationships at http://web.mnstate.edu/colson/ESE/cross-cutting_relationships.html

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The Teacher Resources for Dinosaur Dig are written by Russ and Mary Colson, authors of Learning to Read the Earth and Sky.

Return to Dinosaur Dig by Jacqueline Howland

Return to “Eww, There’s Some Geology in My Fiction.”

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